Language guide

Tom Mulgrew

This documentBasic4GL OverviewBASIC Language SyntaxSyntax differences"Include" filesBasic language featuresCommentsCase insensitivitySeparating instructionsVariables and data typesDeclaring variables (with Dim)Converting between data typesLiteral constantsNamed constantsConstant expressionsStructuresPointersFunction pointersMixing structures, arrays and pointersAllocating dataExpressionsOperatorsExpression operandsBoolean values and expressionsLazy evaluationFlow controlGotoGosubIf .. Then .. Elseif .. Else .. EndifWhile .. WendFor .. nextDo .. loopFunctions and subroutinesSub/End SubFunction/end functionDeclareFunction restrictionsFunction/subroutine pointersDeclaringAssigningCalling a function/subroutine pointerOther function/subroutine pointer operationsLooking up function pointers by nameProgram dataDataReadResetExternal functionsRuntime compilationCompCompFileExecFunctions/subs inside compiled codeCalling functions/subs in runtime codeRuntimeBindCodeRestart/end instructionsEndRun

This document

This document describes the basic syntax of Basic4GL programs.
This document focusses on the language itself, and and as such does not go into the individual functions and constants, or how they are intended to be used.

Basic4GL Overview

Basic4GL is designed to combine a simple, safe and easy to understand programming language based on traditional BASIC.

Basic4GL compiles programs to byte code, which it runs on a virtual machine. This makes Basic4GL a safe language to experiment in as the virtual machine protects the programs from writing to invalid addresses or jumping to uninitialised code, and handles cleaning up resources such as OpenGL textures automatically.
In addition, the Basic4GL virtual machine automatically handles certain setup tasks such as creating an OpenGL capable window (and initialising OpenGL state), handling windows messages, buffering keyboard input.

Basic4GL programs do not need to initialise OpenGL windows, link to libraries, include header files or declare function prototypes.
This means you can cut through all the paperwork and get straight to the code that does the actual work.

The following examples are complete Basic4GL programs.

Example 1, A "Hello world" program:

print "Hello world!"

Example 2, drawing a square in OpenGL:

glTranslatef(0, 0, -5)
    glVertex2f( 1, 1): glVertex2f(-1, 1): glVertex2f(-1,-1): glVertex2f( 1,-1)

BASIC Language Syntax

As well as its own syntax, Basic4GL supports 3 other "traditional" BASIC syntaxes, which are intended to improve compatibility with other BASIC compilers.

These syntaxes must be explicitly enabled, either:

Note: Most of the Basic4GL example programs are written for the regular "Basic4GL" syntax, and may break if the default syntax is changed in the "BASIC rungime settings" dialog. It is safer to add a "language" directive instead.

The syntaxes are:

language basic4gl

This is the default Basic4GL language syntax, as described in this document.

Traditional PRINT only
language traditional_print

A more traditional 'print' command, but otherwise like the "Basic4GL" syntax.

language traditional

A more traditional BASIC syntax, with respect to the 'print' command and omitting brackets from function parameters.

Traditional including variable suffixes
language traditional_suffix

Like "Traditional", but with more traditional rules for variable suffixes (and default variable type if suffix is omitted).

Syntax differences

The specific differences between the syntaxes are as follows


For the "basic4gl" and "traditional_print" syntaxes, division follows the same conversion conventions as other mathematical operators. In particular dividing an integer by another integer will be executed as an integer division with an integer result (the remainder is discarded).

printr 7/2         ' Will print 3 (integer division used)
printr 7.0/2       ' Will print 3.5

For the "traditional" and "traditional_suffix" syntaxes, division is always performed as floating point division, producing a floating point result (even if both operands are integers).

printr 7/2         ' Will print 3.5 (floating point division used)
printr 7.0/2       ' Will print 3.5

"if" statements implicit "endif"

For the "basic4gl" and "traditional_print" syntaxes, every "if" statement must have an "endif", even if the statement is all on one line.

if 5 > 3 then
    printr "Expected"
if 6 < 2 then printr "Unexpected!" endif
if rnd()%10 = 0 then printr "Possible" else printr "Probable" endif

For the "traditional" and "traditional_suffix" syntaxes the "endif" command can be omitted if the "if" statement body is on the same line.

if 5 > 3 then
    printr "Expected"
endif                                  ' Multi-line => requires explicit endif
if 6 < 2 then printr "Unexpected!"                            ' Implicit endif
if rnd % 10 = 0 then printr "Possible" else printr "Probable" ' Implicit endif

Brackets on function calls

The "basic4gl" and "traditional_print" syntaxes require brackets on all calls to functions (either user defined or builtin), except for these 4 commands: cls, print, printr and locate.

sub Mysub()
    dim a = rnd()%5
    locate 10, 12
    print sqrt(a)


For the "traditional" and "traditional_suffix" brackets are omitted when calling any function that:

sub Mysub()
    dim a = rnd % 5
    sleep 1000
    locate 10, 12
    print sqrt(a)


Print command

For the "basic4gl" syntax, the "print" command always leaves the cursor on the same line. To have the cursor move to a newline, you must instead use the "printr" command.

printr "============"
printr " Tom's game"
printr "============"
print "Please enter your name:"

For the "traditional_print", "traditional" and "traditional_suffix" syntaxes, the "print" command leaves the cursor on the same line if it ends with a semicolon (;), otherwise it moves it to the next line.

print "============"
print " Tom's game"
print "============"
print "Please enter your name:";

Default data type

For the "basic4gl", "traditional_print" and "traditional" syntaxes, the default data type of a regular variable declared without a suffix (or "as" type specifier) is integer.
To declare a floating point variable, you must suffix it with a hash (#) character (or declare it "as single").

dim myInt
dim myFloat#
dim myOtherFloat as single

For the "traditional_suffix" syntax, the default data type of a regular variable declared without a suffix (or "as" type specifier) is floating point.
To declare an integer variable, you must suffix it with a percentage (%) character (or declare it "as integer").

dim myFloat
dim myInt%
dim myOtherInt as integer

"Include" files

Basic4GL supports a very simple "include" mechanism.
You can include a file in your main program with:

include filename.ext

Where filename.ext is the filename and extension of the file you wish to include.

IMPORTANT: "include" must be on it's own line, with no leading spaces before the "include" keyword.

When keyed in correctly the line will become highlighted, and filename.ext will be displayed as an underlined hyperlink (which you can click to open up the include file).

Basic4GL will compile your file as if all the lines of filename.ext had been cut and pasted in at the point of the include.

IMPORTANT #2: "include" is not supported by the runtime compilation functions ("Compile()" and "CompileFile()").

Basic language features


Comments are designated with a single quote.
All text from the quote to the end of the line are ignored by the compiler.

' Program starts here
dim a 	'Declare a variable
a = 5 	'Initialise to a value
print a 'Print it to screen

Is equivalent to:

dim a
a = 5
print a

Case insensitivity

Basic4GL is a case insensitive language. This applies to all keywords and variable names, and infact anything except the contents of string constants.

The following lines are all equivalent:

glVertex2f(x, y)
glvertex2f(X, Y)

The following lines are not equivalent:

print "Hello World"
print "hello world"

(because the "Hello world"s are quoted strings).

Separating instructions

Instructions are separated by colons ":" or new-lines.

The following code sample:

dim a$: a$ = "Hello": print a$

Is equivalent to:

dim a$
a$ = "Hello"
print a$

Variables and data types

Basic4GL supports only 3 basic data types (although they can be combined into structures which are described further on).

Variables are declared and allocated explicitly with the "Dim" instruction.
Attempting to use a variable without declaring it with "Dim" will result in a compiler error.

A naming convention is used to designate the type of each variable, as follows:

Declaring variables (with Dim)

All variables must be declared with Dim before use.

The format is:

Dim variable [, variable [, ...]]

For example:

Dim a
Dim name$
Dim a, b, c
Dim xOffset#, yOffset#
Dim ages(20)
Dim a, b, c, name$, xOffset#, yOffset#, ages(20)

Dim is both a declaration to the compiler that the keyword is to be treated as a variable, and an executed instruction. Therefore the Dim instruction must appear before the variable is used.

This program:

a = 5
Dim a

Results in a compiler error, because the compiler encounters 'a' in an expression before it is declared with "Dim".

This program:

goto Skip
Dim a
a = 5

Compiles successfully but results in a run time error, as it attempts to write to 'a' before the "Dim" instruction has executed, and therefore no storage space has yet been allocated for it.

The correct example is (of course):

Dim a
a = 5

Compatibility with other BASICs

Basic4GL also supports the syntax:

Dim variable as type

Where type can be one of:

Or a structure type.

Note: Basic4GL has only one floating point type which is a single precision float (ie a "single"). The "double" keyword is still accepted for compatibility, but Basic4GL still allocates a single precision floating pt number.

Declare and assign

You can also assign a variable's initial value in the dim statement.

The format is:

Dim variable = value [, ...]

For example:

dim a = 5
dim v#(2) = vec3(1, 2, 3)
dim s$ = "Hello"
dim sq2# = sqr(2)

This is equivalent to declaring the variable then assigning its initial value, e.g.

dim a: a = 5
dim v#(2): v# = vec3(1, 2, 3)
dim s$: s$ = "Hello"
dim sq2#: sq2# = sqr(2)

Allocating variable storage

Storage space is allocated when the "Dim" instruction has been executed.
In addition Basic4GL automatically initialises the data as follows (unless an initial value has been specified):

Re-Dimming a variable

Attempting to Dim the same variable twice results in a runtime error.
There is currently no way to re-dim a variable. However, this may be included in a future version of Basic4GL.

Array variables

Basic4GL supports single and multi-dimensional arrays. These are "Dim"med by specifying the array variable name, followed by a number in round brackets. Basic4GL will allocate elements from indices 0, through to and including the value specified in the brackets.


Dim a$(10)
Dim size#(12)
const MaxThings = 12
Dim ThingHeight# (MaxThings), ThingWidth#(MaxThings)
dim count: count = 10
Dim array(count), bigArray (count * 10)

For arrays of more than one dimension, each dimension is specified in its own pair of brackets.


Dim matrix#(3)(3)
matrix#(2)(3) = 1
const width = 20, height = 15
Dim grid(width)(height)

Is mentioned, Basic4GL allocates elements from indices 0, through to and including the value specified in the brackets.
For example:

Dim a(3)

Will allocate four integers, named a(0), a(1), a(2) and a(3), and set their values to 0.

Basic4GL arrays are sized at runtime. You can use any (expression that can be cast to an integer) to specify the number of elements.
However, keep in mind that Basic4GL will stop with a runtime error if you attempt to allocate array:

Basic4GL arrays can be copied by specifying the array name without any brackets or indices. The target array must be the same size as the copied array, otherwise a runtime error will result.

Dim a$(4), b$(4)
b$ = a$ ' Copy entire array from a$ to b$

Likewise some functions accept arrays as parameters, or return them as results:

Dim matrix#(3)(3)
matrix# = MatrixTranslate(-.5, -.5, -2)
glVertex2f(0, 0): glVertex2f(1, 0): glVertex2f(0, 1)

If you specify just one dimension of a 2D array, the result is a 1D array, which can be assigned to/from variables or passed to to/functions like any other 1D array of the same type.

dim vectors#(12)(3), temp#(3)
temp# = vectors#(4)

Likewise, specifying N dimensions of a M dimension array results in a (M - N) dimension array.

Compatibility with other BASICs

Basic4GL also supports the syntax:

Dim variable(dimension [,dimension [...]])

For multidimension arrays.


dim grid(20, 10)
grid(3, 7) = 12

Is exactly equivalent to:

dim grid(20)(10)
grid(3)(7) = 12

Why not automatically allocate variables?

Early designs of Basic4GL were intended to allocate variables automatically the first time they were encountered.
However Basic4GL is case insensitive, and OpenGL uses long constants for bitmasks and flags.

Therefore, mistyping (or miss-spelling) a constant in an OpenGL function call such as:

glClear(GL_DEPTH_BUFER_BIT) ' Missing an "F" in "BUFFER"

Would have resulted in a code that still compiles, but instead of passing the value of GL_DEPTH_BUFFER_BIT into the function, Basic4GL would have created a new variable called GL_DEPTH_BUFER_BIT, initialised the value to 0, and then passed 0 into the function.
This type of error can be very confusing and frustrating, especially when learning a library such as OpenGL.

Therefore variables must be explicitly declared with Dim.

Converting between data types

You can convert a variable, or an expression value to a different type, simply by assigning it to a variable of that type, providing the conversion type is one of the ones below:

Certain expression operators such as +, -, *, / can also result in an automatic conversion of either the left or right operand to match the other, using the following rules:

Literal constants

To use a literal integer in a Basic4GL program, simply specify the integer value. Examples:

Dim a: a = 5
Dim a: a = -5

Likewise to use a literal real:

Dim a#: a# = 3.14159265

Literal integers can also be specified in hexidecimal using the 0x prefix. Examples:

Dim a: a = 0xff
Dim a: a = -0xff

To use a literal string, simply encase the string in double quotes. For example:

Dim helloString$: helloString$ = "Hello world!"

Basic4GL does not support literal prefix notations, such as \n for newline in C/C++.
You can however use the Chr$() function to achieve the same effect, for example:

Dim a$: a$ = "Bob says " + Chr$(34) + "Hello!" + Chr$ (34)
Print a$

Will output:

Bob says "Hello"

Named constants

Basic4GL also has a number of named constants, such as M_PI and GL_CULL_FACE.

Note: Two commonly used constants are "true" and "false", which evaluate to -1 and 0 respectively.

You can add constants using the "Const" instruction.

The format is:

Const name = value [, name = value [, ...]]


For example:

const Things = 20
const Max = 100, Min = 1
const StepCount = 360, StepSize# = 2 * m_pi / StepCount
const major = 3, minor = 7, version$ = major + "." + minor

Constant expressions

Certain instructions require constant expressions, such as the "const" instruction (described above), and the "step" part of the "" instruction.
These expressions must always evaluate to the same value and Basic4GL must be able to calculate this value at the time the program is compiled.

An expression must satisfy these criteria to be considered "constant" by Basic4GL:


m_pi / 180
true and not false
"banana " + "split"
"Pi = " + m_pi

Are all valid constant expressions

Expressions are not considered constant if they contain variables or functions. This holds even for expressions that (to a human) are obviously constant.
For example:

sqrt (2)
length(vec3(1, 1, 1))

Are not valid constant expressions in Basic4GL, even though it is clear to us that they will always evaluate to the same value.


Structures are used to group related information together into a single "data structure".
The format is as follows:

Struc strucname

[dim] field [, field [,...]]
dim] field [,[field [,...]]]



struc SPlayer
    pos#(1), vel#(1)
    dir#, lives, score, deadCounter, inGame
    leftKey, rightKey, thrustKey, shootKey

This defines a data storage format. You can now allocate variables of the new structure type by using a special format of the "Dim" instruction:

Dim strucname variablename


Dim SPlayer player
const maxPlayers = 10
Dim SPlayer players(maxPlayers)

Each variable now stores all the information described in the structure. You can access these individual fields using the "." operator as follows:


For example:

player.pos#(0) = 12.3
players(4).score = players(4).score + 10
i = 3
print players(i).lives

You can also assign variables of the same structure type to one another. This will copy all the fields from one variable to the other.

player(7) = player(6)

Compatibility with other BASICs

Basic4GL also supports the syntax:

Type typename

variable as type [, variable as type [...]]

End type


struc SpaceMartian
    x#, y#

Is equivalent to:

type SpaceMartian
    name as string
    x, y as single
    health(4) as integer
end type

(Except that in the first example the field names now have $ and # post-fixes.)

Arrays inside structures

Structures can contain arrays. Unlike regular arrays, the size of an array in a structure must be fixed at compile time. This means that the array size must be either a numeric constant, or a named constant, or a constant expression.

For example:

struc STest: dim a(10): endstruc
const size = 20
struc STest2: dim array$(size): endstruc

Will work.

However this example:

dim size: size = 20
struc STest2: dim array$(size): endstruc

Will cause a compile time error, because size is now a variable and is not fixed at compile time. (Even though it's obvious to a human that it will always be 20!)


Basic4GL has a pointer syntax which is vaguely similar to C++'s 'reference' type, but a lot more simplified.

Declaring pointers

Pointers are declared by prefixing a "&" character before the variable name in the "Dim" statement.
The syntax is then the same as "Dim"ming a regular variable, except that array dimensions must be specified with "()" (i.e with no number in the brackets).

So whereas:

Dim i, r#, a$, array#(10), SomeStructure s, matrix#(3)(3)

Declares and allocates:

Dim &pi, &pr#, &pa$, &parray#(), SomeStructure &ps, &pmatrix#()()


Setting pointers

Pointer variables are initially unset. Attempting to read or write to the data of an unset pointer results in a runtime error (although you can still compare them to other pointers or the special 'null' constant). To do anything useful you need to point them to a variable, otherwise known as "set"ting them.

Pointers are set using this syntax:

&pointer = &variable


Dim a$, &ptr$
a$ = "Hello world"
&ptr$ = &a$
print ptr$
Dim array(10), &element, i
for i = 1 to 10: &element = &array(i): element = i: next
dim matrix#(3)(3), &basisVector#(), axis, i
matrix# = MatrixIdentity ()
print "Axis? (0-3): ": axis = Val(input$()) ' Enter 4 to crash!
&basisVector# = &matrix#(axis)
for i = 0 to 3: print basisVector#(i) + " ": next

Accessing pointer data

Once a pointer is set, it can be accessed like any other variable, i.e read, assigned to, passed to functions e.t.c. The actual data read from or written to will be that of the variable that it is pointing to.

Dim a, b, &ptr
&ptr = &a
a = 5 ' a is 5, b is 0
b = ptr ' a is 5, b is 5
ptr = b + 1 ' a is 6, b is 5
print "a = " + a + ", b = " + b

Un-setting pointers

You can "un-set" a pointer by assigning it the special constant null, as follows:

Dim val, &ptr
&ptr = &val ' Pointer now set
&ptr = null ' Pointer now un-set

You can also compare a pointer to null.

if &ptr = null then


if &ptr <> null then



Function pointers

Basic4GL also has a pointer to function/subroutine variable type.

These are discussed in more details in the function/subroutine pointers section.

Mixing structures, arrays and pointers

You can mix structures, arrays and pointers mostly in any way you wish.
There are a few limitations to keep in mind however:

You cannot allocate an array of pointers, as:

Dim &ptrs()

will allocate a pointer to an array.

If you really need an array of pointers you can use the following workaround:

struc SPtr: dim &ptr: endstruc
dim SPtr array(100)

Then you can set the pointers using:

&array(5).ptr = &var

(or similar.)

Allocating data

Basic4GL supports a very simple memory allocation scheme. Memory once allocated is permanent (until the program finishes). There is no concept of freeing a block of allocated memory.

This has some obvious limitations, but it does avoid a large number of pointer related bugs.

Data is allocated as follows:

alloc pointername [, arraysize [, arraysize [...]]]

Where pointername is the name of a Basic4GL pointer variable DIMmed earlier.


dim &ptri
alloc ptri ' Allocate an integer
dim &ptrr#
alloc ptrr# ' Allocate a real numer
dim &ptrs$
alloc ptrs$ ' Allocate a string
struc SPlayer: dim x, y, z: endstruc
dim SPlayer &ptrplayer
alloc ptrplayer ' Allocate a player structure

Basic4GL allocates a variable of the type that pointername points to, and then points pointername to the new variable.

To allocate an array, add a comma, and list the dimension sizes separated by commas.


dim &ptrarray()     ' Array size is not specified here!
alloc ptrarray, 100 ' Specified here instead!
dim &ptrMatrix#()()
alloc ptrMatrix, 3, 3

As with DIMming arrays, specifiying N as the array size will actually create N+1 elements: 0 through to N inclusive.
Also the array size is calculated at runtime, and is subject to the same rules as DIMming an array (size must be at least 0 e.t.c).



Basic4GL evaluates infix expressions with full operator precedence.

In most loosely to most tightly bound order:

Operator Description Example
or Bitwise or a# < 0 or a# > 1000
and Bitwise and a# >= 0 and a# <= 1000
xor Bitwise exclusive or a = a xor 255
lor Bitwise lazy or a# < 0 lor a# > 1000
land Bitwise lazy and a# >= 0 and a# <= 1000
not Bitwise not not a# = 5
= Test for equal
= can also be used to compare pointers of the same type, or to compare pointers to null.
a# = 5
<> Test for not equal
<> can also be used to compare pointers of the same type, or to compare pointers to null.
a# <> 5
> Test for greater than a > 10
>= Test for greater or equal a# >= 0
< Test for less than a# < 9.5
<= Test for less or equal a <= 1000
+ Add numeric values, or concatenate strings  
- Subtract  
* Multiply  
/ Divide  
% Remainder  
- (with single operand) Negate a * -b


  1. + and - have equal precedence (except when minus is used to negate a single value).
  2. The comparison operators: =, <>, >, >=, <, <= all have equal precedence.

Operators with equal precedence are evaluated from left to right.

You can force Basic4GL to evaluate expressions in a different order by enclosing parts of them in round brackets. For example:

(5 + 10) / 5

Will add 5 to 10, then divide the result by 5 (giving 3), whereas:

5 + 10 / 5

Will divide first, then add, and the resulting value will be 7.

Operators generally operate on standard integer, real and to a lesser extent string types. However certain operators have been extended to work with 1D and 2D arrays of real numbers for vector and matrix functions. These are explained in the Programmer's Guide. Also the = and <> operators can also be used to compare pointers to each other, or to compare pointers to null.

Expression operands

An expression operand can be any of the following:

Boolean values and expressions

Basic4GL stores boolean values as integers, where 0 is false and anything non 0 is true.

The comparison operators <, <=, =, >=, >, and <> all evaluate to -1 if the comparison is true or 0 if it is false.

The "and" and "or" operators perform a bitwise "and" or "or" of the respective operands.

Effectively this means that "and" and "or" can be used in both boolean expressions and bit manipulation.

Boolean example:

If a < 0 or a > 10 Then Print "Out of range": Endif

Bitwise example:


Lazy evaluation

Basic4GL supports lazy evaluation through the "land" and "lor" operators.
Here "lazy" means that Basic4GL will stop evaluating a boolean (true/false) expression as soon as it knows what the result will be.

For example, the expression:

age# < 15 land not accompanied_by_adult

will not even evaluate "not accompanied_by_adult" if age# were set to 42 (for example), because Basic4GL already knows that "age# < 15" evaluates to false and therefore the whole expression will evaluate to false.

Besides the lazy behaviour, "land" is exactly equivalent to "and" and "lor" is exactly equivalent to "or".

Proper use of lazy evaluation can make your programs more efficient, and can be useful in situations where evaluating all of the expression may produce undesirable results. For example:

if i >= 0 and i <= 10 and array(i) = searchValue then

could halt your program with an "Array index out of range" error if "i" happened to be 11 (assuming "array" is a 0..10 element array). Whereas:

if i >= 0 land i <= 10 land array(i) = searchValue then

will not halt your program, because "array(i)" is only ever evaluated if "i >= 0" and "i <= 10" have already evaluated to true.

Flow control


Jumps directly to a new position in the source code.


Goto labelName

Where "labelName" is a Basic4GL label declared as the first identifier on a line, followed by a colon.
Basic4GL will jump straight to the offset of the "labelName" label, and continue execution.
For example:

Print "Hello "
Goto Loop

Creates an infinite loop, where "Hello" is printed again and again.


Calls a subroutine.


Gosub labelName

Where "labelName" is a Basic4GL label, declared exactly the same way as with the "Goto" instruction.

The subroutine should directly follow the "labelName" label, and be terminated with a "Return" instruction.
When "Return" executes, Basic4GL will jump to the instruction immediately after the "Gosub" instruction.


Dim name$: name$ = "Bob"
locate 10, 10: gosub Name
locate 20, 4:  gosub Name
locate 3, 15:  gosub Name
locate 30, 20: gosub Name

    print name$

To encounter a "Return" instruction, without a corresponding "Gosub" is a runtime error.
A "Gosub" without a "Return" will not cause a runtime error, but will waste stack space.
If too many "Gosub"s are without "Return"s will eventually cause a "stack overflow" runtime error.

If .. Then .. Elseif .. Else .. Endif

Executes a block of code conditionally.


If expression Then
If block


If expression Then
If block
Else block

Basic4GL evaluates "expression". It must evaluate to an integer (usually the result of a boolean expression).
If the expression evalutes to true (non zero), then the "If block" instructions are executed.
Otherwise the "Else block" instructions are executed if present.

Example 1:

If lives < 1 then
Print "Game Over"

Example 2:

If score > highscore Then
Print "New high score!"
highscore = score
Print "Better luck next time."

Basic4GL also supports the "Elseif" keyword, which is equivalent to an "else" followed by an "if", but removes the need for an extra "endif" at the end of the "if" structure.

if expression1 then
elseif expression2 then

Is equivalent to:

if expression then
if expression2 then

Any number of "endif" sections can be placed after the initial "if". You cannot place an "endif" after the "else" section however.

Example 3:

dim a
for a = 0 to 10
    if     a = 0  then printr "Zero"
    elseif a = 1  then printr "One"
    elseif a = 2  then printr "Two"
    elseif a = 3  then printr "Three"
    elseif a = 4  then printr "Four"
    elseif a = 5  then printr "Five"
    elseif a = 6  then printr "Six"
    elseif a = 7  then printr "Seven"
    elseif a = 8  then printr "Eight"
    elseif a = 9  then printr "Nine"
    elseif a = 10 then printr "Ten"
        printr "???"

Example 4:

dim score
print "Enter score (0-100): "
score = Val (Input$ ())
print "Your grade is: "
if     score < 20 then printr "F"
elseif score < 30 then printr "E"
elseif score < 50 then printr "D"
elseif score < 70 then printr "C"
elseif score < 90 then printr "B"
else                   printr "A"

Compatibility with other BASICs

Basic4GL also supports the syntax:

If condition Then
end if

The "if" must follow immediately after the "end", otherwise it will be interpreted as an "end" program instruction.

While .. Wend

Executes a code block repeatedly while an expression is true.


While expression
Code block

This creates a conditional loop. Basic4GL evalutes "expression", which again must evaluate to an integer (and is usually a boolean expression).
If the expression evaluates to false (zero), then Basic4GL will jump straight to the instruction following the "Wend", and continue.
If the expression evaluates to true Basic4GL will execute the code block, then re-evaluate the expression.
Basic4GL will continue executing the code block until the expression evaluates to false.


while lives > 0
' Do gameplay
' Game over

For .. next

Used to create loops with a loop counter variable.


For variable = begin-value To end-value
Code block


For variable = begin-value To end-value step step-constant
Code block

This creates a loop, where "variable" counts from "begin-value" to "end-value". Variable must be a numeric type (integer or real), and cannot be an array element or structure field. Step-constant must be a constant expression (integer or real). If no "step" is given the step-constant defaults to 1.

Basic4GL will count either upwards or downwards depending on whether the step-constant is positive or negative.
If step-constant is positive, the construct is exactly equivalent to:

variable = begin-value
While variable <= end-value
Code block
variable =
variable + step-constant

If step-constant is negative, it is equivalent to:

variable = begin-value
While variable >= end-value
Code block
variable =
variable + step-constant

And if step-constant is zero, it is equivalent to:

variable = begin-value
While variable <> end-value
Code block


Example 1:

dim index
for index = 1 to 10
    printr "Index = " + index

Example 2:

dim count: count = 10
dim squared(count), index
for index = 0 to count
    squared(index) = index * index

Example 3:

dim angle#
glTranslatef (0, 0, -3)
glBegin (GL_LINE_LOOP)
for angle# = 0 to 2 * m_pi step 2 * m_pi / 360
    glVertex2f(sin(angle#), cos(angle#))

Example 4:

dim count
for count = 10 to 1 step -1
    cls: locate 20, 12: printr count
    Sleep (1000)
cls: locate 15, 12: print "Blast off!!"

Do .. loop

Also used to execute a code block a number of times.


Code block


do while condition
Code block


do until condition
Code block


Code block
loop while condition


Code block
loop until condition

If a 'while' or 'until' clause is attached to the 'do' command, it is applied immediately before the code block is executed and may prevent the loop from executing at all.

If it is attached to the 'loop' command, the code block will be executed first, then the 'while' or 'until' clause will be evaluated to determine whether to continue looping. The code block will always be executed at least once.

A 'do' .. 'loop' with no 'while' or 'until' clause creates an infinite loop.

Functions and subroutines

User defined functions and subroutines are created with the "function" and "sub" keywords respectively.
They are blocks of code that are "called", much like when you "gosub" to a label. At this point the computer executes the code inside the function/subroutine and then resumes executing from the instruction after the one that called the function/subroutine.

You are strongly recommeneded to use functions/subroutines instead of gosub/return, as it is generally considered to be better programming practice. Functions/subroutines introduce a number of features not supported by gosub/return:

Sub/End Sub

To create a subroutine, use "Sub" and "End Sub"


Sub name([param[, param[,...]]])


End Sub

Where name is the name of the subroutine, and must not have already been used for a variable, function, other subroutine etc.
param are optional parameters that will be passed to the subroutine, and can be used inside it like variables.


sub MySubroutine()
    print "Hello"
end sub
sub PrintAt(x, y, text$)
    locate x, y
    print text$
end sub

The format for parameters is the same as when DIMming a variable. You can specify integer, real or string (%, # and $ suffixes), structures and pointers.

Array parameters are specified by suffixing the variable with empty brackets "()". Note that you do not specify the array size. To specify a 2D or 3D array, use () and ()() respectively (and so on). For example:

sub PrintTextArray(array$())
    dim i
    for i = 0 to arraymax(array$)
        printr array$(i)
end sub

dim a$(3)
a$(0) = "This"
a$(1) = "is"
a$(2) = "a"
a$(3) = "test"

Return (from subroutine)

Program control returns from a subroutine as soon as its last instruction has executed.
Alternatively you can return immediately from a subroutine with the "return" command.



Calling a subroutine

Subroutines are called the same way as Basic4GL built-in routines and functions.


Local variables

To declare a local variable, simply declare it with dim inside the body of the subroutine.

sub DrawStars(count)
    dim i               ' This is a local variable
    for i = 1 to count
        print "*"
end sub
dim i                   ' This is a global variable
i = 3
print i

Local variables can only be accessed inside the subroutine that they are DIMmed. Their memory is reclaimed as soon as the subroutine finishes.

An important feature of local variables is that if a variable of the same name is DIMmed in two different subroutines, (or if one is DIMmed outside any subroutine), they are treated as two completely different variables, each with its own separate storage. This is very useful for temporary variables (like loop counters), as the variable is guaranteed not to be overwritten by another subroutine that your subroutine may call.

Function/end function

To create a function, use "function" and "end function".


Function name([param[, param[, ...]]])


End Function

Where name is the name of the function, and must not have already been used for a variable, function, other subroutine etc.
param are optional parameters that will be passed to the function, and can be used inside it like variables.

name also determines the "return type" of the function (what kind of value it returns), and can be treated much like a variable in a DIM, in that you can suffix it with (%, #, $) to return an integer, real or string respectively, or precede it with a structure name to return a structure.

To declare a function that returns an array, suffix the declaration with a pair of empty brackets.

A function must explictly return a value with the "return" keyword.

Return (from function)

A function must return a value to the caller with the "return" keyword.


Return expression

Where expression is the expression that will be evaluated, and whose result will be sent back to the caller.


function AddTwoNumbers(n1, n2)
    return n1 + n2
end function
function SumArray(array())
    dim sum, i
    for i = 0 to arraymax(array)
        sum = sum + array(i)
    return sum
end function

Calling a function

A function can be called exactly the same way as a subroutine.
However, a function can also be called within an expression, and its result used as part of the expression in the same way as a constant or variable.

Example 1:

function Reverse$(s$)
    dim result$, i
    for i = 1 to len(s$)
        result$ = result$ + mid$(s$, len(s$) - i + 1, 1)
    return result$
end function

print Reverse$(")-: !ereh saw LG4cisaB")

Example 2:

function Random(min, max)
    return rnd() % (max - min + 1) + min
end function

dim dice(5), i
for i = 1 to 5: dice(i) = Random(1, 6): next
for i = 1 to 5: print dice(i); " ";: next

Example 3:

function UpdateChar$(c$, delta)
    dim a
    a = asc(c$)
    a = a + delta
    if a > 255 then a = a - 256 endif
    if a < 0 then a = a + 256 endif
    return chr$(a)
end function

function UpdateWord$(w$, delta)
    dim result$, i
    for i = 1 to len(w$)
        result$ = result$ + UpdateChar$(mid$(w$, i, 1), delta)
    return result$
end function

dim word$, encoded$, decoded$
input "Word"; word$
encoded$ = UpdateWord$(word$, 1)
printr "Encoded: "; encoded$
decoded$ = UpdateWord$(encoded$, -1)
printr "Decoded: "; decoded$


You can "forward declare" a function or subroutine with the "declare" keyword.


Declare sub name([param[, param[, ...]]])


Declare function name([param[, param[, ...]]])

"Forward declaring" a function/subroutine allows the compiler to compile calls to the function/subroutine before it has compiled the function body.

Function restrictions

Be aware that there are a couple of restrictions on what can be placed inside a function or subroutine:

Function/subroutine pointers

A function/subroutine pointer is a special type of data value that represents a callable function or subroutine.

Function/subroutine pointers can be used for variables, function/sub parameters and return values, and struc fields.


To declare the pointer use the "as" keyword, then the "function"/"sub" keyword and the function/sub parameters and return type. The syntax is the same as when declaring a function/sub except that there is not function/sub name.


' Pointer to function taking an integer and returning an integer
dim fn1 as function(x)
' Subroutine taking 3 strings
dim sub1 as sub(this$, that$, other$)
' Pointer to function taking a vector and returning a vector (array of floating pt numbers)
dim fn2 as function(v#())() as single
' Function taking a function parameter
function ConvertStr(s as string, fn as function(char$) as string) as string
' Structure containing function pointer
struc SMyStruc
	fn as function(x#, y#) as single
end struc


Use the & operator to take the address of a function/sub. This can be assigned to a function pointer variable, or used as a function pointer parameter.




For example:

function Twice(x): return x * 2: end function
dim fn as function(x)
fn = &Twice

The function/sub signature (it's parameters and return value) must match the pointer type's signature, otherwise your program will not compile.

Note that you can only store pointers to user defined functions/subroutines - taking a pointer to a built in function/sub (e.g. "sqrt") is not currently supported. If you really need to do this you can create a "wrapper" function/subroutine and store the pointer to the wrapper. For example:

function doSqrt#(x#): return sqrt(x#):end function
dim fn as function(x#) as single = &doSqrt#
printr fn(16)

Calling a function/subroutine pointer

Calling a function/subroutine via pointer has the same syntax as calling it by name, except that instead of the function/subroutine name, you substitute in the pointer.


sub Stars(n)
	dim i
	for i = 1 to n: print "*": next
end sub

dim ptr as sub(x) = &Stars
sub Find(max, pred as function(i))
	dim i
	for i = 1 to max
		if pred(i) then
			printr i
end sub

function IsEven(n): return (n % 2) = 0: end function
function IsSquare(n): return Int(sqrt(n)) = sqrt(n): end function

printr "Even numbers"
Find(10, &IsEven)

printr "Squares"
Find(10, &IsSquare)

If you attempt to call a function/subroutine pointer that hasn't been assigned a value (or has explicitly been assigned the "null" value), you will get an "Unset pointer" run time error.

Other function/subroutine pointer operations

There are a handful of other operations that can be done with a function/subroutine pointer.

Function/subroutine pointers can be assigned to each other. For example:

sub DoIt(): printr "Done!": end sub

dim s1 as sub()
dim s2 as sub()

s1 = &DoIt
s2 = s1				' Copy from one pointer variable to another


The function/sub pointer types must match. You will get a compile time error if the function/sub pointer has a different return type, or parameter types etc.

You can also assign the special "null" value to a function pointer. This reverts the pointer back to its uninitialised state. Attempting to call the pointer will then result in an "Unset pointer" run time error.

sub DoIt(): printr "Done!": end sub
dim s1 as sub() = &DoIt
s1()				' This will work
s1 = null
s1()				' This will cause a run time error

Finally - similar to pointers to data structures - you can compare two pointers of the same type, or compare a pointer to the "null" value. As with pointers to data structures, you are restricted to using the = and <> operators.


sub FilterAndProcess(array(), filter as function(n), process as sub(n))
	dim i
	for i = 0 to ArrayMax(array)
		' Note: Only call filter if not set to null
		if filter = null lor filter(array(i)) then
end sub

dim a(8)
a(0) = 3
a(1) = 1
a(2) = 4
a(3) = 1
a(4) = 5
a(5) = 9
a(6) = 2
a(7) = 6
a(8) = 5

function Above4(n): return n > 4: end function
sub Output(n): printr ">"; n: end sub

printr "All"
FilterAndProcess(a, null, &Output)			' No filter function

printr "5 and up"
FilterAndProcess(a, &Above4, &Output)

Looking up function pointers by name

You can also lookup a function/subroutine pointer by name using the GetFunctionByName command.


GetFunctionByName(name$, handle)

Where name$ is the name of the function/subroutine to find and handle is an integer handle returned from a successful call to "Comp" or "CompFile".


sub testSub(): printr "Hello world": end sub
dim ptr as sub() = GetFunctionByName("testSub")

For regular code it is usually better to use the & operator rather than GetFunctionByName. The & operator gives you compile-time name and type checking, whereas GetFunctionByName will return null at runtime if no matching function/subroutine is found and will stop with a runtime error if the function/subroutine does not match the type of function pointer you assign it to.

GetFunctionByName is more useful is for runtime script code compiled with "Comp" or "CompFile". For example:

dim script1 = Comp("sub test(): printr 1: end sub")
dim script2 = Comp("sub test(): printr 2: end sub")
dim ptr1 as sub() = GetFunctionByName("test", script1)
dim ptr2 as sub() = GetFunctionByName("test", script2)

Passing a script handle to GetFunctionByName restricts Basic4GL to search only inside the specified compiled script.

Passing 0 as a script handle which instructs Basic4GL to ignore all compiled scripts and search inside the main program.

If no handle is supplied at all, Basic4GL will return the first instance of the function/subroutine that was declared, searching the main code first, then across all scripts in the order they were compiled.

Program data

Basic4GL provides the standard "Data", "Read" and "Reset" mechanism for entering data directly into programs. This is basically a shorthand way of hard-coding data into programs and is typically used to initialise arrays.

The actual data stored is a list of values. Each value is either a string or a number (int or real).


To specify the data elements, use "Data".


Data element [, element [, ...]]


data 12.4, -3.4, 12, 0, 44
data My age, 20, My height, 156
data "A long time ago, in a galaxy far away, yada yada yada"

If the data element can be parsed as a number, it will be stored as such. Otherwise it will be stored as a string.

Strings can either be quoted (enclosed in double quotes) or unquoted. Quoted strings can contain commas (,), colons (:) and single quotes (').
Unquoted strings cannot contain these characters, because:

So it is best to quote strings if you are unsure.


In order to do something with the data, you need to read it into variables, using "Read".


Read variable [, variable [, ...]]

Variable must be a simple variable type, either a string, integer or real. (In otherwords you can't read a structure or an array with a single read statement, although you can write code to read each element individually).

Read copies an element of data into the variable, and then moves the data position along one.
If there is no data, or the program has run out of data, you will get an "Out of DATA" runtime error.
Attempting to read a string value into a number variable (integer or real) will also generate a runtime error.

Example 1:

data age, 22, height, 175, shoesize, 12
dim name$(3), value(3), i
for i = 1 to 3
    read name$(i), value(i)
for i = 1 to 3
    printr name$(i) + "=" + value(i)


"Reset" tells Basic4GL where to start loading data from.


Reset labelname

Where labelname is a Basic4GL program label.
The next "Read" will begin reading data from the first "Data" statement after labelname.

    data 1, 2, 3, 4, 5
    data cat, dog, fish, mouse, horse

dim a$, i
printr "1) This data"
printr "2) That data"
print "Please press 1 or 2"

while a$ <> "1" and a$ <> "2" 
    a$ = Inkey$ ()

if a$ = "1" then 
    reset ThisData
    reset ThatData

for i = 1 to 5
    read a$
    printr a$

External functions

Basic4GL supports a number of external functions.

The use of these functions is documented on other help files, like the Programmers' Guide and Sprite Library Guide.

External functions are called with the following format:

FunctionName ([param [, param [, ...]])


glVertex3f(-2.5, 10, 0)

A small number of functions do not require their arguments to be enclosed in brackets (mainly for historical reasons.)
These functions are: Cls, Print, Printr and Locate.
For example:

Locate 17, 12
Print "Hello"

Runtime compilation

Basic4GL code can also be compiled and executed at runtime. The source can be a file on disk, or a text string in memory.
The runtime compile is the same as the compile time compiler, and accepts all the same code. The only restriction is that you cannot use "include" within runtime compiled code.

The main commands are "Comp" and "Exec" to compile and execute respectively.
(Actually "Comp" is a function, but it's so closely associated with the "Exec" command that I've included it here.)

There is also support for calling functions in runtime-compiled code, using the "Runtime" keyword.


"Comp" compiles a text string and return a handle that can be used to execute the compiled code at runtime.



Where codetext is a text string, or an array of text strings, containing code to be compiled at runtime.

If the text compiled successfully, "Comp" returns a non-zero integer handle to identify the compiled code.
If the compiler encountered an error, "Comp" returns zero, and the error description can be retrieved with CompilerError(), CompilerErrorLine() and CompilerErrorCol().

Example 1:

dim code1, code2
code1 = Comp("printr " + chr$(34) + "Ding" + chr$(34))
code2 = Comp("printr " + chr$(34) + "Dong" + chr$(34))
exec code1
exec code2
exec code1
exec code2

Example 2:

dim prog$(10), code
prog$(0) = "dim x, y"
prog$(1) = "for y = 1 to 10"
prog$(2) = "for x = 1 to y"
prog$(3) = "print " + chr$(34) + "*" + chr$(34) + ";"
prog$(4) = "next"
prog$(5) = "printr"
prog$(6) = "next" 


"CompFile" compiles a file on disk and return a handle that can be used to execute the compiled code at runtime.



Where filename is the filename as a text string.

If the file was read and compiled successfully, "CompFile" returns a non-zero integer handle to identify the compiled code.
If the compiler encountered an error, "CompFile" returns zero, and the error description can be retrieved with CompilerError(), CompilerErrorLine() and CompilerErrorCol().


"Exec" executes runtime-compiled code.



Exec handle

Where handle is an integer handle returned from a successful call to "Comp" or "CompFile".
If no handle is supplied, Exec executes the last code compiled (or bound if "BindCode" has been executed.)

Be warned that any runtime errors will halt your program.

Functions/subs inside compiled code

Normally you cannot have two functions or subs with the same name. However Basic4GL will allow this if the functions/subs are in different compiled code blocks, or if one is in the main program and the other(s) in compiled code blocks.

Basic4GL applies "scoping" logic to determine which function/sub is to be called as follows:

The scoping logic only applies to functions and subs however. Other things like global variables, labels etc are not scoped this way.


' Subroutines in main code 
sub Sub1(): printr "Main 1": end sub
sub Sub2(): printr "Main 2": end sub
' Subroutines in compiled code
Comp("sub Sub1(): printr " + chr$(34) + "Runtime 1" + chr$(34) + ": end sub: Sub1(): Sub2()")

' Execute compiled code

' Call main subroutines

Calling functions/subs in runtime code

Runtime compiled code can call functions/subs in the main program easily. Calling runtime-compiled functions/subs from your main program requires you declare the function with "runtime" first.


The "Runtime" keyword is used to declare a function or sub that can be implemented either:

The syntax is much the same as the "Declare" keyword:


Runtime Sub prototype

Runtime Function prototype

Where prototype defines the function/sub, its parameters and return type (if applicable).


runtime sub MySub()
runtime sub MoveBadGuy(SBadGuy& badguy)
runtime function CalcY#(x#)

Once declared with "runtime", the sub/function can be called from your main program.
Basic4GL will check at runtime to see if the function/sub being called has been implemented, checking the current runtime-compiled code first, then the main program. If the function/sub is found, Basic4GL calls it. Otherwise a runtime error results, and your program stops.

As with "exec", the "current" runtime-compiled code is the last code that was compiled with "Comp", or bound with "BindCode".


runtime sub MySub()

sub MySub()
    printr "Main program"
end sub
' Will call MySub() in main program

dim code
code = Comp("sub MySub(): printr " + chr$(34) + "Runtime compiled code" + chr$(34) + ": end sub")
' Will call MySub() in runtime code 

' Will call MySub() in main program
bindcode 0

' Will call MySub() in runtime code
bindcode code


The "BindCode" command is used to make runtime-compiled code current.
This affects the "Exec" command (when called without a parameter), and where Basic4GL looks for "Runtime" functions.


BindCode 0

BindCode handle

Where handle is an integer handle returned from a successful call to "Comp" or "CompFile".

"BindCode 0" has special meaning. No runtime-compiled code is considered bound. Any "runtime" functions called must therefore be implemented in the main program itself. "Exec" without a parameter will cause a runtime exception.

Restart/end instructions

There are two more Basic4GL instructions that have yet to be discussed.


Ends the BASIC program.

This is essentially the same as when the end of the program is reached. No more BASIC instructions are executed.

Note: Some commands like "end if", "end struct", "end function" also contain the "end" keyword. However Basic4GL is able to determine from the context that this is not an "end" instruction, and will not end the program in this case.

if 5 > 3 then 
    printr "This will print"
end if                          ' This is not an "end" instruction
printr "So will this"			
end                             ' This *is* an "end" instruction
printr "But this bit wont"


The "Run" command causes Basic4GL to reset the virtual machine and restart the program from the top.

All variables are deallocated, the call stack is cleared, and resources (like textures) are automatically freed.

The program will begin executing again as if you had just clicked "Run" in the Basic4GL editor.