This document is aimed at experienced programmers and describes the basic syntax of Basic4GL programs.

This document focuses on the language itself, and as such does not go into the individual functions and constants, or how they are intended to be used.

Basic4GL is designed to combine a simple, safe and easy to understand programming language based on traditional BASIC with the OpenGL graphics library, so that programmers can experiment and learn OpenGL and beginning programmers can learn about programming in general.

The downside is that Basic4GL cannot compete with programs compiled to native machine code (e.g. from a C++ compiler). But this was never the intention.

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, and 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.

print "Hello world!"

glTranslatef (0, 0, -5)
glBegin (GL_QUADS)
glVertex2f ( 1, 1): glVertex2f (-1, 1): glVertex2f (-1,-1): glVertex2f ( 1,-1)
glEnd ()
SwapBuffers ()

As of Basic4GL language version 2.3.2, Basic4GL supports a new "traditional BASIC" syntax.

This syntax is intended to be more compatible with other BASIC compilers, to make porting code between them and Basic4GL a little bit easier, and to make programming in Basic4GL a little easier for people who are used to other BASIC compilers.

The new syntax must be explicitly enabled, otherwise Basic4GL will simply use the standard Basic4GL syntax. You do this by placing the following command at the top of your program:

language traditional

Basic4GL also accepts:

language basic4gl

Which will switch the compiler to the standard Basic4GL syntax. (Although it's not really necessary, as this is the default syntax anyway.)

And also:

language traditional_print

Which is a tradeoff between the standard Basic4GL syntax, except with a more traditional print command syntax.

Syntax differences The differences between the "Traditional BASIC" and old "Basic4GL" syntax are listed below:

"Traditional" BASIC Basic4GL Functions have round brackets only if they return a value. Examples:

a = rnd()%5
sleep 1000
locate 10, 12
glVertex3f -5, 12, 2
print sqrt(2)

All functions have round brackets except for cls, print, printr and locate

Examples:

a = rnd()%5
sleep(1000)
locate 10, 12
glVertex3f(-5, 12, 2)
print sqrt(2)

If a print command ends with a semicolon (;) the cursor will remain on the same line. Otherwise, the cursor will move to the next line.

Example:

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

The cursor always remains on the same line after print. To have the cursor move to the next line, use the printr command instead.

Example:

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

When dividing two integers, they will automatically be converted to floating point first.

Examples:

print "5 goes into 12"; int(5/12); "times"
print "10/8 = "; 10/8
a = 3: b = 4: c# = a / b

When dividing two integers, integer division is used, and the remainder is discarded.

Examples:

printr "5 goes into 12 "; 5/12; " times"
printr "10/8 = "; 10.0 / 8
a = 3: b = 4: c# = (a * 1.0) / b

The syntax documented in these help files is the standard Basic4GL syntax.

When other syntaxes differ, the differences will be described in their corresponding Compatibility with other BASICs section.

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.

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.

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

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)
glvertex2f(X, Y)

The following lines are not equivalent:

print "HELLO WORLD"
print "Hello World"
print "hello world"

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

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$

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:

String variables are post-fixed with a $ character, for example:

Dim a$
a$ = "Hello world"

Real variables are post-fixed with a # character, for example:

Dim value#
value = 1.2345

Integer variables are not post-fixed, for example:

Dim index
index = 10

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
Skip:
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

Basic4GL also supports the syntax:

Dim variable as type

Where type can be one of:

• integer
• string
• single
• double

Storage space is allocated when the Dim instruction has been executed. In addition, Basic4GL automatically initialises the data as follows:

• Integers and reals are initialised to 0.
• Strings are initialised to the empty string "".

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.

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.

Examples:

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.

Examples:

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.

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.

Examples:

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)
glLoadMatrixf (matrix#)
glBegin (GL_TRIANGLES)
glVertex2f (0, 0): glVertex2f (1, 0): glVertex2f (0, 1)
glEnd ()
SwapBuffers ()

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.

Example:

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

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

Basic4GL also supports the syntax:

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

For multidimension arrays.

E.g.

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.

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:

• Integer -> Real
• Real -> Integer
• Integer -> String
• Real -> String

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:

• If one operand is a string, the other operand is converted to a string before the operation is performed.
• If one operand is a real and the other is an integer, the integer is converted to a real before the operation is performed.

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 hexadecimal 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!"

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

You can add constants using the Const instruction.

The format is:

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

Where:

name is the name of the constant, and follows the same naming conventions as standard variables, (including # and $ suffixes for real and string constants respectively). value is a literal constant, another named constant, or a constant expression (defined below) 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

Certain instructions require constant expressions, such as the const instruction (described above), and the step part of the for..next 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:

• The expression must contain only literal constants or named constants.
• These constants can only be combined with the standard operators: +, -, *, /, %, =, <>, >, >=, <, <=, or, and, not.

Examples:

-12
22.4
m_pi
m_pi / 180
true and not false
"banana"
"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 [,...]]]
[...]

EndStruc

Example:

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

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

Examples:

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:

structurename.fieldname

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.

Example:

player (7) = player (6)

Basic4GL also supports the syntax:

Type typename

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

End type

E.g.

struc SpaceMartian
dim name$
dim x#, y#
dim health(4)
endstruc

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.)

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.

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:

• An integer named "i"
• A real named "r#"
• A string named "a$"
• An array of reals named "array#"
• A structure of type "SomeStructure" named "s"
• A 2D array of reals named "matrix#"

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

Declares:

• An pointer to an integer named "pi"
• A pointer to a real named "pr#"
• A pointer to a string named "pa$"
• A pointer to an array named "parray#"
• A pointer to a structure of type "SomeStructure" named "ps"
• A pointer to a 2D array of reals named "pmatrix#"

Pointer variables are initially unset. Attempting to read or write to the data of an unset pointer results in a runtime error. 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

Examples:

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

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

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

    ...

endif
if &ptr <> null then

    ...

endif

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.)

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! (Note: While this has some obvious limitations, it does prevent a large number pointer related bugs. Keep in mind that Basic4GL was never intended to be the next C++...)

Data is allocated as follows:

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

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

Examples:

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.

Examples:

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:

Notes:

+ and - have equal precedence (except when minus is used to negate a single value). 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.

Also, the = and <> operators can also be used to compare pointers to each other, or to compare pointers to null.

An expression operand can be any of the following:

• A variable. E.g. a$
• An array variable. E.g. x# (index)
• A literal constant. E.g. 3.14159265
• A named constant. E.g. M_PI
• A function result. E.g. Sqrt (2)

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:

glClear (GL_DEPTH_BUFFER_BIT or GL_COLOR_BUFFER_BIT)

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.

Jumps directly to a new position in the source code.

Format:

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:

Loop:
Print "Hello "
Goto Loop

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

Calls a subroutine.

Format:

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.

Example:

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

Name:
print name$
Return

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 Gosubs are without Returns will eventually cause a "stack overflow" runtime error

Executes a block of code conditionally.

Format:

If expression Then
If block
Endif

Or:

If expression Then
If block
Else
Else block
Endif

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"
End
Endif

Example 2:

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

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. Thus:

if expression1 then
...
elseif expression2 then
...
endif

Is equivalent to:

if expression then
...
else
if expression2 then
...
endif
endif

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"
else               
printr "???"
endif
next

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"
endif

Basic4GL also supports the syntax:

If condition Then
ifblock
end if

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

Executes a code block repeatedly while an expression is true.

Format:

While expression
Code block
Wend

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.

Example:

While lives > 0
' Do gameplay
...
Wend
' Game over
...

Used to create loops with a loop counter variable.

Format:

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

Or:

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

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 for..next construct is exactly equivalent to:

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

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

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

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

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

Example 1:

Dim index
For index = 1 to 10
Printr "Index = " + index
Next

Example 2:

Dim count: count = 10
Dim squared(count), index
For index = 0 to count
squared (index) = index * index
Next

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#))
next
glEnd ()
SwapBuffers ()

Example 4:

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

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

Format:

do
Code block
loop

Or:

do while condition
Code block
loop

Or:

do until condition
Code block
loop

Or:

do
Code block
loop while condition

Or:

do
Code block
loop until condition

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.

Functions/subroutines introduce a number of features not supported by gosub/return:

• Local variables - Prevent two unrelated parts of code from interfering with each other by modifying each others' variables.
• Parameters - Provide a convenient and less error prone (than using global variables) way to pass data to a routine.
• Return values - Provide a convenient and less error prone (than using global variables) way to pass data back from a routine.
• Better encapsulation - A function/subroutine can only be executed by calling it explicitly. You do not have to setup gotos to "jump around" the routine to prevent it from executing when it shouldn't.

To create a subroutine, use Sub and End Sub

Format:

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.

Examples:

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 ().

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$(i)
next
end sub
dim a$(3)
a$(0) = "This"
a$(1) = "is"
a$(2) = "a"
a$(3) = "test"
PrintTextArray(a$)

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.

Format:

Return

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

Format:

name([value1[,value2[,...]]])

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

Example:

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

dim i                   ' This is a global variable
i = 3
DrawStars(20)
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 for..next loop counters), as the variable is guaranteed not to be overwritten by another subroutine that your subroutine may call.

To create a function, use function and end function.

Format:

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 explicitly return a value with the return keyword.

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

Format:

Return expression

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

Examples:

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)
next     
return sum
end 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$) - i + 1, 1)
  next
  return result$
end function

print Reverse$("?efil laer eht siht sI")

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$, i, 1), delta)
  next
  return result$
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.

Format:

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

Or:

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.

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

• You cannot define a label inside a function/subroutine.
• You cannot use the goto or gosub commands inside a function/subroutine.

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.

Format:

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

Examples:

Dim 12.4, -3.4, 12, 0, 44
Dim My age, 20, My height, 156
Dim "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:

• Comma starts a new data element
• Colon starts a new instruction
• Single starts a program comment

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.

Format:

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

Variable must be a simple variable type, either a string, integer or real. (In other words, 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.

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)
next
for i = 1 to 3
printr name$(i) + "=" + value(i)
next

Reset tells Basic4GL where to start loading data from.

Format:

Reset labelname

Where labelname is a Basic4GL program label.

The next Read will begin reading data from the first Data statement after labelname.

ThisData:
  data 1, 2, 3, 4, 5

ThatData:
  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$ ()
wend

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

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

Basic4GL supports a number of external functions.

External functions are called with the following format:

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

Examples:

Beep()
glClear (GL_COLOR_BUFFER_BIT)
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:

Cls
Locate 17, 12
Print "Hello"

When "traditional BASIC" syntax is used, functions that do not return a value must not have their parameters enclosed in brackets.

Some examples:

sleep 1000
glBegin GL_TRIANGLES
SprSetX x#
glVertex3f 10, 4, 2

Functions which do return a value must still have their parameters enclosed in brackets (or have empty brackets if there are no parameters)

Examples are:

a = rnd() % 10
texture = LoadTexture(filename$)
a$ = inkey$()

Some functions return a value, which can be assigned to a variable, used in an expression or as a parameter to another external function.

Examples:

print Sqrt (2)
if ScanKeyDown (VK_UP) then ...
locate (TextCols()-Len(a$))/2, TextRows()/2: Print a$

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.

Format:

Comp(codetext)

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"
Compile(prog$)
exec

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

Format:

CompFile(filename)

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.

Format:

Exec

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.)

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:

• If calling code is in the main program, the function/sub is assumed to be in the main program
• If calling code is runtime-compiled, the function/sub is first assumed to be in the runtime-compiled code then in the main program (if not found in the runtime code)

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

Example:

' 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
exec
printr                 
' Call main subroutines
Sub1()
Sub2()

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:

• In the main program
• In one or more sections of runtime compiled code
• Or, all of the above

The syntax is much the same as the Declare keyword.

Format:

Runtime Sub prototype

Runtime Function prototype

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

Examples:

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.

Example:

runtime sub MySub()
sub MySub()
printr "Main program"
end sub

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

' Will call MySub() in runtime code
MySub()            
' Will call MySub() in main program
bindcode 0
MySub()                
' Will call MySub() in runtime code
bindcode code
MySub()

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.

Format:

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.

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

Causes Basic4GL to stop executing the program.

Causes Basic4GL to deallocate all variables, reset OpenGL, deallocate any resources (such as OpenGL textures), clear the Gosub-Return stack and begin executing the program again from the top.

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

Basic4GL, Copyright (C) 2003-2007 Tom Mulgrew

Language guide

16-Feb-2008 Tom Mulgrew

Documentation modified for Markdown (.md) formatting by Nathaniel Nielsen