A variable is a “container” name that can hold a numerical or string value which can be referenced or changed by the program (as opposed to CONSTant values which never change).
Variable names
Variables in QB64 can be any name except the names of QB64 or QBasic keywords and may not contain spaces or non-alphabetical/non-numerical characters (except “.” and “_”). Numerical characters cannot be used as the first character of a variable or array name! QB64 reserves the use of a leading underscore to QB64 procedural or variable type names only!
Variable values can be passed to sub-procedures by using the name as a SUB or FUNCTION parameter in a CALL. Variable names in the main program module can be passed to sub-procedures by using DIM SHARED without using them as CALL parameters.
Variable names dimensioned in SUB or FUNCTION procedures can be the same as names used in the main procedure and can hold different values. SHARED sub-procedure variables can only be shared with the main program module!
Dot variable names are normally used with TYPE variable definitions. The first name before the dot is the name of the variable DIM AS the type. The name after the dot is the name of the variable assigned inside of the TYPE.
Variable types
Variables can be defined as a specific type using a variable type suffix or by using a DIM or REDIM(for dynamic arrays only) statement AS a variable type of _BIT, _BYTE, INTEGER, LONG, SINGLE, DOUBLE, _INTEGER64, _FLOAT or STRING in QB64.
Groups of variable names can be type defined by the first letter or list of letters of the names using DEFINT, DEFLNG, DEFSNG, DEFDBL, DEFSTR or _DEFINE AS in QB64.
$DYNAMIC arrays can be resized and can retain their remaining element values when used with REDIM _PRESERVE in QB64. ERASE or CLEAR removes the array entirely from memory!
$STATIC arrays cannot be resized, but cannot be removed either. ERASE or CLEAR will clear the array element values only!
Variable values
All numerical variables default to 0 and all string variables default to “” at the start of a program and when first referenced inside of a SUB or FUNCTION procedure except when the variable is defined as a STATIC value.
Arrays variable names can hold many values in one variable name by specifying a reference index enclosed in parentheses.
STATIC sub-procedure values keep their value after the sub-procedure is exited. They will hold an empty value when first used.
Variables are used to hold program data information that is attained through the program flow, either by user input, calculations or by other ways of communication (as with I/O, memory, TCP/IP or files).
Assignment of variable values can be done using the = assignment symbol (variable1.number = 500, for example).
To pass variable values to a sub-procedure without altering or affecting the variable name’s value, parenthesis can be used around the variable name in the CALL.
Example(s)
Example of different usages of variables:
max = 1000
DIM d(max)
FOR c = 1 TO max
d(c) = c + d(c - 1)
NEXT
PRINT "Show the result of the addition from 1 to n (1+2+3...+n)"
PRINT "n = (0-" + LTRIM$(STR$(max)) + "): ";
INPUT "", n
IF n <= max AND n >= 0 THEN PRINT d(n) ELSE PRINT "Invalid value (only 0 to" + STR$(max) + " is permitted)."
Show the result of the addition from 1 to n (1+2+3...+n)
n = (1-1000): 10
55