A '''variable''' is a named "container" that can hold a numerical or string value and which can be referenced or changed by the program (as opposed to [[CONST]]ant values which never change).
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 single leading underscore for QB64 procedural or variable type names.
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|dimensioned]] [[AS]] the type. The name after the dot is the name of the variable assigned inside of the [[TYPE]].
<center>'''Variable types'''</center>
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!
Variables are initialized to empty values (0 for numerical values, and "" for strings) at the start of a program, or when first referenced inside of a [[SUB]] or [[FUNCTION]] call.
[[STATIC]] variables are also initialized to empty values, however, STATIC variables will retain their values in between SUB or FUNCTION calls, and exist for the life of the program.
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 communicaton (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]].
{{PageExamples}}
''Example of different usages of variables:''
{{CodeStart}}
max = 1000
{{Cl|DIM}} d(max)
{{Cl|FOR...NEXT|FOR}} c = 1 {{Cl|TO}} max
d(c) = c + d(c - 1)
{{Cl|NEXT}}
{{Cl|PRINT}} "Show the result of the addition from 1 to n (1+2+3...+n)"
{{Cl|IF...THEN|IF}} n <= max {{Cl|AND (boolean)|AND}} n >= 0 {{Cl|THEN}} {{Cl|PRINT}} d(n) {{Cl|ELSE}} {{Cl|PRINT}} "Invalid value (only 0 to" + {{Cl|STR$}}(max) + " is permitted)."
{{CodeEnd}}
{{OutputStart}}
Show the result of the addition from 1 to n (1+2+3...+n)
