QB64-PE/internal/help/IF...THEN.txt

'''IF...THEN''' statements make Boolean True or False program evaluations to automate program decision making.



{{PageSyntax}}
:: IF ''condition'' THEN ''action'' {{Cb|ELSE}} evaluation = 0


''Block {{PageSyntax}}
:: IF ''condition statement'' [[THEN]]
::: ''action''
:: [[ELSEIF]] ''condition statement'' [[THEN]] ''action'' 
:: [[ELSE]] ''action''
:: [[END IF]]


* The ''conditional'' evaluation by '''IF''' must be true(-1) or a non-zero numerical value for the THEN ''action'' statements to be executed.
* Multiple conditional evaluations can be made using inclusive [[AND (boolean)|AND]] or alternative [[OR (boolean)|OR]] conditional expressions.
* IF statements can also have alternative evaluations using [[ELSEIF]] and [[ELSE]] conditions. 
* When the '''IF''' statement and/or code to be run is more than code line, an [[END IF]] statement '''MUST''' be used.
* With multiple code lines to run, end the IF statement with THEN and place all of the code on lines below that line.
* Multiple code line block statements require that the [[IF...THEN]], [[ELSEIF]], [[ELSE]] and [[END IF]] be on separate lines.
* '''The IDE may return an error of [[NEXT]] without [[FOR]] or [[LOOP]] without [[DO...LOOP|DO]] when [[END IF]] does not end a statement block!'''
* The '''QB64''' IDE may red line the IF THEN statement line until END IF closes the statement block.
* Use '''[[colon]]s''' to execute multiple statements in a one line IF statement. You cannot use [[AND]] statements after [[THEN]]!
* An '''[[underscore]]''' can be used anywhere after the code on one line to continue it to the next line in '''QB64 ONLY'''.



{{Template:RelationalTable}}


<center> When evaluating a number value, no IF value > 0 operation is necessary for values not 0. Use: IF value THEN </center>


<center>'''Boolean Conditional Operators:'''</center>


:::::* [[AND (boolean)]] can be used to add extra conditions to a boolean statement evaluation.
:::::* [[OR (boolean)]] can be used to add alternate conditions to a boolean statement evaluation.
:::::* Parenthesis are allowed inside of boolean statements to clarify an evaluation.


<center>'''Mathematical Logical operators:'''</center>
<center>* Truth table of the 6 BASIC Logical Operators:</center>


{{Template:LogicalTruthTable}}

<center>* '''Note that Basic returns -1 for True and 0 for False.'''</center>



''Example 1:'' One line IF statement
{{CodeStart}} '' ''
IF x > 100 THEN PRINT x '' ''
{{CodeEnd}}


''Example 2:'' IF statement blocks require that the IF THEN and END IF statements be separate from the code executed.
{{CodeStart}} '' ''
IF x > 100 THEN
  y = 200
  PRINT y
  PRINT x
END IF '' ''
{{CodeEnd}}


''Example 3:'' True or False evaluation of a numerical value executes only when the value is not 0. '''Cannot evaluate [[STRING]] values!'''
{{CodeStart}} '' ''
IF x THEN PRINT x  '' ''
{{CodeEnd}}
:Example will only print if a numerical value is True (positive or negative). (Equivalent to: IF x > 0 OR x < 0 THEN evaluation)


''Example 4:'' Multiple evaluations using parenthesis to determine the order.
{{CodeStart}} '' ''
IF (value% > 100 AND value% < 200) OR value% = 50 THEN PRINT "OK" '' ''
{{CodeEnd}}


''Example 5:'' Using multiple IF options in a one line statement.
{{CodeStart}} '' ''
x = 100        'change x value to see other evaluations
IF x > 200 THEN PRINT "High" {{Cl|ELSEIF}} x < 0 THEN PRINT "Low" {{Cl|ELSE}} PRINT "OK" '' ''
{{CodeEnd}}


''Example 6:'' [[STRING]] values can be compared using greater than, less than, not equal to or equal to operators only.
{{CodeStart}} '' ''
PRINT "Press a letter key: ";
Key$ = {{Cl|INPUT$}}(1)
PRINT Key$ 
IF Key$ >= {{Cl|CHR$}}(65) AND Key$ <= {{Cl|CHR$}}(90) THEN PRINT "A to Z"
{{CodeEnd}}
: ''Explanation:'' Long [[STRING]] expression values are compared by their cumulative [[ASCII]] code values.


''See also:'' 
*[[ELSEIF]], [[ELSE]]
*[[AND (boolean)]], [[OR (boolean)]] 
*[[NOT]], [[SELECT CASE]], [[Boolean]]


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