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

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



''Line'' {{PageSyntax}}
:: '''IF''' ''condition statement'' '''THEN''' ''action'' [[ELSE]] other_action
:: '''IF''' ''condition statement'' [[GOTO]] ''linelabel''


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


''GOTO {{PageSyntax}}
:: '''IF''' x > 0 [[GOTO]] {line_number|line_label}


* 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.
* [[THEN]] is not required when [[GOTO]] is used to send program flow to a line number or label.
* 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'''.
* '''NOTE:''' [[STRING]] values can only be evaluated in an IF statement if a value is compared to a literal or [[CHR$]] string value. '''QB64 may not compile literal IF string statements or indicate an [[IDE]] coding error!''' Use [[LEN]] or [[ASC]] to compare strings numerically.



{{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:'' In a one line IF statement, only [[REM]] can be used to comment out the action without an [[END IF]] error:
{{CodeStart}} '' ''
{{Cl|INPUT}} "Enter a number over or under 100: ", x
{{Cl|IF...THEN|IF}} x > 100 {{Cl|THEN}} {{Cl|PRINT}} x 
{{Cl|IF...THEN|IF}} x > 100 {{Cl|THEN}} {{Cl|REM}} {{Cl|PRINT}} x '' '
{{CodeEnd}}


''Example 2:'' IF statement blocks require that the IF THEN and END IF statements be separate from the code executed.
{{CodeStart}} '' ''
{{Cl|INPUT}} "Enter a number over or under 100: ", x
{{Cl|IF...THEN|IF}} x > 100 {{Cl|THEN}}
  y = 200
  {{Cl|PRINT}} y
  {{Cl|PRINT}} x
{{Cl|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}} '' ''
{{Cl|INPUT}} "Enter a number or just hit Enter: ", x
{{Cl|IF...THEN|IF}} x {{Cl|THEN}} {{Cl|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}} '' ''
{{Cl|INPUT}} "Enter a number over or under 100 or 50: ", value
{{Cl|IF...THEN|IF}} (value% > 100 {{Cl|AND (boolean)|AND}} value% < 200) {{Cl|OR (boolean)|OR}} value% = 50 {{Cl|THEN}} {{Cl|PRINT}} "OK" '' ''
{{CodeEnd}}


''Example 5:'' Using multiple IF options in a one line statement.
{{CodeStart}} '' ''
{{Cl|INPUT}} "Enter a number over or under 200: ", x
{{Cl|IF...THEN|IF}} x > 200 {{Cl|THEN}} {{Cl|PRINT}} "High" {{Cl|{{Cl|ELSEIF}}}} x < 0 {{Cl|THEN}} {{Cl|PRINT}} "Low" {{Cl|{{Cl|ELSE}}}} {{Cl|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.


<center>'''Qbasic decimal point value comparison errors'''</center>
* Floating decimal point numerical values may not be compared as exactly the same value. QB64 will compare them the same.
:''Example:'' Qbasic will print ''unequal'' in the IF comparison code below even though it is exactly the same value printed.
{{CodeStart}} '' ''
x# = 5 / 10
y# = 6 / 10
z# = x# + y#
{{Cl|PRINT}} x#, y#, z#
{{Cl|IF...THEN|IF}} x# + y# = z# {{Cl|THEN}} {{Cl|PRINT}} "equal" {{Cl|ELSE}} {{Cl|PRINT}} "unequal" '' ''
{{CodeEnd}}
: Note: QB64 will make the calculation correctly and print ''equal''. Change older program code that relies on the error accordingly.


''See also:'' 
* [[ELSEIF]], [[ELSE]]
* [[AND (boolean)]], [[OR (boolean)]] 
* [[NOT]], [[GOTO]]
* [[SELECT CASE]]
* [[Boolean]] {{text|(numerical comparisons return a True or False value)}}


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