mirror of
https://github.com/QB64-Phoenix-Edition/QB64pe.git
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6e01fc8dce
Added new _STRCMP and _STRICMP commands for quick string comparisons. Cleaned up QB64 to finish removing the QUI (quick user insert) code and folders. Altered UCASE and LCASE routines to be faster in some situations for us.
106 lines
4 KiB
Plaintext
106 lines
4 KiB
Plaintext
'''IF...THEN''' statements make Boolean True or False program evaluations to automate program decision making.
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{{PageSyntax}}
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:: IF ''condition'' THEN ''action'' {{Cb|ELSE}} evaluation = 0
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''Block {{PageSyntax}}
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:: IF ''condition statement'' [[THEN]]
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::: ''action''
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:: [[ELSEIF]] ''condition statement'' [[THEN]] ''action''
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:: [[ELSE]] ''action''
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:: [[END IF]]
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* The ''conditional'' evaluation by '''IF''' must be true(-1) or a non-zero numerical value for the THEN ''action'' statements to be executed.
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* Multiple conditional evaluations can be made using inclusive [[AND (boolean)|AND]] or alternative [[OR (boolean)|OR]] conditional expressions.
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* IF statements can also have alternative evaluations using [[ELSEIF]] and [[ELSE]] conditions.
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* When the '''IF''' statement and/or code to be run is more than code line, an [[END IF]] statement '''MUST''' be used.
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* With multiple code lines to run, end the IF statement with THEN and place all of the code on lines below that line.
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* Multiple code line block statements require that the [[IF...THEN]], [[ELSEIF]], [[ELSE]] and [[END IF]] be on separate lines.
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* '''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!'''
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* The '''QB64''' IDE may red line the IF THEN statement line until END IF closes the statement block.
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* Use '''[[colon]]s''' to execute multiple statements in a one line IF statement. You cannot use [[AND]] statements after [[THEN]]!
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* An '''[[underscore]]''' can be used anywhere after the code on one line to continue it to the next line in '''QB64 ONLY'''.
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{{Template:RelationalTable}}
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<center> When evaluating a number value, no IF value > 0 operation is necessary for values not 0. Use: IF value THEN </center>
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<center>'''Boolean Conditional Operators:'''</center>
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:::::* [[AND (boolean)]] can be used to add extra conditions to a boolean statement evaluation.
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:::::* [[OR (boolean)]] can be used to add alternate conditions to a boolean statement evaluation.
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:::::* Parenthesis are allowed inside of boolean statements to clarify an evaluation.
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<center>'''Mathematical Logical operators:'''</center>
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<center>* Truth table of the 6 BASIC Logical Operators:</center>
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{{Template:LogicalTruthTable}}
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<center>* '''Note that Basic returns -1 for True and 0 for False.'''</center>
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''Example 1:'' One line IF statement
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{{CodeStart}} '' ''
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IF x > 100 THEN PRINT x '' ''
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{{CodeEnd}}
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''Example 2:'' IF statement blocks require that the IF THEN and END IF statements be separate from the code executed.
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{{CodeStart}} '' ''
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IF x > 100 THEN
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y = 200
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PRINT y
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PRINT x
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END IF '' ''
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{{CodeEnd}}
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''Example 3:'' True or False evaluation of a numerical value executes only when the value is not 0. '''Cannot evaluate [[STRING]] values!'''
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{{CodeStart}} '' ''
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IF x THEN PRINT x '' ''
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{{CodeEnd}}
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:Example will only print if a numerical value is True (positive or negative). (Equivalent to: IF x > 0 OR x < 0 THEN evaluation)
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''Example 4:'' Multiple evaluations using parenthesis to determine the order.
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{{CodeStart}} '' ''
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IF (value% > 100 AND value% < 200) OR value% = 50 THEN PRINT "OK" '' ''
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{{CodeEnd}}
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''Example 5:'' Using multiple IF options in a one line statement.
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{{CodeStart}} '' ''
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x = 100 'change x value to see other evaluations
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IF x > 200 THEN PRINT "High" {{Cl|ELSEIF}} x < 0 THEN PRINT "Low" {{Cl|ELSE}} PRINT "OK" '' ''
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{{CodeEnd}}
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''Example 6:'' [[STRING]] values can be compared using greater than, less than, not equal to or equal to operators only.
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{{CodeStart}} '' ''
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PRINT "Press a letter key: ";
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Key$ = {{Cl|INPUT$}}(1)
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PRINT Key$
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IF Key$ >= {{Cl|CHR$}}(65) AND Key$ <= {{Cl|CHR$}}(90) THEN PRINT "A to Z"
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{{CodeEnd}}
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: ''Explanation:'' Long [[STRING]] expression values are compared by their cumulative [[ASCII]] code values.
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''See also:''
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*[[ELSEIF]], [[ELSE]]
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*[[AND (boolean)]], [[OR (boolean)]]
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*[[NOT]], [[SELECT CASE]], [[Boolean]]
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{{PageNavigation}} |