IF...THEN

IF...THEN statements make boolean (true or false) evaluations to automate program decision making.

Syntax

Single-line

IF conditionStatement THEN {code} ELSE {alternativeCode}

IF conditionStatement GOTO lineLabel

Block

IF conditionStatement THEN

{code}

⋮

ELSEIF conditionStatement2 THEN

{code}

⋮

ELSE

{code}

⋮

END IF

Description

• The conditionStatement evaluation by IF must be true (-1) or a non-zero numerical value for the THEN {code} to be executed.
• Multiple conditional evaluations can be made using inclusive AND or alternative 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 when END IF does not end a statement block.
• The QB64 IDE will indicate an error in the IF statement line until END IF closes the statement block.
• Use colons to execute multiple statements in a single-line IF statement.
• An underscore can be used anywhere after the code on a single-line to continue it to the next line in QB64.
• 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.

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

Boolean Conditional Operators:

• 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.

Mathematical Logical operators: * Truth table of the 6 BASIC Logical Operators:

Template:LogicalTruthTable

* Note that Basic returns -1 for True and 0 for False.

Examples

Example 1: In a one line IF statement, only REM can be used to comment out the action without an END IF error:

INPUT "Enter a number over or under 100: ", x IF x > 100 THEN PRINT x IF x > 100 THEN REM PRINT x '

Example 2: IF statement blocks require that the IF THEN and END IF statements be separate from the code executed.

INPUT "Enter a number over or under 100: ", x IF x > 100 THEN y = 200 PRINT y PRINT x END IF

Example 3: True or False evaluation of a numerical value executes only when the value is not 0. Cannot evaluate STRING values.

INPUT "Enter a number or just hit Enter: ", x IF x THEN PRINT x

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.

INPUT "Enter a number over or under 100 or 50: ", value IF (value% > 100 AND value% < 200) OR value% = 50 THEN PRINT "OK"

Example 5: Using multiple IF options in a one line statement.

INPUT "Enter a number over or under 200: ", x IF x > 200 THEN PRINT "High" [[ELSEIF|ELSEIF]] x < 0 THEN PRINT "Low" [[ELSE|ELSE]] PRINT "OK"

Example 6: STRING values can be compared using greater than, less than, not equal to or equal to operators only.

PRINT "Press a letter key: "; Key$ = INPUT$(1) PRINT Key$ IF Key$ >= CHR$(65) AND Key$ <= CHR$(90) THEN PRINT "A to Z"

Explanation: Long STRING expression values are compared by their cumulative ASCII code values.

QBasic decimal point value comparison errors

• Floating decimal point numerical values may not be compared as exactly the same value. QB64 will compare them the same.

Example: QBasic would print unequal in the IF comparison code below even though it is exactly the same value printed.

x# = 5 / 10 y# = 6 / 10 z# = x# + y# PRINT x#, y#, z# IF x# + y# = z# THEN PRINT "equal" ELSE PRINT "unequal"

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 (numerical comparisons return a true or false value)

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