IF...THEN
From QB64 Wiki
IF...THEN statements make Boolean True or False program evaluations to automate program decision making.
- IF true <> 0 THEN 'execute code statement
- The evaluation by IF must be true(-1) or non-zero for the THEN code statements to be run.
- 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 and END IF statements be the only ones on each line.
- Qbasic's IDE may return an error pointing somewhere else if you forget to use END IF with a statement block!
- Use colons to execute multiple statements in a one line IF statement. You cannot use AND on the other side of THEN!
- An underscore can be used anywhere after the code on one line to continue it to the next line in QB64 ONLY.
- > is Greater than
- < is Less than
- = is Equal to
- >= is Greater than or Equal to
- <= is Less than or Equal to
- <> is Not Equal to
- 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.
- The results of the bitwise logical operations, where A and B are operands, and T and F indicate that a bit is set or not set:
Operands Operations A B NOT B A AND B A OR B A XOR B A EQV B A IMP B T T F T T F T T T F T F T T F F F T F F T T F T F F T F F F T T
- Note that the Relational Operations return negative one (-1, all bits set) and zero (0, no bits set) for true and false, respectively. This allows relational tests to be inverted and combined using the bitwise logical operations.
Example 1: One line IF statement
IF x > 100 THEN PRINT x
Example 2: IF statement blocks require that the IF THEN and END IF statements be separate from the code executed.
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!
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.
IF (value% > 100 AND value% < 200) OR value% = 50 THEN PRINT "OK"
Example 5: Using multiple IF options in a one line statement.
x = 100 'change x value to see other evaluations IF x > 200 THEN PRINT "High" ELSEIF x < 0 THEN PRINT "Low" 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"
See also: