Boolean

Boolean statements are numerical evaluations that return True (-1 or NOT 0) or False (0) values that can be used in other calculations.

Basic Returns:

• True evaluations return -1. NOT 0 = -1 in Basic. Can be used to increment a value.
• For positive True results, subtract it, multiply it by a negative value or use ABS.
• False evaluations return 0. Watch out for "Division by 0" errors!

Template:RelationalTable

• When evaluating a True value, an IF value < 0 statement is NOT necessary for return values not 0.

Truth table of the BASIC Logical Operators:

Boolean Conditional Operators:

• AND can be used to add extra conditions to a boolean statement evaluation. Both must be True.
• OR can be used to add alternate conditions to a boolean statement evaluation. One must be True.
• Parenthesis are allowed inside of boolean statements to clarify an evaluation.
• Note that Basic returns -1 for True and 0 for False.

Example 1: Using 2 different boolean evaluations to determine a leap year.

INPUT "Enter a year greater than 1583: ", annum$ Y = VAL(annum$) leap1 = (Y MOD 4 = 0 AND Y MOD 100 <> 0) OR (Y MOD 400 = 0) leap2 = (Y MOD 4 = 0) - (Y MOD 100 = 0) + (Y MOD 400 = 0) PRINT "Year = "; annum$, "Leap1 = "; leap1, "Leap2 = "; leap2

Explanation: Both boolean evaluations will return -1 if the year is a leap year. It is not simply every four years as many people think. That is checked by the first evaluation (Y MOD 4 = 0) of each. In new century years like 1900 (which was not a leapyear) there is only one leap year every 400 years. 100 is used with MOD to see if there is a remainder. When that is true, the boolean return of that part of the first evaluation will be 0. The second returns -1 (which is actually added). In both evaluations the result of (Y MOD 400 = 0) indicates a century leap year.

Entry year = 2000:

leap1 = (-1 AND 0) OR -1 = -1 ' the AND evaluation returns False(0) so the OR value is used.

leap2 = (-1) - (-1) + (-1) = -1 + 1 + -1 = -1

Entry year = 1900:

leap1 = (-1 AND 0) OR 0 = 0 OR 0 = 0

leap2 = (-1) - (-1) + (0) = -1 + 1 + 0 = 0

Example 2: Moving an ASCII character using the arrow keys and boolean statements to determine the new coordinate.

SCREEN 12 COLOR 7 LOCATE 11, 20: PRINT "Using Screen 12 here to be in 80 X 30 coordinates mode" LOCATE 13, 6: PRINT "Simple Example of Alternative programming without IF-THEN-ELSE Statements" LOCATE 15, 1: PRINT "Use the four Cursor keys to move the yellow cursor, text will not be disturbed" LOCATE 17, 12: PRINT "When you END the program with the ESC key, cursor will disappear" cordx% = 40 cordy% = 15 DO oldcordx% = cordx% oldcordy% = cordy% p% = SCREEN(cordy%, cordx%) 'get ASCII character code at present position COLOR 14: LOCATE cordy%, cordx%: PRINT CHR$(178); 'print cursor character to position WHILE cordx% = oldcordx% AND cordy% = oldcordy% AND k$ <> CHR$(27) k$ = INKEY$ cordx% = cordx% + (k$ = (CHR$(0) + "K") AND cordx% > 1) + ABS(k$ = (CHR$(0) + "M") AND cordx% < 80) cordy% = cordy% + (k$ = (CHR$(0) + "H") AND cordy% > 1) + ABS(k$ = (CHR$(0) + "P") AND cordy% < 30) WEND COLOR 7: LOCATE oldcordy%, oldcordx%: PRINT CHR$(p%); 'replace overwritten screen characters LOOP UNTIL k$ = CHR$(27)

Code by AlgoreIthm

See also:

• IF...THEN, SELECT CASE
• Binary, ABS, SGN
• AND, OR, XOR

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