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88 lines
3.5 KiB
Markdown
88 lines
3.5 KiB
Markdown
**Boolean** statements are numerical evaluations that return True (-1 or NOT 0) or False (0) values that can be used in other calculations.
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*Basic Returns:*
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* True evaluations return -1. [NOT](NOT) 0 = -1 in Basic. Can be used to increment a value.
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* For positive True results, subtract it, multiply it by a negative value or use [ABS](ABS).
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* False evaluations return 0. Watch out for [ERROR Codes](ERROR-Codes)!
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MISSING: Relational Table
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* When evaluating a True value, an IF value < 0 statement is NOT necessary for return values not 0.
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** Truth table of the BASIC Logical Operators:**
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MISSING: Logical Truth Table
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**Boolean Conditional Operators:**
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* [AND (boolean)](AND-(boolean)) can be used to add extra conditions to a boolean statement evaluation. Both must be True.
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* [OR (boolean)](OR-(boolean)) can be used to add alternate conditions to a boolean statement evaluation. One must be True.
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* Parenthesis are allowed inside of boolean statements to clarify an evaluation.
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* **Note that Basic returns -1 for True and 0 for False.**
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Using 2 different boolean evaluations to determine a leap year.
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```vb
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INPUT "Enter a year greater than 1583: ", annum$
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Y = VAL(annum$)
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leap1 = (Y MOD 4 = 0 AND Y MOD 100 <> 0) OR (Y MOD 400 = 0)
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leap2 = (Y MOD 4 = 0) - (Y MOD 100 = 0) + (Y MOD 400 = 0)
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PRINT "Year = "; annum$, "Leap1 = "; leap1, "Leap2 = "; leap2
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```
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*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](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.
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Entry year = 2000
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> leap1 = (-1 AND 0) OR -1 = -1 ' the AND evaluation returns False(0) so the OR value is used.
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> leap2 = (-1) - (-1) + (-1) = -1 + 1 + -1 = -1
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Entry year = 1900
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> leap1 = (-1 AND 0) OR 0 = 0 OR 0 = 0
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> leap2 = (-1) - (-1) + (0) = -1 + 1 + 0 = 0
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Moving an [ASCII](ASCII) character using the arrow keys and boolean statements to determine the new coordinate.
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```vb
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SCREEN 12
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COLOR 7
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LOCATE 11, 20: PRINT "Using Screen 12 here to be in 80 X 30 coordinates mode"
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LOCATE 13, 6: PRINT "Simple Example of Alternative programming without IF-THEN-ELSE Statements"
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LOCATE 15, 1: PRINT "Use the four Cursor keys to move the yellow cursor, text will not be disturbed"
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LOCATE 17, 12: PRINT "When you END the program with the ESC key, cursor will disappear"
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cordx% = 40
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cordy% = 15
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DO
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oldcordx% = cordx%
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oldcordy% = cordy%
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p% = SCREEN(cordy%, cordx%) 'get ASCII character code at present position
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COLOR 14: LOCATE cordy%, cordx%: PRINT CHR$(178); 'print cursor character to position
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WHILE cordx% = oldcordx% AND cordy% = oldcordy% AND k$ <> CHR$(27)
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k$ = INKEY$
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cordx% = cordx% + (k$ = (CHR$(0) + "K") AND cordx% > 1) + ABS(k$ = (CHR$(0) + "M") AND cordx% < 80)
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cordy% = cordy% + (k$ = (CHR$(0) + "H") AND cordy% > 1) + ABS(k$ = (CHR$(0) + "P") AND cordy% < 30)
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WEND
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COLOR 7: LOCATE oldcordy%, oldcordx%: PRINT CHR$(p%); 'replace overwritten screen characters
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LOOP UNTIL k$ = CHR$(27)
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```
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<sub>Code by AlgoreIthm</sub>
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## See Also
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* [IF...THEN](IF...THEN), [SELECT CASE](SELECT-CASE)
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* [Binary](Binary), [ABS](ABS), [SGN](SGN)
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* [AND](AND), [OR](OR), [XOR](XOR)
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