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