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    PB/WIN - NOT operator

    Purpose
    The NOT operator works as a bitwise arithmetic operator.
    Syntax
    NOT p
    Remarks
    PowerBASIC's NOT operator returns the one's-complement of an integer-class expression. When dealing with the absolute values 0 and -1, the NOT operator "reverses" the two values, performing a Boolean-like operation.

    PowerBASIC accepts any non-zero value as a logical TRUE value; therefore, subtle logic problems can arise in a program when the NOT operator is used to perform Boolean logic tests with operand values that are not limited to just 0 and -1.

    Consider the following two test conditions:

    Code:
    test1 = 0          ' test1 is FALSE (zero)
    IF NOT test1 THEN  ' TRUE (-1 is non-zero)
    test2 = 1          ' test2 is TRUE (1 is non-zero)
    IF NOT test2 THEN  ' still TRUE (-2 is non-zero)
    Because NOT performs a bitwise operation on test2, it does not reverse the logical TRUE/FALSE value of test2, rather, it returns -2 (the one's-complement of 1) and this is evaluated as a logical TRUE value.

    In cases where a proper logical (Boolean) evaluation is required, and the operand may be a value other than 0 and -1, the ISFALSE operator should be used in place of the NOT operator:

    Code:
    test3 = 1             ' test3 is TRUE (non-zero)
    IF ISFALSE test3 THEN ' ISFALSE detects test3 is
    [statements]          ' TRUE so the IF test fails
    The two's-complement of a value can be obtained with the following algorithm:

    Code:
    y = (NOT x) + 1
    Using NOT as a logical operator
    NOT returns 0 (FALSE) if and only if its operand is exactly -1 (TRUE). Generally, you should use the ISFALSE operator instead of NOT, when you are testing for logical falsity.

    Truth Table
    Code:
    +------------+
    | x  | NOT x |
    +------------+
    | 0  | -1    |
    +------------+
    | -1 | 0     |
    +------------+
    Using NOT as a bitwise arithmetic operator
    NOT performs a one's-complement or bit reversal of each bit in an integral-class value. Here is a sample:

    Code:
    x% = NOT 16383%  ' Result is -16384
    See AlsoReferences
    Last edited by Gary Beene; 28 Oct 2014, 09:57 PM.
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