NOT
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NOT is a boolean logical operator that will change a false statement to a true one and vice-versa.
Syntax
- True = -1: False = NOT True
Description
- In QBasic, True = -1 and False = 0 in boolean logic and evaluation statements.
- NOT evaluates a value and returns the bitwise opposite, meaning that NOT 0 = -1.
- Often called a negative logic operator, it returns the opposite of a value as true or false.
- Values are changed by their bit values so that each bit is changed to the opposite of on or off. See example 3 below.
Table 3: The relational operations for condition checking. In this table, A and B are the Expressions to compare. Both must represent the same general type, i.e. they must result into either numerical values or STRING values. If a test succeeds, then true (-1) is returned, false (0) if it fails, which both can be used in further Boolean evaluations. ┌─────────────────────────────────────────────────────────────────────────┐ │ Relational Operations │ ├────────────┬───────────────────────────────────────────┬────────────────┤ │ Operation │ Description │ Example usage │ ├────────────┼───────────────────────────────────────────┼────────────────┤ │ A = B │ Tests if A is equal to B. │ IF A = B THEN │ ├────────────┼───────────────────────────────────────────┼────────────────┤ │ A <> B │ Tests if A is not equal to B. │ IF A <> B THEN │ ├────────────┼───────────────────────────────────────────┼────────────────┤ │ A < B │ Tests if A is less than B. │ IF A < B THEN │ ├────────────┼───────────────────────────────────────────┼────────────────┤ │ A > B │ Tests if A is greater than B. │ IF A > B THEN │ ├────────────┼───────────────────────────────────────────┼────────────────┤ │ A <= B │ Tests if A is less than or equal to B. │ IF A <= B THEN │ ├────────────┼───────────────────────────────────────────┼────────────────┤ │ A >= B │ Tests if A is greater than or equal to B. │ IF A >= B THEN │ └────────────┴───────────────────────────────────────────┴────────────────┘ The operations should be very obvious for numerical values. For strings be aware that all checks are done case sensitive (i.e. "Foo" <> "foo"). The equal/not equal check is pretty much straight forward, but for the less/greater checks the ASCII value of the first different character is used for decision making: E.g. "abc" is less than "abd", because in the first difference (the 3rd character) the "c" has a lower ASCII value than the "d". This behavior may give you some subtle results, if you are not aware of the ASCII values and the written case: E.g. "abc" is greater than "abD", because the small letters have higher ASCII values than the capital letters, hence "c" > "D". You may use LCASE$ or UCASE$ to make sure both strings have the same case. |
Table 4: The logical operations and its results.
In this table, A and B are the Expressions to invert or combine.
Both may be results of former Boolean evaluations.
┌────────────────────────────────────────────────────────────────────────┐
│ Logical Operations │
├───────┬───────┬───────┬─────────┬────────┬─────────┬─────────┬─────────┤
│ A │ B │ NOT B │ A AND B │ A OR B │ A XOR B │ A EQV B │ A IMP B │
├───────┼───────┼───────┼─────────┼────────┼─────────┼─────────┼─────────┤
│ true │ true │ false │ true │ true │ false │ true │ true │
├───────┼───────┼───────┼─────────┼────────┼─────────┼─────────┼─────────┤
│ true │ false │ true │ false │ true │ true │ false │ false │
├───────┼───────┼───────┼─────────┼────────┼─────────┼─────────┼─────────┤
│ false │ true │ false │ false │ true │ true │ false │ true │
├───────┼───────┼───────┼─────────┼────────┼─────────┼─────────┼─────────┤
│ false │ false │ true │ false │ false │ false │ true │ true │
└───────┴───────┴───────┴─────────┴────────┴─────────┴─────────┴─────────┘
Note: In most BASIC languages incl. QB64 these are bitwise operations,
hence the logic is performed for each corresponding bit in both
operators, where true or false indicates whether a bit is set or
not set. The outcome of each bit is then placed into the respective
position to build the bit pattern of the final result value.
As all Relational Operations return negative one (-1, all bits set) for
true and zero (0, no bits set) for false, this allows us to use these
bitwise logical operations to invert or combine any relational checks,
as the outcome is the same for each bit and so always results into a
true (-1) or false (0) again for further evaluations.
|
Examples
Example 1: Alternating between two conditions in a program loop.
DO switch = NOT switch 'NOT changes value from -1 to 0 and vice-versa LOCATE 10, 38 IF switch THEN PRINT "True!" ELSE PRINT "False" SLEEP k$ = INKEY$ LOOP UNTIL k$ = CHR$(27) ' escape key quit |
Example 2: Reading a file until it reaches the End Of File.
DO WHILE NOT EOF(1) INPUT #1, data1, data2, data3 LOOP |
- Explanation: EOF will return 0 until a file ends. NOT converts 0 to -1 so that the loop continues to run. When EOF becomes -1, NOT converts it to 0 to end the loop.
Example 3: So why does NOT 5 = -6? Because NOT changes every bit of a value into the opposite:
PRINT NOT 5 PRINT ReadBits 5 ReadBits -6 SUB ReadBits (n AS INTEGER) 'change type value and i bit reads for other whole type values FOR i = 15 TO 0 STEP -1 'see the 16 bit values IF n AND 2 ^ i THEN PRINT "1"; ELSE PRINT "0"; NEXT PRINT END SUB |
-6 0000000000000101 1111111111111010 |
- Explanation: The bit values of an INTEGER are 2 _BYTEs and each bit is an exponent of 2 from 15 to 0 (16 bits). Thus comparing the numerical value with those exponents using AND reveals the bit values as "1" for bits on or "0" for bits off as text.
'16 bit INTEGER values from -32768 to 32767 a% = &B0000000000000101 PRINT a% b% = &B1111111111111010 PRINT b% '8 bit BYTE values from -128 to 127 a%% = &B00000101 PRINT a%% b%% = &B11111010 PRINT b%% |
See also