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117 lines
3.3 KiB
Markdown
117 lines
3.3 KiB
Markdown
[NOT](NOT) is a [Boolean](Boolean) logical operator that will change a false statement to a true one and vice-versa.
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## Syntax
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> *True* = -1: *False* = [NOT](NOT) True
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## Description
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* In QBasic, True = -1 and False = 0 in boolean logic and evaluation statements.
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* [NOT](NOT) evaluates a value and returns the bitwise opposite, meaning that `NOT 0 = -1`.
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* Often called a negative logic operator, it returns the opposite of a value as true or false.
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* Values are changed by their bit values so that each bit is changed to the opposite of on or off. See example 3 below.
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**Relational Operators:**
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| Symbol | Condition | Example Usage |
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| -- | -- | -- |
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| = | Equal | IF a = b THEN |
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| <> | NOT equal | IF a <> b THEN |
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| < | Less than | IF a < b THEN |
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| > | Greater than | IF a > b THEN |
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| <= | Less than or equal | IF a <= b THEN |
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| >= | Greater than or equal | IF a >= b THEN |
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The results of the bitwise logical operations, where *A* and *B* are operands, and *T* and *F* indicate that a bit is set or not set:
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| A | B | | [NOT](NOT) B | A [AND](AND) B | A [OR](OR) B | A [XOR](XOR) B | A [EQV](EQV) B | A [IMP](IMP) B |
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| - | - | - | - | - | - | - | - | - |
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| T | T | | F | T | T | F | T | T |
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| T | F | | T | F | T | T | F | F |
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| F | T | | F | F | T | T | F | T |
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| F | F | | T | F | F | F | T | T |
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**[Relational Operations](Relational-Operations) return negative one (-1, all bits set) and zero (0, no bits set) for *true* and *false*, respectively.**
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This allows relational tests to be inverted and combined using the bitwise logical operations.
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## Example(s)
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Alternating between two conditions in a program loop.
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```vb
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DO
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switch = NOT switch 'NOT changes value from -1 to 0 and vice-versa
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LOCATE 10, 38
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IF switch THEN PRINT "True!" ELSE PRINT "False"
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SLEEP
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k$ = INKEY$
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LOOP UNTIL k$ = CHR$(27) ' escape key quit
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```
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Reading a file until it reaches the End Of File.
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```vb
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DO WHILE NOT EOF(1)
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INPUT #1, data1, data2, data3
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LOOP
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```
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> *Explanation:* [EOF](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.
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So why does **NOT 5 = -6**? Because NOT changes every bit of a value into the opposite:
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```vb
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PRINT NOT 5
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PRINT
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ReadBits 5
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ReadBits -6
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SUB ReadBits (n AS INTEGER) 'change type value and i bit reads for other whole type values
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FOR i = 15 TO 0 STEP -1 'see the 16 bit values
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IF n AND 2 ^ i THEN PRINT "1"; ELSE PRINT "0";
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NEXT
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PRINT
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END SUB
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```
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```text
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-6
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0000000000000101
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1111111111111010
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```
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> *Explanation:* The bit values of an [INTEGER](INTEGER) are 2 [_BYTE](_BYTE)s and each bit is an exponent of 2 from 15 to 0 (16 bits). Thus comparing the numerical value with those exponents using [AND](AND) reveals the bit values as "1" for bits on or "0" for bits off as text.
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> QB64 can use [&B](&B) to convert the above [_BIT](_BIT) values back to [INTEGER](INTEGER) or [_BYTE](_BYTE) values as shown below:
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```vb
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'16 bit INTEGER values from -32768 to 32767
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a% = &B0000000000000101
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PRINT a%
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b% = &B1111111111111010
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PRINT b%
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'8 bit BYTE values from -128 to 127
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a%% = &B00000101
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PRINT a%%
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b%% = &B11111010
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PRINT b%%
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```
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## See Also
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* [_BIT](_BIT), [&B](&B), [_BYTE](_BYTE)
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* [AND](AND), [XOR](XOR), [OR](OR)
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* [Binary](Binary), [Boolean](Boolean)
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* [Mathematical Operations](Mathematical-Operations)
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