Editing Negation (section)

==Programming language and ordinary language==
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As in mathematics, negation is used in [[computer science]] to construct logical statements.

<syntaxhighlight lang="cpp">
if (!(r == t))
{
    /*...statements executed when r does NOT equal t...*/
}
</syntaxhighlight>

The [[exclamation mark]] "<code>!</code>" signifies logical NOT in [[B (programming language)|B]], [[C Programming Language|C]], and languages with a C-inspired syntax such as [[C++]], [[Java (programming language)|Java]], [[JavaScript]], [[Perl]], and [[PHP]]. "<code>NOT</code>" is the operator used in [[ALGOL 60]], [[BASIC]], and languages with an ALGOL- or BASIC-inspired syntax such as [[Pascal programming language|Pascal]], [[Ada programming language|Ada]], [[Eiffel (programming language)|Eiffel]] and [[Seed7]]. Some languages (C++, Perl, etc.) provide more than one operator for negation. A few languages like [[PL/I]] and [[Ratfor]] use <code>¬</code> for negation. Most modern languages allow the above statement to be shortened from <code>if (!(r == t))</code> to <code>if (r != t)</code>, which allows sometimes, when the compiler/interpreter is not able to optimize it, faster programs.

In computer science there is also ''[[bitwise negation]]''. This takes the value given and switches all the [[binary numeral system|binary]] 1s to 0s and 0s to 1s.  This is often used to create [[signed number representations|ones' complement]] (or "<code>~</code>" in C or C++) and [[two's complement]] (just simplified to "<code>-</code>" or the [[negative sign]], as this is equivalent to taking the [[Negation (arithmetic)|arithmetic negation]] of the number).

To get the absolute (positive equivalent) value of a given integer the following would work as the "<code>-</code>" changes it from negative to positive (it is negative because "<code>x < 0</code>" yields true)

<syntaxhighlight lang="cpp">
unsigned int abs(int x)
{
    if (x < 0)
        return -x;
    else
        return x;
}
</syntaxhighlight>

To demonstrate logical negation:

<syntaxhighlight lang="cpp">
unsigned int abs(int x)
{
    if (!(x < 0))
        return x;
    else
        return -x;
}
</syntaxhighlight>

Inverting the condition and reversing the outcomes produces code that is logically equivalent to the original code, i.e. will have identical results for any input (depending on the compiler used, the actual instructions performed by the computer may differ).

In C (and some other languages descended from C), double negation (<code>!!x</code>) is used as an [[Programming idiom|idiom]] to convert <code>x</code> to a canonical Boolean, ie. an integer with a value of either 0 or 1 and no other. Although any integer other than 0 is logically true in C and 1 is not special in this regard, it is sometimes important to ensure that a canonical value is used, for example for printing or if the number is subsequently used for arithmetic operations.<ref>{{cite web |last1=Egan |first1=David |title=Double Negation Operator Convert to Boolean in C |url=https://dev-notes.eu/2019/10/Double-Negation-Operator-Convert-to-Boolean-in-C/ |website=Dev Notes}}</ref>
	
The convention of using <code>!</code> to signify negation occasionally surfaces in ordinary written speech, as computer-related [[slang]] for ''not''.  For example, the phrase <code>!voting</code> means "[[not voting]]".  Another example is the phrase <code>!clue</code> which is used as a synonym for "no-clue" or "clueless".<ref>[[Eric S. Raymond|Raymond, Eric]] and Steele, Guy.  [https://books.google.com/books?id=g80P_4v4QbIC&pg=PA18&lpg=PA18 The New Hacker's Dictionary], p. 18 (MIT Press 1996).</ref><ref>Munat, Judith.  [https://books.google.com/books?id=UOPXXYslemYC&pg=PA148&lpg=PA148 Lexical Creativity, Texts and Context], p. 148 (John Benjamins Publishing, 2007).</ref>