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copied!<p>A <a href="https://en.wikipedia.org/wiki/Bitwise_NOT#NOT" rel="nofollow">bitwise NOT</a> is also known as a <a href="https://en.wikipedia.org/wiki/1s_complement" rel="nofollow">1s complement</a>.</p> <p>You can convert a 1s complement number to <a href="https://en.wikipedia.org/wiki/Two%27s_complement" rel="nofollow">2s complement</a> by adding 1.</p> <p>Both 1s complement and 2s complement are different ways of representing negative numbers, but these days 2s complement is almost universally used.</p> <p>So, to negate a number in 2s complement form you can simply apply a bitwise NOT and add 1, i.e.</p> <pre><code>-x == ~x + 1 </code></pre> <p>Conversely, to convert a 2s complement number to 1s complement you can negate it and subtract 1:</p> <pre><code>~x == -x - 1 </code></pre> <p>Note that positive numbers have the same representation in 1s complement and 2s complement representations - it is only negative numbers for which there is a difference. E.g. for 8 bit integer:</p> <pre><code>Decimal 1s complement 2s complement 0 00000000 00000000 -0 11111111 n/a 1 00000001 00000001 -1 11111110 11111111 2 00000010 00000010 -2 11111101 11111110 127 01111111 01111111 -127 10000000 10000001 -128 n/a 10000000 </code></pre> <p>Note that 1s complement has two representations for decimal 0: +0 and -0, while 2s complement has a unique zero representation. 2s complement can also represent an additional negative value that is not available in 1s complement (-128 in the case of 8 bit integers).</p>

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