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    <p>This is something I have written a while ago for fun. It uses a <a href="http://en.wikipedia.org/wiki/Two%27s_complement" rel="noreferrer">two's complement</a> representation and implements addition using repeated shifts with a carry bit, implementing other operators mostly in terms of addition.</p> <pre><code>#include &lt;stdlib.h&gt; /* atoi() */ #include &lt;stdio.h&gt; /* (f)printf */ #include &lt;assert.h&gt; /* assert() */ int add(int x, int y) { int carry = 0; int result = 0; int i; for(i = 0; i &lt; 32; ++i) { int a = (x &gt;&gt; i) &amp; 1; int b = (y &gt;&gt; i) &amp; 1; result |= ((a ^ b) ^ carry) &lt;&lt; i; carry = (a &amp; b) | (b &amp; carry) | (carry &amp; a); } return result; } int negate(int x) { return add(~x, 1); } int subtract(int x, int y) { return add(x, negate(y)); } int is_even(int n) { return !(n &amp; 1); } int divide_by_two(int n) { return n &gt;&gt; 1; } int multiply_by_two(int n) { return n &lt;&lt; 1; } int multiply(int x, int y) { int result = 0; if(x &lt; 0 &amp;&amp; y &lt; 0) { return multiply(negate(x), negate(y)); } if(x &gt;= 0 &amp;&amp; y &lt; 0) { return multiply(y, x); } while(y &gt; 0) { if(is_even(y)) { x = multiply_by_two(x); y = divide_by_two(y); } else { result = add(result, x); y = add(y, -1); } } return result; } int main(int argc, char **argv) { int from = -100, to = 100; int i, j; for(i = from; i &lt;= to; ++i) { assert(0 - i == negate(i)); assert(((i % 2) == 0) == is_even(i)); assert(i * 2 == multiply_by_two(i)); if(is_even(i)) { assert(i / 2 == divide_by_two(i)); } } for(i = from; i &lt;= to; ++i) { for(j = from; j &lt;= to; ++j) { assert(i + j == add(i, j)); assert(i - j == subtract(i, j)); assert(i * j == multiply(i, j)); } } return 0; } </code></pre>
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    1. POHow to add two numbers without using ++ or + or another arithmetic operator
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    1. USPeter Mortensen
    1. USJason Creighton
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    1. POHow to add two numbers without using ++ or + or another arithmetic operator
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      1. VTUpMod
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    1. CO+1 Wow, that's insane dude! I thought about this type of thing while I was taking Digital design, but never actually wrote code for it!
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      1. USNoMoreZealots
    2. COOf course you'll need to unroll the loop to get rid of incrementing index :)
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      1. USEugene
    3. CO@Eugene: haha, good point. Obviously it's not a 100% pure implementation...there's probably some clever way to re-write it without incrementing an index. Also, in `multiply`, I use some comparison operators, which might be considered cheating, although due to the fact that I'm basically just checking for negative values, those would be pretty trivial to translate to bit-frobbing.
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