Editing Subtraction (section)

==In computing==
The [[method of complements]] is a technique used to subtract one number from another using only the addition of positive numbers. This method was commonly used in [[mechanical calculator]]s, and is still used in modern [[computers]].

{| class="wikitable floatright"
! Binary<br>digit
! Ones' <br>complement
|-align="center"
| 0
| 1
|-align="center"
| 1
| 0
|}
To subtract a binary number ''y'' (the subtrahend) from another number ''x'' (the minuend), the ones' complement of ''y'' is added to ''x'' and one is added to the sum. The leading digit "1" of the result is then discarded.

The method of complements is especially useful in binary (radix 2) since the ones' complement is very easily obtained by inverting each bit (changing "0" to "1" and vice versa). And adding 1 to get the two's complement can be done by simulating a carry into the least significant bit. For example:

   01100100  (x, equals decimal 100)
 - 00010110  (y, equals decimal 22)

becomes the sum:

   01100100  (x)
 + 11101001  (ones' complement of y)
 +        1  (to get the two's complement)
 ——————————
  101001110

Dropping the initial "1" gives the answer: 01001110 (equals decimal 78)