What are some algorithms for subtraction?

Question

I am trying to find some algorithms for subtraction and I have to explain them as simple as possible. Can anyone help?

Answer 1

For subtraction, you could use a left-to-right algorithm instead of right-to-left. Another "non-standard" algorithm is using 9's complement:
851929
-556516
------------
295413
becomes
. 851929
+443484
-------------
1295413
Throw away the leading 1 and you have
295413

There are many alternate algorithms for multiplication, usually using the property of distribution. The most universal method is one I was taught as the Basque method. It is quite easy to use with 2-digit numbers, and only a little more difficult for 3-digit numbers. Generalizing, with a, b, c, and d representing single digits,
10a + b times
10c + d equals
100ac +
10ad +
10cb +
bd
Note the similarity to the FOIL method of expanding binomial multiplication.