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# Recursive multiplication and Karatsuba

James Charles
Greenhorn
Posts: 1
Hey everyone, I've got a question where I'm sort of stuck on writing a java code for a recursive "school method" multiplication where we have input 'n' and two n digit numbers 'a' and 'b' where the output is m=a.b. I also have to use an array representation for representing numbers. Could anyone please help me out? Thanks

The basic format is a=a1.B^(k) + a0 and b=b1.B^(k) + b0 where a.b=a1.b1.B^(2k)+(a1.b0+a0.b1).B^(k)+a0.b0 and k=floor(n/2)

Also then how would you use this and implement a Karatsuba multiplication if say n>3.

sudheer kiran
Ranch Hand
Posts: 237
hi james,will you please send more description

Campbell Ritchie
Marshal
Posts: 56529
172
Welcome to the Ranch

What is a Karatsuba multiplication? I have never heard of it before.

I am going to have to tell you to use the high-tech hardware I so often recommend. A sheet of paper, a (soft) pencil and a (large) eraser.
Write down how you would do it on paper, and get it down to words of one syllable. Then you will know how to convert it to code. You may have to write passing half a number. Remember you will get down to where k is 0, and you will have to stop your recursion at that point.

Darryl Burke
Bartender
Posts: 5167
11

 It is sorta covered in the JavaRanch Style Guide.