Multiplying two long long ints C

Question

I am working on a program in C as a part of Homework in which I have to get the product of two long numbers which are taken as character string. eg: 123456789021 and 132456789098. Since it is taken as a string, I converted them to long long int for the multiplication. But the resulting product will be very large(larger than long long int I guess). Can anyone please suggest me a method to perform this multiplication?

Answer 1

Here's one approach: Consider how you would multiply these numbers by hand, on paper. Implement this method in C. You will have to discover:

• how to break up an integer (represented as a string) into digits
• how to convert each digit back to an integer 0 <= d < 10
• how to manage arrays of digits (ie. how big should you make the arrays?)
• how to write the loop(s) you might need to implement multiplication
• how to manage carrying products from one digit to the next
• how to convert those digits back to characters for output

Answer 2

usually big integers represented as byte arrays. You can look at Microsoft's BigInteger implementation in DLR. I think they've used algorithms developed by Knuth