# multiplication

Discussion in 'C Programming' started by helPlease, Apr 30, 2007.

1. ### helPleaseGuest

I want to multiply two very long integers (say of 20 digits).I am
storing them in character array.Then what mechanism should i follow
to mutiply two such numbers.If anybody has any suggestions please post.

helPlease, Apr 30, 2007

2. ### Barry SchwarzGuest

On 30 Apr 2007 04:29:26 -0700, helPlease <>
wrote:

>I want to multiply two very long integers (say of 20 digits).I am
>storing them in character array.Then what mechanism should i follow
>to mutiply two such numbers.If anybody has any suggestions please post.

Take a pencil and paper and multiply two four-digit numbers. Carefully
show **every** step. Notice that while the total number of steps is
dependent on the number of digits in the two numbers, each individual
step is dependent only on one digit from each number (and any
potential carry from the previous step).

As a preliminary step to multiplying, work out the algorithm to add
two numbers of arbitrary length. Then implement the algorithm in a
function and test extensively. Notice that the sum of two n-digit
numbers has at most n+1 digits.

Follow the same process for multiplication. Since you will need to do
some addition, make use of the function previously developed. Notice
that the product of two n-digit numbers can have up to 2n digits.

If/when you run into problems, post your code and ask specific
questions. Here is a starting hint. Decide if the elements of your
character array will contain the integer value or the character value
of the digits, '1' is not the same as 1. One simplifies input/output;
the other simplifies computation.

And please don't post the same question multiple times with different
subjects.

Remove del for email

Barry Schwarz, Apr 30, 2007

3. ### user923005Guest

On Apr 30, 4:29 am, helPlease <> wrote:
> I want to multiply two very long integers (say of 20 digits).I am
> storing them in character array.Then what mechanism should i follow
> to mutiply two such numbers.If anybody has any suggestions please post.

For only 20 digits, schoolbook method will be fine.
If you need bigger numbers, then try news:sci.crypt where they talk
about that sort of thing all day long.

user923005, Apr 30, 2007

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