How to apply modulo operation on a char array in C?

Question

Edited:

I have a big number that C does not have a type for it natively. I have to use a char array to hold it. As an example, I create a 32-byte array. It represents a large number up to 2 ^ 256.

unsigned char num[32]; // The size could be any number for this question.

I want to apply modulo operation on it, for example, I want to mod the big number by a small divisor and get an integer type result.

int divisor = 1234; // Note that the divisor is much smaller than the big number
int result;

// do something here
// to produce a result
// like result = number mod divisor

I do not want to use other library. How can I do it?

Said like this, it has little sense. What do you expect in result? — rocambille
– rocambille, Commented Jul 25, 2016 at 15:25
and there are many ways...do you want to know how to convert a string to an integer? — Sourav Ghosh
– Sourav Ghosh, Commented Jul 25, 2016 at 15:26
I suspect this is a question about implementing the % operation in arbitrary-precision arithmetic without using a library meant for that purpose — user2404501
– user2404501, Commented Jul 25, 2016 at 15:26
I want to treat the char array as a big number and apply modulo. — Luke
– Luke, Commented Jul 25, 2016 at 15:27
Your first job: use unsigned char instead. char could be signed 1's complement. That will completely mess things up. — Bathsheba
– Bathsheba, Commented Jul 25, 2016 at 15:30

Community · Accepted Answer · 2017-05-23 11:51:28Z

5

To perform mod an a large number, use mod one unsigned char (@Bathsheba) at a time.

% is C's remainder operator. For positive operands it has the same functionality as mod.

unsigned mod_big(const unsigned char *num, size_t size, unsigned divisor) {
  unsigned rem = 0;
  // Assume num[0] is the most significant
  while (size-- > 0) {
    // Use math done at a width wider than `divisor`
    rem = ((UCHAR_MAX + 1ULL)*rem + *num) % divisor;
    num++;
  }
  return rem;
}

edited May 23, 2017 at 11:51

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answered Jul 25, 2016 at 16:07

chux

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1 Comment

Armen Michaeli Over a year ago

Useful answer to an interesting question, if you ask me.

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