user102008 user102008 - 3 months ago 19
C Question

Python-style integer division & modulus in C

In Python and Ruby, signed integer division truncates towards negative infinity, and signed integer modulus has the same sign the second operand:

>>> (-41) / 3
-14
>>> (-41) % 3
1


However, in C and Java, signed integer division truncates towards 0, and signed integer modulus has the same sign as the first operand:

printf("%d\n", (-41) / 3); /* prints "-13" */
printf("%d\n", (-41) % 3); /* prints "-2" */


What is the simplest and most efficient way in C to perform the same kind of division and modulus as in Python and Ruby?

Answer

The direction for rounding with signed integer division is not specified in older C standards. However, in C99 it is specified to round towards zero.

Here's portable code which works with all versions of the C standards and CPU architectures:

int py_div(int a, int b)
{
  if (a < 0)
    if (b < 0)
      return -a / -b;
    else
      return -(-a / b) - (-a % b != 0 ? 1 : 0);
  else if (b < 0)
      return -(a / -b) - (a % -b != 0 ? 1 : 0);
    else
      return a / b;
}

int py_mod(int a, int b)
{
  if (a < 0)
    if (b < 0)
      return -(-a % -b);
    else
      return -a % b - (-a % -b != 0 ? 1 : 0);
  else if (b < 0)
      return -(a % -b) + (-a % -b != 0 ? 1 : 0);
    else
      return a % b;
}

I did some superficial tests and it appears to give the same results as Python. This code may not be maximally efficient, but a good C compiler can probably optimize it adequately, especially if you put the code in a header as static functions.

You may also want to take a look at this closely related question: Integer division rounding with negatives in C++.

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