Integer remainder for negative numbers

Hello, I'm looking for a way in C to get the remainder of a negative number such that the remainder is positive, i.e. using truncation toward negative infinity rather than truncation toward zero.

Example; using the modulus operator, -10%3 returns -1 as it is equivalent to the following:

-10 - 3*int(-10/3) = -10 -3(-3) = -1

I want an expression that returns 2:

-10 - 3*floor(-10/3) = -10 - 3(-4) = 2

I'd like to avoid using a floating point function (floor()); integer math only.

Thanks, Stan

Use the positive version of your dividend. You can use a unary - in C to negate something. Say x is -10, y is 3:

Code:

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remainder = x - y * -(-x / y);

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Use the abs() labs() or llabs() function as approriate to your variable type.

Code:

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int x = -10;
int y;

y = abs( x % 3);

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Code:

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#include <stdio.h>

int main()
{

    int x = -10;
    int y;
    const int mod_value = 3;

    y = ((x%mod_value)+mod_value)%mod_value;
    printf("%d\n", y);
    return 0;
}

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Code stolen from CT used in my modified solution.

Tim S.

Quote:

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Originally Posted by stahta01

Code:

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#include <stdio.h>

int main()
{

    int x = -10;
    int y;
    const int mod_value = 3;

    y = ((x%mod_value)+mod_value)%mod_value;
    printf("%d\n", y);
    return 0;
}

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Code stolen from CT used in my modified solution.

Tim S.

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That one worked, thanks.