Modulo returns wrong result when using negatives

Question

I want to calculate: (-15 % 3) which should be 0 but instead i'm getting 1:

When i explicitly do:

int IntFcn (const void *key, size_t tableSize)
{
    printf("%d\n",(*(int*)key)); // prints -15
    printf("%d\n",tableSize); // prints 3
    printf("%d\n",(-15) % 3); // prints 0
}

I get the right result (0) but when i try to use the variables below i get 1:

int IntFcn (const void *key, size_t tableSize)
{
    printf("%d\n",(*(int*)key)); // prints -15
    printf("%d\n",tableSize); // prints 3
    printf("%d\n",((*(int*)key) % tableSize)); // prints 1
    return ((*(int*)key) % tableSize);
}

Why is this happening?

Don't printf() a size_t argument as "%d". Either cast to (unsigned int) and use a "%u" spec, or use the "%zu" specifier. Note: printf() is a varargs function, so you should be very carefull with the types of the arguments. — wildplasser
– wildplasser, Commented Dec 26, 2012 at 11:28

SomeWittyUsername · Accepted Answer · 2012-12-26 11:30:11Z

6

In your second case the second operand of modulo is unsigned integer thus the first operand is promoted to unsigned as well before performing the modulo. So the result will be (unsigned)(-15) % 3 which is equal (for 32-bit int) to 4294967281 % 3 == 1.

answered Dec 26, 2012 at 11:30

SomeWittyUsername

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4 Comments

Tom Over a year ago

how can i overcome this if i want this to work on both 32/64 bit machines?

SomeWittyUsername Over a year ago

It's not related to 32/64 bits. It's a matter of signed/unsigned arithmetic operations. Your operand is being promoted to unsigned when you don't expect it to. Cast explicitly to int to prevent the effect: ((*(int*)key) % (int)tableSize)

Mats Petersson Over a year ago

I would suggest that, assuming tablesize may get LARGE on a 64-bit machine [hence the use of size_t?], that using casts to long would be a better choice than int. But otherwise agree.

SomeWittyUsername Over a year ago

@MatsPetersson I agree in general. But in this case it's probably ok since key to that table is int

Adeel Ahmed · Accepted Answer · 2012-12-26 11:44:14Z

1

try this..

return ((*(int*)key) % (int)tableSize);

answered Dec 26, 2012 at 11:44

Adeel Ahmed

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