Wrong modulus result when using unsigned int

Question

I'll consider the following snippet :

int a =-9;
int m = 10;
unsigned int m1 =10;

std::cout<<a%m<<std::endl;
std::cout<<a%m1<<std::endl;

the result is :

-9
 7

The 7 is so strange. Can anyone have an explanation to it ?

If it was C question, I would point to [usual arithmetic conversions](http://port70.net/~nsz/c/c11/n1570.html#6.3.1.8) — Eugene Sh., Jul 30 '18 at 17:40
Wrong duplicate, I believe. It's more about signed/vs unsigned operations... — Eugene Sh., Jul 30 '18 at 17:41
@EugeneSh. Seems you are correct, `-9` as unsigned int is `4294967287` — Borgleader, Jul 30 '18 at 17:44
I agree, what is strange is that the unsigned modulus, will treat the left operand as unsigned aswell. — Stéphane Cauchie, Jul 30 '18 at 17:49
Possible duplicate of [C : Modulus operator on unsigned int gives unexpected output](https://stackoverflow.com/questions/16122213/c-modulus-operator-on-unsigned-int-gives-unexpected-output) — Joseph Quinsey, Jul 30 '18 at 18:12
See also: [signed int modulo unsigned int produces nonsense results](//stackoverflow.com/q/43293686) — Joseph Quinsey, Jul 30 '18 at 18:17
Please do not spam tags. Your question is clearly about C++, hence do not cross post to C as these are 2 distinct languages that mainly share the capability of C++ being able to call *most* C libraries. — Antti Haapala -- Слава Україні, Jul 30 '18 at 19:56

score 3 · Accepted Answer · answered Jul 30 '18 at 19:54

Per C++ 2017 (draft n4659) 8 11, the usual arithmetic conversions are performed on the operands of a % m. Per 11.5.3, when one operand has signed integer type and the other has unsigned integer type of greater or equal rank, the signed operand is converted to the type of the unsigned operand.

This causes the int value of −9 to be converted to unsigned. This conversion performed modulo one more than the maximum value of unsigned, which is apparently 4,294,967,295 (2³²−1) in your implementation, so the conversion is performed modulo 4,294,967,296, which produces 4,294,967,287. (Performing a conversion modulo 4,294,967,296 means that 4,294,967,296 is added or subtracted as many times as needed to produce a result in the range from 0 to 4,294,967,295, inclusive.)

Then the % operator operates in the usual way; the remainder of dividing 4,294,967,287 by 10 is 7.

Wrong modulus result when using unsigned int

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