Calculating modulus in a function gives different answer from using % operator directly [duplicate]

Question

my_mod.cpp:

#include <iostream> using namespace std; unsigned int my_mod(int a, unsigned int b) { int result; cout << "\ta\t=\t" << a << endl; cout << "\tb\t=\t" << b << endl; cout << "\ta%b\t=\t" << a%b << endl; result = a%b; if (result < 0) {result += b;} return result; } int main() { cout << "-1%5 = " << -1%5 << endl; cout << "my_mod(-1,5) = " << my_mod(-1,5) << endl; return 0; }

compiled via: g++ ./my_mod.cpp

results in:

-1%5 = -1 a = -1 b = 5 a%b = 0 my_mod(-1,5) = 0

What the actual hell is happening here I just can't understand what possibly could go on?! This can't be due to the global scope, right?! I mean it is exactly the same %-expression ... how can they yield 0 and -1?! (Instead of the desired 4, by the way.)

Please, if anybody can, explain this to me ... it just took me days to narrow down an error in a wider context to this. Seriously, I'm about to cry.

How can I have my (global) own modulus returning 4 in the example above??

This is supposed to implement periodic boundary condition, so resulting in 4 there would be handy. — JohnSmithOptional
– JohnSmithOptional, Commented Aug 30, 2013 at 11:51

Matthew Mcveigh · Accepted Answer · 2013-08-30 12:08:30Z

3

It's because you're using an unsigned int, the signed int (-1) gets promoted to -1 % UINT_MAX so your operation becomes (-1 % UINT_MAX) % 5 = 0 (thanks to jrok for this more detailed reason)

try

cout << "\ta%b\t=\t" << a%(int)b << endl; result = a%(int)b;

with function signature of: int my_mod(int a, unsigned int b)

Or just use a function signature of: int my_mod(int a, int b)

edited Aug 30, 2013 at 12:08

answered Aug 30, 2013 at 11:40

Matthew Mcveigh

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2 Comments

jrok Over a year ago

It's actualy because of integer promotion, to be pedantic. -1 promoted to unsigned int yields UINT_MAX which is evenly divisible by 5.

JohnSmithOptional Over a year ago

OK, now that was just as fast as it was embarrassing for me. Sorry, I don't know how I could fail to see this. I haven't slept much since this error screw things over, granted. But this is bad. Thank you very, very much!

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