math - Modulo operator in Objective-C returns the wrong result

Question

I'm a little freaked out by the results I'm getting when I do modulo arithmetic in Objective-C. -1 % 3 is coming out to be -1, which isn't the right answer: according to my understanding, it should be 2. -2 % 3 is coming out to -2, which also isn't right: it should be 1.

Is there another method I should be using besides the % operator to get the correct result?

Answer 1

Objective-C is a superset of C99 and C99 defines a % b to be negative when a is negative. See also the Wikipedia entry on the Modulo operation and this StackOverflow question.

Something like (a >= 0) ? (a % b) : ((a % b) + b) (which hasn't been tested and probably has unnecessary parentheses) should give you the result you want.