Question

I’m using PHP for a project and I expect -1 % 4 to return 3. However, the final result is -1 in PHP and I don’t know why:

php > echo -1 % 4;
-1

I checked in Ruby IRB and the result is 3:

irb(main):001:0> puts -1 % 4
3

Answer 1

Because it’s defined in terms of division, such that:

a%b == a - (a/b)*b

For divisions with negative results, there are two possible definitions; either you round toward zero, or you round toward negative infinity. Different programming languages have made that choice differently.

See http://en.wikipedia.org/wiki/Modulo_operator#Remainder_calculation_for_the_modulo_operation for more information.