Why is modulus different in different programming languages?

Question

Perl

print 2 % -18;

-->

-16

Tcl

puts [expr {2 % -18}]

-->

Answer 1

Wikipedia's "Modulo operation" page explains it quite well. I won't try to do any better here, as I'm likely to make a subtle but important mistake.

The rub of it is that you can define "remainder" or "modulus" in different ways, and different languages have chosen different options to implement.