Problem using modulo with negative numbers in decryption program

Question

I\'m rather new to C and have recently been working on making a simple encryption/decryption program. I managed to get the encryption fine, but I\'ve hit a road block with the

Answer 1

Instead of using asciinum % 26, use (asciinum + 26) % 26, this will have you using modulus on positive numbers, at the cost of an extra addition each time through the cycle.

Answer 2

In pre-C99 C, the behaviour of % for negative numbers is implementation-defined. In C99 onwards, it's defined, but doesn't do what you want.

The easiest way out is to do:

((asciinum + 26) % 26)

Assuming asciinum can never get lower than -26.