What is the correct idiom for mapping the $0$ value in a modulo $n$ expression back to $n$.
For example if I want to be sure that any integer value maps back to the index for a character in the alphabet, I might use something like
letterIndex[l_] = First@FirstPosition[ToUpperCase /@ Alphabet, l]; letterIndex[l_, rot_] := Mod[letterIndex[l] + rot, 26] /. 0 -> 26;
But this look stupid to me and I feel like I'm missing something idiomatic to Mathematica (or to modular equivalences — which have always frustrated me — in general).