What is an off-by-one error and how do I fix it?

Question

What is an off-by-one error? If I have one, how do I fix it?

Answer 1

A common off-by-one confusion arises because some languages enumerate vectors from zero (C, for example) and other languages from one (R, for example). Thus, a vector x of size n has members running from x[0] to x[n-1] in C but from x[1] to x[n] in R.

You are also confronted with the off-by-one challenge when coding the common idiom for cyclic incrementation:

In C:

i = (i+1)%n

In R:

i <- (i-1)%%n + 1