Story time! When Octave was getting a lot of traffic from the first Coursera machine learning class, when it was just Andrew Ng doing a course and before the C

Story time! When Octave was getting a lot of traffic from the first Coursera machine learning class, when it was just Andrew Ng doing a course and before the Coursera org existed, we were getting a lot of novice users who would do something like some_matrix(i,j) = 5 and get cryptic errors about how matrices cannot be indexed by complex numbers. This is because by default, in Matlab and Octave `i` and `j` are functions that evaluate to the imaginary unit. You have to overwrite the function names with `i=2, j=3` or whatever beforehand, which novices often forget. This was happening often enough that I pushed some patches to warn, "did you forget to assign i or j?" if someone tried to index matrices with complex numbers. Point is, people in Matlab and Octave often unintentionally try to index by complex numbers.