Nitpick: the index could come from program input. For example: - if one were to write ‘ed’ in C++, that could be the code that handles going to the n-th line.

Nitpick: the index could come from program input. For example: - if one were to write ‘ed’ in C++, that could be the code that handles going to the n-th line. - in a lzw decompressor, it could be an integer read from the to-be-decompressed data stream. In both cases, using at rather than doing a range check first can be defended because failing the range check is an exceptional case.