I have a very large .csv file with a numerical matrix saved within. I need to calculate the mean of many selections of values in each row. (e.g. In each row, the mean of the values at index 1, 3, 52, 123; then, in the same line, values 2, 3, 12, 29, 67, etc...)

My file is HUGE, even in row length (a row has like, 8000+ items), so I need this to be fast. I know the indexes I need to average at the start of the computation, but don't have enough memory to load the whole file at once.