Question

I’m trying to use a for line in data: loop to assign the first 3 lines of a file to 3 different variables (one line to each variable) and have it iterate for every 3 lines, so that if there were 9 lines in the file, each variable would contain 3 different lines throughout the iterations, but I cannot figure it out whatsoever.

So far my basic code looks something like this:

for line in infile:

  to_line = infile.readline()
  from_line = infile.readline()
  header_line = infile.readline()

But it’s simply not working. It’s assigning every other line of my file to each variable (i.e. 4 lines to each variable and not all lines are being assigned).

The premise of the program is to process an input file containing 9 lines (a “to” line, “from” line, and “subject” line for each email [3 emails total]) and then sort them based on whether or not they are spam or “good” mail.

A sticky stipulation that I have for this is that it’s for a college course and I’m not allowed to use anything that we have yet to go over in class (we’re 8 weeks in so we have a small knowledge base of basic information to use). If anyone responds with anything that I cannot use, I will let you know.

Answer 1

This isn’t working because the beginning of the loop (i.e., for line in infile:) reads the next line from infile, which you basically throw away by not using it. An alternative to this would be reading all lines upfront into a list, and then working through the list:

# read all lines from the file into a list, where each list element is one line
lines = infile.readlines()

# iterate through the list three lines at a time, until you run out of lines
line_number = 0
while line_number < len(lines):
    to_line = lines[line_number]
    from_line = lines[line_number+1]
    header_line = lines[line_number+2]
    line_number += 3