Question

The purpose of my code is to iterate over each element in an array, convert the element to a string, and return lines from another file that contain this string. My code is:

    for element in myarray:
         elementstring=''.join(element)
         for line in myfile:
              if elementstring in line:
                  print line

If the code is run, it will only work for the first element. Can someone explain why this is?

Answer 1

This is happening because when you read through the lines of a file once, you reach the end of the file and there are no lines left to read. You need to close the file and re-open it for reading for each element.

Here’s one way of doing that:

for element in myarray:
    elementstring=''.join(element)
    with open('path/to/myfile') as myfile:
        for line in myfile:
            if elementstring in line:
                print line

Alternatively, if this is a small enough file, you can reduce the runtime by avoiding several reads from disk by caching the lines in the file before hand like so:

myfile = [line.rstrip('\n') for line in open('path/to/myfile')]
for element in myarray:
    elementstring=''.join(element)
    for line in myfile:
        if elementstring in line:
            print line