Python: remember line index when reading lines from text file - python-3.x

Question

I'm extracting data in a loop from a text file between two strings with Python 3.6. I've got multiple strings of which I would like to extract data between those strings, see code below:
for i in range(0,len(strings1)):
with open('infile.txt','r') as infile, open('outfile.txt', 'w') as outfile:
copy = False
for line in infile:
if line == strings1[i]:
copy = True
elif line == strings2[i]:
copy = False
elif copy:
outfile.write(line)
continue
To decrease the processing time of the loop, I would like to modify my code such that after it has extracted data between two strings, let's say strings1[1] and strings2[1], it remembers the line index of strings2[1] and starts the next iteration of the loop at that line index. Therefore it doesn't have to read the whole file during each iteration. The string lists are build such that the previous strings will never occur after a current string, so modifying my code to what I want won't break the loop.
Does anyone how to do this?
===========================================================================
EDIT:
I've got a file in a format such as:
the first line
bla bla bla
FIRST some string 1
10 10
15 20
5 2.5
SECOND some string 2
bla bla bla
bla bla bla
FIRST some string 3
10 10
15 20
5 2.5
SECOND some string 4
The file goes on like this for many lines.
I want to extract the data between 'FIRST some string 1' and 'SECOND some string 2', and plot this data. When that is done, I want to do the same for the data between 'FIRST some string 3' and 'SECOND some string 4' (thus also plot the data). All the 'FIRST some string ..' are stored in strings1 list and all the 'SECOND some string ..' are stored in strings2 list.
To decrease computational time, I would like to modify the code such that after the first iteration, it knows that it can start from line with string 'some string 2' and not from 'the first line' AND also that when during the first iteration, it knows that it can stop the first iteration when it has found 'SECOND some string 2'.
Does anyone how to do this? Please let me know when something is unclear.

Answer 1

The key issue is you're reopening your files in a for loop, of course it will reiterate the files from the beginning each time. I wouldn't open the files in a for loop, that's horribly inefficient. You can load the files into memory first and then loop through strings1.
There are some other issues, namely here:
copy = False
for line in infile:
if line == strings1[i]:
copy = True
elif line == strings2[i]:
copy = False
elif copy:
outfile.write(line)
continue
The elif copy: line will never execute in the first iteration of the second loop because copy is only ever True once the line == strings1[i] is met. After that condition is met, for the rest of the iterations it will always write the lines from infile to outfile. Unless this is precisely what you're trying to achieve the logic doesn't work.
Without a full context it's hard to understand what exactly you're looking for.
But maybe what you want to do instead is simply this:
with open('infile.txt','r') as infile, open('outfile.txt', 'w') as outfile:
for line in infile.readlines():
if line.rstrip('\n') in strings1:
outfile.write(line)
What this code is doing:
1.) Open both files into memory.
2.) Iterate through the lines of the infile.
3.) Check if the iterated line, stripping the trailing newline character is in the list strings1, assuming your strings1 is a list that doesn't have any trailing newline characters. If each item in strings1 already has a trailing \n, then don't rstrip the line.
4.) If line occurs in strings1, write the line to outfile.
This looks to be the gist of what you're attempting.