CODEWITHSUNDEEP
pythonfile
The Nightman
The Nightman

Reputation: 5759

Filter some lines from a textfile in python

I am currently creating a list from an input file like this:

list = inputFile.read().splitlines()

Then after that manually iterating through and making a second list of the items/lines I care about (which are lines 2,6,10,14,18...). Is there a faster way to do this just with splitlines() so automatically, list contains only the lines I care about?

Upvotes: 3

Views: 1795

Answers (4)

itzMEonTV
itzMEonTV

Reputation: 20339

You can use enumerate also

f = inputFile.read()
to_read = [ 2,6,10,14,18]
for i,j in enumerate(f, 1):
    if i in to_read
        #your code here

Enumerate is a built in Python function to give an index to each object in an iterable. 

>>> l = iter(["a", "b", "c"])
>>> [x for x in enumerate(l)]
[(0, 'a'), (1, 'b'), (2, 'c')]

Upvotes: 1

rassa45
rassa45

Reputation: 3550

Try doing this if you want to read in the whole file. 

readfile = file.read()
for x in range(2,len(readfile.split("\n"),4)):

    line = readfile[x]
    #Do stuff with x

Running through the file line by line instead of reading it in is better for memory. 

count = 1
for line in file:
    if count <= 2:
        continue
    if count % 4 != 2 or (count % 4 == 0 and count <= 4):
        continue
        count += 1
    count += 1
    #Work with line

Upvotes: 1

Kasravnd
Kasravnd

Reputation: 107287

The more pythonic way is that don't read all the lines at once. You can use enumerate to iterate over your file object and keep the expected lines :

with open(file_name) as f:
  list_of lines=[line for index,line in enumerate(f) if index in set_of_indices]

Note that here its better to put your line numbers in a set object which its membership checking order is O(1).

As mentioned in comment if you have a huge set of indices as a more optimized way in terms of memory use you can use a generator to preserve your lines, instead of a list comprehension :

with open(file_name) as f:
  list_of lines=(line for index,line in enumerate(f) if index in set_of_indices)

Upvotes: 1

alecxe
alecxe

Reputation: 473833

itertools.islice(iterable, start, stop[, step]) is the tool for the job:

from itertools import islice

for line in islice(inputFile, 2, None, 4):
    print line

Upvotes: 6

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