CODEWITHSUNDEEP
python-3.x
MarkMcN
MarkMcN

Reputation: 23

Python3 Iterate over a file printing multiple lines

Say I have a text file(test.txt) with the following lines 

line0
line1
line2
line3
line4
line5
line6
line7
line8
line9

It's easy to iterate over it

fh=open("./test.txt")
for x in fh:
    print(x)
fh.close()

What I'm trying to do is print lines 2 lines, like this, 

line0 
line2
line1
line3

So essentially I'm trying to access the current line and the a line 2 lines down from the current location. I have been messing around with the itererator but not got anywhere. I'd welcome all input please. Thanks

Upvotes: 1

Views: 443

Answers (5)

hiro protagonist
hiro protagonist

Reputation: 46849

also an option which does not read the whole file in one go but just stores 2 lines at a time:

txt = """line0
line1
line2
line3
line4
line5
line6
line7
line8
line9"""

with StringIO(txt) as file:
    a, b = next(file), next(file)
    for c in file:
        print(a, end='')
        print(c, end='')
        a, b = b, c

with the output

line0
line2
line1
line3
line2
line4
line3
line5
line4
line6
line5
line7
line6
line8
line7
line9

you'd have to replace the StringIO part with your file of course.

Upvotes: 0

Mark Tolonen
Mark Tolonen

Reputation: 177664

Here is a generator that handles any number of lines including odd length files:

def alternate(f):
    write = False
    for line in f:
        if not write:
            keep = line
        else:
            yield line
            yield keep
        write = not write
    if write: # handle odd-line-count files.
        yield line

with open('test.txt') as f:
    for line in alternate(f):
        print(line,end='')

Example input (odd length):

line0
line1
line2
line3
line4
line5
line6
line7
line8
line9
line10

Output:

line1
line0
line3
line2
line5
line4
line7
line6
line9
line8
line10

Upvotes: 0

blhsing
blhsing

Reputation: 106553

You can use itertools.tee to replicate the file iterator, skip the first two values of the second iterator returned by tee, and then zip the two iterators to produce the desired pairs of lines so that you can chain and join them for output:

from itertools import tee, chain
l, n = tee(fh)
next(n)
next(n)
print(''.join(chain.from_iterable(zip(l, n))))

With your sample input, this outputs:

line0
line2
line1
line3
line2
line4
line3
line5
line4
line6
line5
line7
line6
line8
line7
line9

Upvotes: 0

Daniel Fonnegra García
Daniel Fonnegra García

Reputation: 392

and Welcome to Stackoverflow

You can try this.

left = next(fh)
center = next(fh)
while True:
   try:
     right = next(fh)
   except StopIteration:
     break
   print('{}\n{}'.format(left, right)
   left = center
   center = right

This returns

line0
line2
line1
line3
line2
line4
line3
line5

and so on...

Upvotes: 1

Lev Leontev
Lev Leontev

Reputation: 2615

The easiest way would be to just read the whole file:

fh = open("./test.txt")
lines = fh.readlines()
for i in range(len(lines) - 2):
    print(lines[i])
    print(lines[i + 2])

Upvotes: 2

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