Read every second line and print to new file

Question

I am trying to read every second line in a CSV file and print it in a new file. Unfortunately i am getting a blank line which i am unable to remove.

lines = open( 'old.csv', "r" ).readlines()[::2]
file = open('new.csv', "w")
n = 0
for line in lines:
    n += 1
    if ((n % 2) == 1):
            print >> file, line

The code i am using is simply by looking at the modolus value of n to decide if its actually every second line or not. I have even tried with strip() and rstrip() which still takes the blank lines.

Answer 1

In answer to your question, your blank line is coming from:

print >> file, line

Using print like that automatically outputs a new line, either use sys.stdout.write or, use a trailing comma to suppress the newline character, eg:

print >> file, line,

Anyway, the better way to approach this overall is to use itertools.islice for:

from itertools import islice

with open('input') as fin, open('output', 'w') as fout:
    fout.writelines(islice(fin, None, None, 2))

And if necessary, filter out the blank lines first, then take every 2nd from that...

non_blanks = (line for line in fin if line.strip())
fout.writelines(islice(non_blanks, None, None, 2))

Much more convenient and flexible than mucking about with modulus and such.

Answer 2

Try taking a look at the python library for csv files. It is pretty comprehensive and should help you do what you are looking to do more cleanly.

Answer 3

A recommendation: to clean up your code a bit, and get rid of the need to manually increment and declare a counter variable, use:

for (line_index, line) in enumerate(lines): ...