Merge two rows in a csv file in Python

Question

I am very new to Python and I am trying to do a very simple merge of every two lines in a csv file. Basically I want it like this:

Input:

[H1],1,2,3,4
[H2],5,6,7,8
[H1],a,b,c,d
[H2],e,f,g,h

Output:

[H1],1,2,3,4,[H2],5,6,7,8
[H1],a,b,c,d,[H2],e,f,g,h

This is a brief example, but the csv file has up to 167 columns with the two lines combined. This is what I have:

import csv
f = open("sample.csv",'rU').read().split("\n")
reader = csv.reader(f)
for row in reader:
    if row[0].startswith("[H1]"): 
        i=[]
        while i<167: n = row.append([",".join(row[i]+row[i+1])])
        print n

However when I run it I get the following error:

    print n
NameError: name 'n' is not defined

Any help is appreciated, thanks.

row.append does not return anything. What is n supposed to be? If i = [], what is while i < 167 supposed to mean? — user1907906
– user1907906, Commented Jul 21, 2014 at 9:27
You are comparing a list (i) with a number (167). So that while loop either never runs → no defining of n, or will be an endless loop. — BlackJack
– BlackJack, Commented Jul 21, 2014 at 9:29
I am trying to save the concatenated row into 'n'. For i=[], that was a mistake, I just want it to loop through 167 times. — covfefe
– covfefe, Commented Jul 21, 2014 at 9:32

user1907906 · Accepted Answer · 2014-07-21 09:37:47Z

1

Input i.csv:

1,2,3
foo,bar,baz
4,5,6
qux,quux.quuux

Python codce:

with open("i.csv") as f:
    reader = csv.reader(f)
    i = 0
    for row in reader:
        if i % 2 == 0:
            newRow = row
        else:
            newRow = newRow + row
            print(newRow)
        i = i + 1

Output:

['1', '2', '3', 'foo', 'bar', 'baz']
['4', '5', '6', 'qux', 'quux', 'quuux']

answered Jul 21, 2014 at 9:37

user1907906

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1 Comment

BlackJack Over a year ago

Not binding the open file to a name makes it impossible to close it after it's not needed anymore.

nvd · Accepted Answer · 2014-07-21 09:38:05Z

1

import csv

f = open("sample.csv",'rU').read().split("\n")
reader = csv.reader(f)

i = 0

for row in reader:
    if i % 2 == 0:
        line = row
    else:
        line = line + row
        print ", ".join(line)

    i += 1

edited Jul 21, 2014 at 9:38

user1907906

answered Jul 21, 2014 at 9:37

nvd

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1 Comment

nvd Over a year ago

First line number is even, second odd (i). You need even number of lines in the CSV file to make it work for all pairs.

Pablo Francisco Pérez Hidalgo · Accepted Answer · 2014-07-21 09:33:40Z

0

Writing while i<167: n = row.append([",".join(row[i]+row[i+1])]) is like writing:

while i<167:
    n = row.append([",".join(row[i]+row[i+1])])

So the scope of n is the loop block. Your print n is out of that scope thus raising NameError.

You could add n = None right before the while:

n = None
while i<167: n = row.append([",".join(row[i]+row[i+1])])
print n

Or move print n into the loop block:

while i<167:
    n = row.append([",".join(row[i]+row[i+1])])
    print n

Note that any of these changes will avoid your program interruption by the out of scope error but you will print a lot of lines containing None because append returns None: https://docs.python.org/2/tutorial/datastructures.html

edited Jul 21, 2014 at 9:33

answered Jul 21, 2014 at 9:28

Pablo Francisco Pérez Hidalgo

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5 Comments

user1907906 Over a year ago

What does row.append return?

BlackJack Over a year ago

@Tichodroma It returns None, so it doesn't make sense.

Pablo Francisco Pérez Hidalgo Over a year ago

@Tichodroma It returns None but that is not the origin of the exception. Just another problem with the OP code.

user1907906 Over a year ago

@BlackJack Right, that's why I am asking.

user1907906 Over a year ago

@PabloFranciscoPérezHidalgo So your answer removes the scope problem but still the OP doesn't have a working solution. Of course, that's because we don't know waht the OP is trying to do.

BlackJack · Accepted Answer · 2014-07-21 09:36:18Z

0

Here is a way to combine the line pairs:

import csv
from itertools import izip


def main():
    with open('sample.csv', 'rb') as input_file:
        reader = csv.reader(input_file)
        for even_row, odd_row in izip(reader, reader):
            combined_row = even_row + odd_row
            print combined_row


if __name__ == '__main__':
    main()

answered Jul 21, 2014 at 9:36

BlackJack

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