CSV Columns to Arrays in Python

Question

I'm having a hard time dealing with what seems to me to be a simple problem. I'm trying to import a csv and split its columns into arrays that I can run different operations on, then zip() back together.

import csv

data = csv.reader(open('test.csv', 'rb'), delimiter=",", quotechar='|')
column1, column2 = [], []

for row in data:
    column1.extend(row[0])
    column2.extend(row[1])

print column1
print column2

This code prints two arrays with elements that are individual chars rather than strings. When I try to do this with a single column, column1.extend(row) does what I want.

I'm interested in ways to solve this particular problem or to generalize this to n number of columns.

Nolen Royalty · Accepted Answer · 2012-04-04 22:57:25Z

6

You need to change column1.extend(row[0]) to column1.append(row[0]) (and the same for column2, clearly). Extend is for adding the contents of one list to another, append is for adding a single element. Extend is telling python to treat the string as a list of its chars and append each char.

>>> lst = []
>>> lst.extend("foo")
>>> lst
['f', 'o', 'o']
>>> lst.append("foo")
>>> lst
['f', 'o', 'o', 'foo']

answered Apr 4, 2012 at 22:57

Nolen Royalty

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Comments

tumultous_rooster · Accepted Answer · 2016-03-09 19:24:18Z

3

another option:

column1 = [row[0] for row in data]

edited Mar 9, 2016 at 19:24

tumultous_rooster

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answered Mar 9, 2016 at 14:05

Aldur

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Comments

Pico · Accepted Answer · 2012-04-04 22:59:02Z

1

column1 and column2 are lists. The method you want to use is append() not extend().

Check the official documentation: http://docs.python.org/tutorial/datastructures.html

answered Apr 4, 2012 at 22:59

Pico

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Comments

C2H5OH · Accepted Answer · 2012-04-04 22:59:33Z

1

Each row index is a string, so if you want to add it to your columns you either do this:

column1.append(row[0])

or this:

column1.extend([row[0]])

answered Apr 4, 2012 at 22:59

C2H5OH

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