How to read the first row of an array in Python

Question

I am new to learning Python, here is my current code:

#!/usr/bin/python

l = []
with open('datad.dat', 'r') as f:
  for line in f:
    line = line.strip()
    if len(line) > 0:
      l.append(map(float, line.split()))
print l[:,1]

I attempted to do this but made the mistake of using FORTRAN syntax, and received the following error:

  File "r1.py", line 9, in <module>
print l[:,1]

TypeError: list indices must be integers, not tuple

How would I go about getting the first row or column of an array?

If you need to do a lot of numeric computation, look into numpy. It's an extremely powerful tool. — user2357112
– user2357112, Commented Apr 22, 2015 at 9:50

Padraic Cunningham · Accepted Answer · 2015-04-22 09:51:23Z

6

To print the first row use l[0], to get columns you will need to transpose with zip print(list(zip(*l))[0]).

In [14]: l = [[1,2,3],[4,5,6],[7,8,9]]  
In [15]: l[0] # first row
Out[15]: [1, 2, 3]    
In [16]: l[1] # second row
Out[16]: [4, 5, 6]   
In [17]: l[2] # third row
Out[17]: [7, 8, 9]    
In [18]: t =  list(zip(*l)) 
In [19]  t[0] # first column
Out[19]: (1, 4, 7)    
In [20]: t[1] # second column
Out20]: (2, 5, 8)   
In [21]: t[2] # third column
Out[21]: (3, 6, 9)

The csv module may also be useful:

import csv

with open('datad.dat', 'r') as f:
   reader = csv.reader(f)
   l = [map(float, row) for row  in reader]

edited Apr 22, 2015 at 9:51

answered Apr 22, 2015 at 9:34

Padraic Cunningham

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2 Comments

user2357112 Over a year ago

Performing a full transpose every time you want to select a column would be wasteful, though. If you only need one column, a list comprehension like [row[0] for row in l] would be better; if you need to select columns repeatedly, it would be a good idea to store the transposed form.

Padraic Cunningham Over a year ago

@user2357112, it is just an example, I was not actually recommending transposing every time you want a column ,edited.

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