How do I load specific rows from a .txt file in Python?

Question

Say I have a .txt file with many rows and columns of data and a list containing integer values. How would I load the row numbers in the text file which match the integers in the list?

To illustrate, say I have a list of integers:

a = [1,3,5]

How would I read only rows 1,3 and 5 from a text file into an array?

The loadtxt routine in numpy let's you both skip rows and use particular columns. But I can't seem to find a way to do something along the lines of (ignoring incorrect syntax):

new_array = np.loadtxt('data.txt', userows=a, unpack='true')

Thank you.

Answer 1

Just to expand on my comment

$ cat file.txt
line 0
line 1
line 2
line 3
line 4
line 5
line 6
line 7
line 8
line 9
line 10

Python:

#!/usr/bin/env python

a = [1, 4, 8]

with open('file.txt') as fd:
    for n, line in enumerate(fd):
        if n in a:
            print line.strip()

output:

$ ./l.py 
line 1
line 4
line 8

Answer 2

Given this file:

1,2,3
4,5,6
7,8,9
10,11,12
13,14,15
16,17,18
19,20,21

You can use the csv module to get the desired np array:

import csv
import numpy as np

desired=[1,3,5]
with open('/tmp/test.csv', 'r') as fin:
    reader=csv.reader(fin)
    result=[[int(s) for s in row] for i,row in enumerate(reader) if i in desired]

print(np.array(result))   

Prints:

[[ 4  5  6]
 [10 11 12]
 [16 17 18]]