np.loadtxt() How to load every other line from txt file? Python

Question

I have a txt file of data that I only want to load in the even lines from.

Is there a way to do this in python without using loops?

Here are the first 10 lines of my data file:

1 25544U 98067A   98324.28472222 -.00003657  11563-4  00000+0 0    10
2 25544  51.5908 168.3788 0125362  86.4185 359.7454 16.05064833    05
1 25544U 98067A   98324.33235038  .11839616  11568-4  57349-2 0    28
2 25544  51.6173 168.1099 0123410  88.0187 273.4932 16.04971811    11
1 25544U 98067A   98324.45674522 -.00043259  11566-4 -18040-4 0    32
2 25544  51.5914 167.4317 0125858  91.3429 269.4598 16.05134416    30
1 25544U 98067A   98324.51913017  .00713053  11562-4  34316-3 0    48
2 25544  51.5959 167.1152 0123861  87.8179 273.5890 16.05002967    44
1 25544U 98067A   98324.51913017  .00713053  11562-4  34316-3 0    59
2 25544  51.5959 167.1152 0123861  87.8179 273.5890 16.05002967    44

atru · Answer 1 · 2017-10-05T21:27:32.953

One way to do it is to use a counter and modulo operator:

fname = 'load_even.txt'

data = [];
cnt = 1;
with open(fname, 'r') as infile:
    for line in infile:
        if cnt%2 == 0:
            data.append(line)
        cnt+=1

This reads the file line by line, increasing the counter cnt after each line, and appending that line to data only if the counter value is even which in this case corresponds to an even line number.

For a particular case of numpy array you can use this:

import numpy as np

fname = 'load_even.txt'

data = [];
cnt = 1;
with open(fname, 'r') as infile:
    for line in infile:
        if cnt%2 == 0:
            data.append(line.split())
        cnt+=1

data = np.asarray(data, dtype = float)

ezgoodey · Accepted Answer · 2017-10-05T21:44:19.693

The np.loadtxt() doesn't have the ability to skip lines unless they are the first N lines. Otherwise you will want to use np.genfromtxt():

with open(filename) as f:
    iter = (line for line in f if is_even_line(line))
    data = np.genfromtxt(iter)

where is_even_line() is a function that returns a boolean, if the given line is even. In your case, since the first column indicates whether the line is odd or even, is_even_line() could look like this:

def is_even_line(line):
    return line[0] == '2'

score 0 · Answer 3 · answered Oct 23 '17 at 15:03

0

Here's how I ended up doing it:

import numpy as np
import matplotlib.pyplot as plt


filename = 'zarya2000data.txt'
a = np.genfromtxt(filename)

evens = []
odds = []
N = 8 #30 #5826 #number of lines


for i in range(N):   #2913*2
    if np.mod(i,2) == 0:
        evens.append(a[i,:])
    else:
        odds.append(a[i,:])

oddsArray = np.asarray(odds)
evensArray = np.asarray(evens)

print 'evensArray', evensArray
print''
print 'oddsArray', oddsArray  
print ''

answered Oct 23 '17 at 15:03

Rose

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1
7
20

How about simply this : `evens = a[0::2]` and `odds = a[1::2]` , that would be the proper way and should also be faster. – jadsq Oct 23 '17 at 15:23

np.loadtxt() How to load every other line from txt file? Python

3 Answers3