How to write a text file into a numpy array or list?

Create a list or array from a text file in Python

Here’s our task: We have a text file containing numerical data. We would like to read the file contents into a Numpy array and a Python list. Use the np.loadtxt() function to write your text into an array and the file object read() function to populate a Python list

#1 Data Preparation

Assume that you have a text file that contains the following comma delimited values:

0,1,2,3,4,32,4,2,4,62,1

Here’s is the code in order to create the file:

numbers = "0,1,2,3.5,4.5,32.1,4,2.2,4,62,1"

file_path =r'C:\WorkDir\numbers.txt'

with open (file_path, 'w') as my_file:
     my_file.write(numbers)
        
  

Aside: Note that you’ll need to convert ensure that the numbers variable is a string and not a tuple or list here as other wise you will receive a type error:

TypeError: write() argument must be str, not list 
# or tuple, or int if passed to the file object write function

Related: How to read a list into a text file with Python

#2 Read text file into Numpy array

We can use the numpy loadtxt() method in order to read a text or comma separated csv file into an ndarray object. Remember to import the numpy library into your namespace before invoking np.loadtxt().

import numpy as np
numbers_array = np.loadtxt(r'C:\WorkDir\numbers.txt', delimiter=',')

We can then easily look into the array contents:

print(numbers_array)

This will return the following ndarray object:

[ 0.   1.   2.   3.5  4.5 32.1  4.   2.2  4.  62.   1. ]

Note: unless a delimiter is specified you will get a value error:

ValueError: could not convert string to float

#3 Read csv or txt into list

Similarly, we can also add the contents of a file into a Python:

with open (file_path, 'r') as my_file:
    numbers_lst = my_file.read().split(',')

print(numbers_lst)

This will result in a Python list of strings. We can use a simple list comprehension to convert it to integers.

['0', '1', '2', '3.5', '4.5', '32.1', '4', '2.2', '4', '62', '1']

Explanation: We first open the text or csv file for read only, we then use the read() function to add the content of the file into a string object. Last, because the values in our file are delimited by commas, we use the string split method to populate our list.

Note: use the readlines() function to read each row in your file into a list element, and the readline() function in order to read each row in your file line by line.