Save and load NumPy array to CSV with custom delimiter

Last update on March 26 2025 13:12:27 (UTC/GMT +8 hours)

20. Array Append to List Conversion

Write a NumPy array to a CSV file with a custom delimiter and then read it back into a NumPy array using the same delimiter.

Sample Solution:

Python Code:

import numpy as np

# Create a NumPy array
array_to_save = np.array([[1, 2, 3], [4, 5, 6], [7, 8, 9]])

# Define the file name to save the CSV file
csv_file_name = 'array_data.csv'

# Save the NumPy array to a CSV file with a custom delimiter
np.savetxt(csv_file_name, array_to_save, delimiter=';', fmt='%d')

# Load the data back from the CSV file using the same delimiter
loaded_array = np.loadtxt(csv_file_name, delimiter=';')

# Print the loaded array to verify it matches the original array
print('Loaded array:\n', loaded_array)

Output:

Loaded array:
 [[1. 2. 3.]
 [4. 5. 6.]
 [7. 8. 9.]]

Explanation:

• Import Libraries:
• Imported numpy as np for handling arrays and file operations.
• Create NumPy Array:
• Created a 3x3 NumPy array named array_to_save.
• Define File Name:
• Set the CSV file name to array_data.csv.
• Save Array to CSV with Custom Delimiter:
• Used np.savetxt to save array_to_save to the CSV file, specifying ; as the custom delimiter and formatting the numbers as integers ('%d').
• Load Data from CSV with Same Delimiter:
• Used np.loadtxt to read the data from the CSV file into loaded_array, using ; as the delimiter.
• Print Loaded Array:
• Printed the loaded array to verify it matches the original array.

For more Practice: Solve these Related Problems:

• Write a Numpy program to convert a NumPy array to a list, append additional elements, and then convert the list back to an array verifying dimensions.
• Write a Numpy program to append one NumPy array to another via a Python list intermediary and then perform a vectorized addition on the result.
• Write a Numpy program to merge a Python list with a NumPy array by appending and then reshaping the combined structure into a matrix.
• Write a Numpy program to append a multidimensional NumPy array to a list and then flatten the list into a single-dimensional array for further operations.

Python-Numpy Code Editor:

Previous: Save a NumPy array as an image file using Python.