Time for action – reading and writing files

As an example of file I/O, we will create an identity matrix and store its contents in a file.

Note

In this and other chapters, we will use the following line by convention to import NumPy:

import numpy as np

Perform the following steps to do so:

  1. The identity matrix is a square matrix with ones on the main diagonal and zeros for the rest (see https://www.khanacademy.org/math/precalculus/precalc-matrices/zero-identity-matrix-tutorial/v/identity-matrix).

    The identity matrix can be created with the eye() function. The only argument that we need to give the eye() function is the number of ones. So, for instance, for a two-by-two matrix, write the following code:

    i2 = np.eye(2)
print(i2)

    The output is:

    [[ 1.  0.]
[ 0.  1.]]
  2. Save the data in a plain text file with the savetxt() function. Specify the name of the file that we want to save the data in and the array containing the data itself:
    np.savetxt("eye.txt", i2)

A file called eye.txt should have been created in the same directory as the Python script.

What just happened?

Reading and writing files is a necessary skill for data analysis. We wrote to a file with savetxt(). We made an identity matrix with the eye() function.

Note

Instead of a filename, we can also provide a file handle. A file handle is a term in many programming languages, which means a variable pointing to a file, like a postal address. For more information on how to get a file handle in Python, please refer to http://www.diveintopython3.net/files.html.

You can check for yourself whether the contents are as expected. The code for this example can be downloaded from the book support website: https://www.packtpub.com/books/content/support (see save.py)

import numpy as np

i2 = np.eye(2)
print(i2)

np.savetxt("eye.txt", i2))
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