## 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...