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# Indexing with Booleans

Boolean indexing is indexing based on a boolean array and falls under the category of fancy indexing.

## How to do it...

We will apply this indexing technique to an image:

1. Image with dots on the diagonal.

This is in some way similar to the Fancy indexing recipe in this chapter. This time, we select modulo `4` points on the diagonal of the image:

```def get_indices(size):
arr = np.arange(size)
return arr % 4 == 0```

Then we just apply this selection and plot the points:

```lena1 = lena.copy()
xindices = get_indices(lena.shape[0])
yindices = get_indices(lena.shape[1])
lena1[xindices, yindices] = 0
plt.subplot(211)
plt.imshow(lena1)```
2. Select the array values between quarter and three quarters of the maximum value, and set them to `0`:
`lena2[(lena > lena.max()/4) ...`

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