Is it possible to index a NumPy array with a boolean expression?

Question

Suppose I have the following array:

a = np.array([0,1,0],
             [1,0,0],
             [0,0,1])

Would it be possible to do something like the this:

a[==0] = -1

to set all zeros to -1? I know you could something like the following to achieve the same effect:

b = a == 0
a[b] = -1

but I was just wondering if there was a way to skip the creation of the Boolean array for this.

Not with boolean indexing, which by definition needs that boolean array. But alternatively : 2*a-1 for such an array of 1s and 0s. — Divakar
– Divakar, Commented Oct 31, 2017 at 9:20

ikkuh · Accepted Answer · 2017-10-31 09:23:49Z

1

Your syntax was almost correct. This seems to work:

a[a == 0] = -1

answered Oct 31, 2017 at 9:23

ikkuh

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Comments

Maarten Fabré · Accepted Answer · 2017-10-31 09:26:27Z

1

using np.where

np.where(a==0, -1, a)

answered Oct 31, 2017 at 9:26

Maarten Fabré

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