How do I do conditional array arithmetic on a numpy array?

Question

I'm trying to get a better grip on numpy arrays, so I have a sample question to ask about them:

Say I have a numpy array called a. I want to perform an operation on a that increments all the values inside it that are less than 0 and leaves the rest alone. for example, if I had:

a = np.array([1,2,3,-1,-2,-3])

I would want to return:

([1,2,3,0,-1,-2])

What is the most compact syntax for this?

Thanks!

for a readable tutorial, see scipy.org/Cookbook/Indexing

denis
– denis

2010-08-05 11:41:32 +00:00
Commented Aug 5, 2010 at 11:41 — denis
– denis, Commented Aug 5, 2010 at 11:41

unutbu · Accepted Answer · 2010-08-04 20:01:25Z

41

In [45]: a = np.array([1,2,3,-1,-2,-3])

In [46]: a[a<0]+=1

In [47]: a
Out[47]: array([ 1,  2,  3,  0, -1, -2])

answered Aug 4, 2010 at 20:01

unutbu

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John Kugelman · Accepted Answer · 2024-03-27 00:31:01Z

12

To mutate it:

a[a<0] += 1

To leave the original array alone:

a + (a<0)

edited Mar 27, 2024 at 0:31

answered Aug 4, 2010 at 20:05

John Kugelman

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