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

Question

Hey all,

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!

Answer 1

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])

Answer 2

To mutate it:

a[a<0] += 1

To leave the original array alone:

a+[a<0]