python: creating numpy nonzero index, value pair

Question

I can get the index of non-zero numpy arrays as follows:

a = np.array([0., 1., 0., 2.])
i = np.nonzero(a)

This returns (array([1, 3]),). I can get the corresponding values as:

v = a[i]

Now what I would like to do is create a list with each of the (index, value) tuples. I guess one way to do so is to write a for loop as follows;

l = list()
for x in range(0, len(v)):
    l.append((i[0][x], v[x]))

However, I was wondering if there is a better way to do this without writing the loop.

Sorry, that was a type from me. I was doing something in ipython and messed up the variable names during copying. Thanks for pointing it out. Apologies. — Luca
– Luca, Commented Jul 22, 2015 at 14:19

loopbackbee · Accepted Answer · 2015-07-22 14:25:36Z

2

If you really want tuples, you can get them with:

indexes= np.where(a!=0)[0]   #or np.nonzero(a)[0]
values=a[indexes]
zip(values, indexes)         #or list(zip(values, indexes)) in python 3, if you need to access more than once

answered Jul 22, 2015 at 14:25

loopbackbee

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wiseveri · Accepted Answer · 2015-07-24 07:30:06Z

1

You could use a list comprehension

l = [(x, v[x]) for x in i[0]]

or use zip as the other answers suggest

edited Jul 24, 2015 at 7:30

answered Jul 22, 2015 at 14:30

wiseveri

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3 Comments

loopbackbee Over a year ago

this is somewhat slower than zip, but it's arguably more readable. The order inside the tuple is swapped compared to the example

Luca Over a year ago

I do not think the order is swapped. Here v is iterating over the indexes. I think the confusion is due to the fact that v is something different here than my original example.

wiseveri Over a year ago

Sorry for that, I changed the variable names to better reflect the original usage in the OP.

Anand S Kumar · Accepted Answer · 2015-07-22 14:27:04Z

0

I am not sure if numpy has a library to do that, but you can do this using zip() function. Example -

>>> i = [1,2,3,4]
>>> v = [5,6,7,8]
>>> list(zip(i,v))
[(1, 5), (2, 6), (3, 7), (4, 8)]

You do not need list() for Python 2.x , as zip returns a list of tuples in Python 2.x .

For your case, you can try

answered Jul 22, 2015 at 14:27

Anand S Kumar

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