How to iterate over a numpy array, getting two values per loop?

Question

I envision something like

import numpy as np
x = np.arange(10)
for i, j in x:
     print(i,j)

and get something like

But I get this traceback:

Traceback (most recent call last):
  File "/home/andreas/.local/share/JetBrains/Toolbox/apps/PyCharm-P/ch-0/223.8214.51/plugins/python/helpers/pydev/pydevconsole.py", line 364, in runcode
    coro = func()
  File "<input>", line 1, in <module>
TypeError: cannot unpack non-iterable numpy.int64 object

I also tried to use np.nditer(x) and itertools with zip(x[::2], x[1::2]), but that does not work either, with different error messages.

This should be super simple, but I can't find solutions online.

What is the purpose of the iteration? There is likely a more efficient (numpy-esque) approach. — s3dev
– s3dev, Commented Jan 14, 2023 at 13:02

WingedSeal · Accepted Answer · 2023-01-15 13:27:17Z

4

You were trying to put 0 into i and j which is not possible. To achieve that result you'll have to reshape your numpy array using either x = x.reshape((5,2)) or x.shape = 5, 2. Then you can unpack it like that.

To visualize, this is what your current code is doing:

i, j = 0
...
i, j = 1
...

And this is what will happen if you reshape it:

i, j = [0, 1]
...
i, j = [2, 3]
...

Edit:

import numpy as np
N = 10

x = np.arange(N).reshape((N/2, 2))
for i, j in x:
     print(i,j)

edited Jan 15, 2023 at 13:27

answered Jan 14, 2023 at 13:07

WingedSeal

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4 Comments

Andreas Schuldei Over a year ago

oh, i understand, thank you for explaining it in simple terms!

Andreas Schuldei Over a year ago

your solution is best so far. if you would please provide a snippet, so i can accept your answer. @wingedseal

WingedSeal Over a year ago

I'm on mobile and won't be able to access my laptop for quite a while

Andreas Schuldei Over a year ago

I coded my solution based on your suggestion and am in no hurry. Just do it later.

cards · Accepted Answer · 2023-01-14 13:27:48Z

0

Trying to be faithful to the original attempt. zipping tuples of pairs of even and odds:

import numpy as np
x = np.arange(10)

for i, j in zip(x[::2], x[1::2]):
     print(i,j)

answered Jan 14, 2023 at 13:27

cards

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2 Comments

Andreas Schuldei Over a year ago

Does that run for you? I tried that (as mentioned) and got an error.

cards Over a year ago

yes, running numpy 1.23, test it also here

Colin Kwong · Accepted Answer · 2023-01-14 13:33:25Z

0

Is it what you want?

for x in range(0,10,2):
    print(x, x+1)

or your values are maintained in the numpy array x, then

for i in range(0, len(x), 2):
    print(x[i],x[i+1])

edited Jan 14, 2023 at 13:33

answered Jan 14, 2023 at 13:23

Colin Kwong

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