Elementwise insertion of array into array using numpy

Question

Let's say I have an array of years created like

years = np.arange(2024,2051,1)

And I have an array of months created like

months = np.arange(1,13,1)

What is the most pythonic way of arriving at an elementwise concatenation such that the end result is

[[2024,1],[2024,2],[2024,3]...[2024,12], [2025,1],[2025,2],[2025,3]...[2025,12], ... ]

?

Thanks

Your subject line talks about 'insertion', while question asks about 'concatenate'. Have you considered making np.zeros((n.m,2)) array, and assigning the arrays to it? That's a kind of insertion. — hpaulj
– hpaulj, Commented Mar 16, 2024 at 18:27

Chrysophylaxs · Accepted Answer · 2024-03-16 09:35:21Z

2

I'd say the most pythonic is:

out = [[year, month] for year in years for month in months]

There is also itertools.product, but it will give you a list of tuples.

from itertools import product
out = list(product(years, months))

If you want to be full numpy, you can take advantage of broadcasting.

import numpy as np
out = np.dstack(np.broadcast_arrays(*np.ix_(years, months)))

np.ix_ reshapes the input arrays to be (n_years, 1), (1, n_months)
np.broadcast_arrays creates views of (n_years, n_months), (n_years, n_months)
np.dstack concatenates the above such that you get a (n_years, n_months, 2) array.

Edit:

I just realised that np.ix_ + np.broadcast_arrays is more or less a np.meshgrid:

out = np.dstack(np.meshgrid(years, months, indexing="ij"))

edited Mar 16, 2024 at 9:35

answered Mar 16, 2024 at 9:29

Chrysophylaxs

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1 Comment

hpaulj Over a year ago

In a recent answer to a similar question, I found that the best (fastest) use of broadcasting was to assign the arrays to a predefined zeros array. stackoverflow.com/a/78110268/901925

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