# How to I join arrays that are in a list or from a generator?

**URL:** <https://numba.discourse.group/t/how-to-i-join-arrays-that-are-in-a-list-or-from-a-generator/1798>\
**Category:** Support: How do I do ...?\
**Created:** [February 24, 2023, 10:31pm UTC](https://numba.discourse.group/t/how-to-i-join-arrays-that-are-in-a-list-or-from-a-generator/1798 "2023-02-24T22:31:15Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![patrick](https://avatars.discourse-cdn.com/v4/letter/p/b2d939/32.png) [@patrick](https://numba.discourse.group/u/patrick)\
**Post date:** [February 24, 2023, 10:31pm UTC](https://numba.discourse.group/t/how-to-i-join-arrays-that-are-in-a-list-or-from-a-generator/1798/1 "2023-02-24T22:31:15Z")

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I am having trouble neatly joining numpy arrays that are in a list or from a generator in numba jited functions. The simplest example is as follows:

```auto
import numpy as np
from numba import njit

@njit
def my_program():
    array_1 = np.array([0,3])
    array_2 = np.array([0,4,2,3])
    array_3 = np.array([9,1,3,3,5,9])
    
    list_of_arrays = [array_1, array_2, array_3]
    
    return join_arrays(list_of_arrays)

my_program()

```

Where `join_arrays` should return a 1-D numpy array like `np.array([0,3,0,4,2,3,9,1,3,3,5,9])`.

I have tried:

```auto

@njit
def join_arrays_a(list_of_arrays):
    return np.hstack(list_of_arrays)

@njit
def join_arrays_b(list_of_arrays):
    return np.hstack(tuple(list_of_arrays))

@njit
def join_arrays_c(arrays):
    tot_len = sum(list(map(len, arrays)))
    new_array = np.zeros(shape=(tot_len,), dtype=np.int64)
    prev_array_lens = 0
    for array in arrays:
        for i in range(len(array)):
            new_array[prev_array_lens+i] = array[i]
        prev_array_lens += len(array)
    return new_array

```

`join_arrays_c` works but I would hope that there was a better way to do this, or some utility functions somewhere so that I do not have to write so much code like this. Am I missing something?  
`join_arrays_a` and `join_arrays_b` don’t work when used in the `my_program` instead of `join_arrays_c`.

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**Author:** ![nelson2005](https://yyz2.discourse-cdn.com/free1/user_avatar/numba.discourse.group/nelson2005/32/47_2.png) [@nelson2005](https://numba.discourse.group/u/nelson2005)\
**Post date:** [February 25, 2023, 12:17am UTC](https://numba.discourse.group/t/how-to-i-join-arrays-that-are-in-a-list-or-from-a-generator/1798/2 "2023-02-25T00:17:51Z")

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Is this the real problem? Numba seems like an odd solution here and likely won’t add much speed (if any)

It looks like you’re trying to re-implement [numpy,concatenate](https://numpy.org/doc/stable/reference/generated/numpy.concatenate.html), which seems like the right tool for this job.

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<div class="post-metadata">

**Author:** ![patrick](https://avatars.discourse-cdn.com/v4/letter/p/b2d939/32.png) [@patrick](https://numba.discourse.group/u/patrick)\
**Post date:** [February 25, 2023, 2:23am UTC](https://numba.discourse.group/t/how-to-i-join-arrays-that-are-in-a-list-or-from-a-generator/1798/3 "2023-02-25T02:23:19Z")

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Hi Nelson, much like `hstack`, `numpy.concatenate` has the same restriction that the arrays must be in a tuple when called within a jitted function and so does not solve this problem. And indeed, I am not expecting numba to speed up this function when there are only a small number of arrays, but for a large number I would think that this would be much slower than avoiding any python object creation.

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<div class="post-metadata">

**Author:** ![nelson2005](https://yyz2.discourse-cdn.com/free1/user_avatar/numba.discourse.group/nelson2005/32/47_2.png) [@nelson2005](https://numba.discourse.group/u/nelson2005)\
**Post date:** [February 25, 2023, 7:51pm UTC](https://numba.discourse.group/t/how-to-i-join-arrays-that-are-in-a-list-or-from-a-generator/1798/4 "2023-02-25T19:51:57Z")

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Thanks for the clarification. I don’t know of anything better than what you’ve demonstrated