# List of different sized arrays as a parameter

**URL:** <https://numba.discourse.group/t/list-of-different-sized-arrays-as-a-parameter/58>\
**Category:** Community Support\
**Created:** [June 18, 2020, 6:59pm UTC](https://numba.discourse.group/t/list-of-different-sized-arrays-as-a-parameter/58 "2020-06-18T18:59:16Z")\
**Posts on this page:** 6\
**Page:** 1

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**Author:** ![randompast](https://yyz2.discourse-cdn.com/free1/user_avatar/numba.discourse.group/randompast/32/44_2.png) [@randompast](https://numba.discourse.group/u/randompast)\
**Post date:** [June 18, 2020, 6:59pm UTC](https://numba.discourse.group/t/list-of-different-sized-arrays-as-a-parameter/58/1 "2020-06-18T18:59:16Z")

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I created a list of numpy arrays of different sizes: k = [1, np.ones(2), np.ones((2,2)), np.ones((2,2,2)), …]. @njit understandably doesn’t like this but I’d like to be able to pass this as an argument. Is there pattern for this? One messy approach might be to flatten and concatenate these and rewrite everything in terms of slices or resize them after passing the 1D array.

```
@njit
def f(k):
    pass

@njit
def g(n):
    k = [np.ones([2]*i) for i in range(n) ]
    f(k)
g(6)

```

`TypeError: can't unbox heterogeneous list: array(float64, 1d, C) != array(float64, 2d, C)`

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**Author:** ![sklam](https://yyz2.discourse-cdn.com/free1/user_avatar/numba.discourse.group/sklam/32/9_2.png) [@sklam](https://numba.discourse.group/u/sklam)\
**Post date:** [June 18, 2020, 7:19pm UTC](https://numba.discourse.group/t/list-of-different-sized-arrays-as-a-parameter/58/2 "2020-06-18T19:19:37Z")

</div>

This is hitting several limitations of Numba:

- Numba treats arrays with difference number of dimensions as different types.
- The list only support homogeneous types; i.e. `[1d_array, 2d_array]` would be heterogeneous.
- Variables need to be type-stable; i.e `for x in iterable` cannot have `x` being multiple types (can’t be 1d\_array and 2d\_array in different iteration).

When you have dynamic types like that, the only solution I can think of is to not use `@njit` on `g()`. Instead write:

```python
@njit
def f(k):
    ....

def g(n):
   for i in range(n):
       k = np.ones([2]*i)
       f(k)

g(6)

```

That way, `f()` is specialized on the different argument types.

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<div class="post-metadata">

**Author:** ![randompast](https://yyz2.discourse-cdn.com/free1/user_avatar/numba.discourse.group/randompast/32/44_2.png) [@randompast](https://numba.discourse.group/u/randompast)\
**Post date:** [June 18, 2020, 7:38pm UTC](https://numba.discourse.group/t/list-of-different-sized-arrays-as-a-parameter/58/3 "2020-06-18T19:38:26Z")

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Thanks for elaborating on what was causing the issue. The problem is that I need to pass this list of k values since it’s the kernel for a multidimensional convolution. It’s a [volterra series operator](https://en.wikipedia.org/wiki/Volterra_series). The basis of that is that I have something like this k[0] + conv\_1d1o(k[1], x) + conv\_1d2o(k[2], x, x) + … Maybe I could generalize [these convolutions](https://github.com/randompast/python-convolution-comparisons) better. With your suggested approach I’d need to continually recalculate k every time I were to use it. Additionally, I’d like to have @njit applied to g(n). Another option is to pass each one separately as an argument f(k[0], k[1], …) for every function that I pass k to and just limit the generalization to maybe 3 or so k’s. Each function in f has @njit applied, so maybe that’s the best I could have hoped for.

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<div class="post-metadata">

**Author:** ![luk-f-a](https://yyz2.discourse-cdn.com/free1/user_avatar/numba.discourse.group/luk-f-a/32/56_2.png) [@luk-f-a](https://numba.discourse.group/u/luk-f-a)\
**Post date:** [June 18, 2020, 8:21pm UTC](https://numba.discourse.group/t/list-of-different-sized-arrays-as-a-parameter/58/4 "2020-06-18T20:21:40Z")

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if you can create `k` as a tuple of arrays in interpreted code, and pass it to `g()` already as a tuple you can write

```auto
def g(ks):
   for k in literal_unroll(ks):
       f(k)

```

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<div class="post-metadata">

**Author:** ![sklam](https://yyz2.discourse-cdn.com/free1/user_avatar/numba.discourse.group/sklam/32/9_2.png) [@sklam](https://numba.discourse.group/u/sklam)\
**Post date:** [June 18, 2020, 8:59pm UTC](https://numba.discourse.group/t/list-of-different-sized-arrays-as-a-parameter/58/5 "2020-06-18T20:59:35Z")

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Not sure if this would work for your case, but maybe the arrays can be created to have the maximum number of dimension by padding the leading dimension with ones. For example, if max ndim is 3, 1d array would have shape of `(1, 1, N)` and 2d array would have shape of `(1, M, N)`.

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<div class="post-metadata">

**Author:** ![luk-f-a](https://yyz2.discourse-cdn.com/free1/user_avatar/numba.discourse.group/luk-f-a/32/56_2.png) [@luk-f-a](https://numba.discourse.group/u/luk-f-a)\
**Post date:** [June 18, 2020, 10:11pm UTC](https://numba.discourse.group/t/list-of-different-sized-arrays-as-a-parameter/58/6 "2020-06-18T22:11:00Z")

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@sklam’s solution above is probably the best.