# A little help with broadcasting

**URL:** <https://discourse.pymc.io/t/a-little-help-with-broadcasting/10844>\
**Category:** v5\
**Created:** [November 17, 2022, 5:30pm UTC](https://discourse.pymc.io/t/a-little-help-with-broadcasting/10844 "2022-11-17T17:30:33Z")\
**Posts on this page:** 4\
**Page:** 1

<div class="post-metadata">

**Author:** ![arkottke](https://yyz2.discourse-cdn.com/flex036/user_avatar/discourse.pymc.io/arkottke/32/398_2.png) [@arkottke](https://discourse.pymc.io/u/arkottke)\
**Post date:** [November 17, 2022, 5:30pm UTC](https://discourse.pymc.io/t/a-little-help-with-broadcasting/10844/1 "2022-11-17T17:30:33Z")

</div>

I have two distributions with different shapes (`eq` and `region`). I thought I could use broadcasting to combine them together to compute `n`. However, I couldn’t reshape them to create something that aesara thought was broadcastable. Eventually, I used the code below and it seems to have worked. Is there a better way to do this?

```auto
mag = pm.Normal(
        "mag",
        mu=events.loc[id_eq, "mag"].values,
        sigma=events.loc[id_eq, "mag_sd"].values,
        dims="eq",
    )
n_a = pm.LogNormal("n_a", mu=np.log(0.6884), sigma=0.05, dims="region")

foo = n_b * pm.math.tanh(mag - n_c)
n = at.stack([n_a[0] * foo, n_a[1] * foo])

```

---

<div class="post-metadata">

**Author:** ![jessegrabowski](https://yyz2.discourse-cdn.com/flex036/user_avatar/discourse.pymc.io/jessegrabowski/32/5010_2.png) [@jessegrabowski](https://discourse.pymc.io/u/jessegrabowski)\
**Post date:** [November 17, 2022, 5:52pm UTC](https://discourse.pymc.io/t/a-little-help-with-broadcasting/10844/2 "2022-11-17T17:52:15Z")

</div>

What is the shape of your desired output? Scalar operations like multiplication are not defined between two vectors of different shapes. Calling the shape of `n_a` as `(n, 1)` and the shape of `foo` as `(m, 1)`, it looks like you are taking the outer product so that `n` has shape `(n, m)`, then raveling it into a single `(n * m, 1)` vector.

You could do this using broadcasting by adding extra dimensions to your variables, i.e. `n_a[:, None] * foo[None, :]` will have shape `(n, m)`. You can also use `at.outer` to get the same result without having to think about the indexing.

---

<div class="post-metadata">

**Author:** ![arkottke](https://yyz2.discourse-cdn.com/flex036/user_avatar/discourse.pymc.io/arkottke/32/398_2.png) [@arkottke](https://discourse.pymc.io/u/arkottke)\
**Post date:** [November 17, 2022, 6:11pm UTC](https://discourse.pymc.io/t/a-little-help-with-broadcasting/10844/3 "2022-11-17T18:11:08Z")

</div>

The intended shape would be [region, eq] in size. Thanks! Both of those solutions work. I had previously tried to use `.reshape(1, n_region)` to change the shape, which I thought was consistent with `[None, :]`.

---

<div class="post-metadata">

**Author:** ![jessegrabowski](https://yyz2.discourse-cdn.com/flex036/user_avatar/discourse.pymc.io/jessegrabowski/32/5010_2.png) [@jessegrabowski](https://discourse.pymc.io/u/jessegrabowski)\
**Post date:** [November 17, 2022, 6:14pm UTC](https://discourse.pymc.io/t/a-little-help-with-broadcasting/10844/4 "2022-11-17T18:14:20Z")

</div>

Reshape will add a dimension in Aesara, but only if you give the shape as a tuple. So `.reshape(1, -1)` doesn’t work, but `.reshape((1, -1))` does. This is a rare difference between aesara and numpy syntax.