# PyMC3 elementwise if condition
**URL:** https://discourse.pymc.io/t/pymc3-elementwise-if-condition/8670
**Category:** Questions
**Tags:** aesara
**Created:** [January 27, 2022, 8:48am UTC](https://discourse.pymc.io/t/pymc3-elementwise-if-condition/8670 "2022-01-27T08:48:19Z")
**Posts on this page:** 7
**Page:** 1
### Author:  [@michnag](https://discourse.pymc.io/u/michnag)
#### Post date: [January 27, 2022, 8:48am UTC](https://discourse.pymc.io/t/pymc3-elementwise-if-condition/8670/1 "2022-01-27T08:48:19Z")
I have the following model, where I have a feature space `X` of shape (NxTxK) which should be multiplied elementwise by `alpha` or `beta` depending on the sign of the value in `phi` which is also (NxTxK). The parameters `alpha` and `beta` are drawn from a prior distribution.
With numpy, the syntax would be: `X = np.where(phi == 1, X*alpha, X*beta)`.
So far I used the following code which however gave me an error message.
```auto
with pm.Model() as model:
alpha = pm.Beta(name="alpha", alpha=0.5, beta=0.5)
beta = pm.Beta(name="beta", alpha=0.5, beta=0.5)
X= pm.Deterministic(name="X", var=pm.math.where(pm.math.eq(phi, 1), X* alpha, X * beta))
b= pm.Normal(name="b", mu=0, sigma=1, shape=n_vars_1)
eps = pm.InverseGamma(name="eps", alpha=9.0, beta=4.0)
y_hat = pm.Deterministic(name="y_hat", var=pm.math.sum(X * b, axis=2)
y_like = pm.Normal(name="y_like", mu=y_hat, sd=eps, observed=y)
```
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### Author:  [@ricardoV94](https://discourse.pymc.io/u/ricardoV94)
#### Post date: [January 27, 2022, 12:04pm UTC](https://discourse.pymc.io/t/pymc3-elementwise-if-condition/8670/2 "2022-01-27T12:04:24Z")
What is the error message you are getting?
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### Author:  [@michnag](https://discourse.pymc.io/u/michnag)
#### Post date: [January 27, 2022, 12:25pm UTC](https://discourse.pymc.io/t/pymc3-elementwise-if-condition/8670/3 "2022-01-27T12:25:35Z")
TypeError: Condition given to the op has to be a scalar with 0 standing for False, anything else for True
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### Author:  [@junpenglao](https://discourse.pymc.io/u/junpenglao)
#### Post date: [January 27, 2022, 2:53pm UTC](https://discourse.pymc.io/t/pymc3-elementwise-if-condition/8670/4 "2022-01-27T14:53:03Z")
Try `aesara.tensor.switch`
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### Author:  [@ricardoV94](https://discourse.pymc.io/u/ricardoV94)
#### Post date: [January 27, 2022, 8:21pm UTC](https://discourse.pymc.io/t/pymc3-elementwise-if-condition/8670/5 "2022-01-27T20:21:56Z")
`where` is a provided alias to `switch` so it should have worked. Just to make sure check with `theano.tensor.switch` or `aesara.tensor.switch` depending on which version you are using.
What is `phi`?
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### Author:  [@michnag](https://discourse.pymc.io/u/michnag)
#### Post date: [January 27, 2022, 9:12pm UTC](https://discourse.pymc.io/t/pymc3-elementwise-if-condition/8670/6 "2022-01-27T21:12:10Z")
Phi is a TensorVariable that is drawn from a Bernoulli.
If I determine phi before initializing the model as a numpy array the code works.
Could it be that `where` or `switch`does not work for TensorVariables?
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### Author:  [@ricardoV94](https://discourse.pymc.io/u/ricardoV94)
#### Post date: [January 27, 2022, 9:25pm UTC](https://discourse.pymc.io/t/pymc3-elementwise-if-condition/8670/7 "2022-01-27T21:25:47Z")
Those should definitely work with TensorVariables. Can you show the complete code, including the creation of `Phi`?