Select {#dev_guide_op_select}
=============================

## General

The select operation returns a tensor filled with the elements from the second or
the third input, depending on the condition (the first input) value.

    dst[i] = cond[i] ? src_0[i] : src_1[i]

Broadcasting is supported.

### Broadcasting rules

If the auto_broadcast attribute is not none, the select operation takes a
two-step broadcast before performing the selection:

* **Step 1**: Input tensors src_0 and src_1 are broadcasted to dst_shape
  according to the Numpy [broadcast rules](https://numpy.org/doc/stable/user/basics.broadcasting.html).

* **Step 2**: Then, the cond tensor will be one-way broadcasted to the
  dst_shape of broadcasted src_0 and src_1. To be more specific, we align the
  two shapes to the right and compare them from right to left. Each dimension
  should be either equal or the dimension of cond should be 1.

* **example**:

  * cond={4, 5}, dst_shape={2, 3, 4, 5} => dst = {2, 3, 4, 5}
  * cond={3, 1, 5}, dst_shape={2, 3, 4, 5} => dst = {2, 3, 4, 5}
  * cond={3,5}, dst_shape={2, 3, 4, 5} => dst = invalid_shape

## Operation attributes

| Attribute Name                                               | Description                                                | Value Type | Supported Values           | Required or Optional |
|:-------------------------------------------------------------|:-----------------------------------------------------------|:-----------|:---------------------------|:---------------------|
| [auto_broadcast](@ref dnnl::graph::op::attr::auto_broadcast) | Specifies rules used for auto-broadcasting of src tensors. | string     | `none`, `numpy` (default)  | Optional             |

## Execution arguments

The inputs and outputs must be provided according to the following index order when
constructing an operation.

### Inputs

| Index | Argument Name | Required or Optional |
|:------|:--------------|:---------------------|
| 0     | `cond`        | Required             |
| 1     | `src_0`       | Required             |
| 2     | `src_1`       | Required             |

@note All input shapes should match and no broadcasting is allowed if the
`auto_broadcast` attribute is set to `none`, or can be broadcasted according to the
broadcasting rules mentioned above if `auto_broadcast` attribute is set to `numpy`.

### Outputs

| Index | Argument Name | Required or Optional |
|:------|:--------------|:---------------------|
| 0     | `dst`         | Required             |

## Supported data types

Select operation supports the following data type combinations.

| Cond    | Src_0 / Src_1 | Dst  |
|:--------|:--------------|:-----|
| boolean | f32           | f32  |
| boolean | bf16          | bf16 |
| boolean | f16           | f16  |