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# output_descriptor

Descriptors of the output, hidden output, and cell-state output data.

```
var output_descriptor: BNNSLSTMDataDescriptor
```

## Discussion

You must define the layout of this descriptor as one of either:

- [`BNNSDataLayoutSNE`](/documentation/Accelerate/BNNSDataLayoutSNE) with the shape (num_directions*[`hidden_size`](/documentation/Accelerate/BNNSLayerParametersLSTM/hidden_size),[`batch_size`](/documentation/Accelerate/BNNSLayerParametersLSTM/batch_size),[`seq_len`](/documentation/Accelerate/BNNSLayerParametersLSTM/seq_len))
- [`BNNSDataLayoutNSE`](/documentation/Accelerate/BNNSDataLayoutNSE) with the shape (num_directions*[`hidden_size`](/documentation/Accelerate/BNNSLayerParametersLSTM/hidden_size),[`seq_len`](/documentation/Accelerate/BNNSLayerParametersLSTM/seq_len),[`batch_size`](/documentation/Accelerate/BNNSLayerParametersLSTM/batch_size))

BNNS can upack the first dimension as a 2D array with [`hidden_size`](/documentation/Accelerate/BNNSLayerParametersLSTM/hidden_size) as the major dimension. That is, BNNS can interpret the dimension as a C array with the shape `[num_directions][` [`hidden_size`](/documentation/Accelerate/BNNSLayerParametersLSTM/hidden_size) `]`.

If [`lstm_flags`](/documentation/Accelerate/BNNSLayerParametersLSTM/lstm_flags) includes [`BNNSLayerFlagsLSTMBidirectional`](/documentation/Accelerate/BNNSLayerFlagsLSTMBidirectional), BNNS defines `num_directions` as `2`, or otherwise `1`.

If you don’t specify a layout, BNNS uses [`BNNSDataLayoutSNE`](/documentation/Accelerate/BNNSDataLayoutSNE).

Use C style multidimensional array notation to define the hierarchy of the [`hidden_desc`](/documentation/Accelerate/BNNSLSTMDataDescriptor/hidden_desc) and [`cell_state_desc`](/documentation/Accelerate/BNNSLSTMDataDescriptor/cell_state_desc) descriptors as `[num_layers][num_directions][batch_size][hidden_size]`.

---

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