---
title: "BNNSDirectApplyTopK(_:_:_:_:_:_:_:_:_:_:)"
framework: accelerate
role: symbol
role_heading: Function
path: "accelerate/bnnsdirectapplytopk(_:_:_:_:_:_:_:_:_:_:)"
---

# BNNSDirectApplyTopK(_:_:_:_:_:_:_:_:_:_:)

Applies a top-k filter directly to an input.

## Declaration

```swift
func BNNSDirectApplyTopK(_ K: Int, _ axis: Int, _ batch_size: Int, _ input: UnsafePointer<BNNSNDArrayDescriptor>, _ input_batch_stride: Int, _ best_values: UnsafeMutablePointer<BNNSNDArrayDescriptor>, _ best_values_batch_stride: Int, _ best_indices: UnsafeMutablePointer<BNNSNDArrayDescriptor>?, _ best_indices_batch_stride: Int, _ filter_params: UnsafePointer<BNNSFilterParameters>?) -> Int32
```

## Parameters

- `K`: The number of entries the operation finds.
- `axis`: The axis along which the operation finds top-k entries.
- `batch_size`: Number of input-output pairs to process.
- `input`: The descriptor of the input.
- `input_batch_stride`: Increment, in values, between inputs.
- `best_values`: The descriptor of the k best values generated by the operation.
- `best_values_batch_stride`: Increment, in values, between best values tensors.
- `best_indices`: The descriptor of the indices of the k best values generated by the operation.
- `best_indices_batch_stride`: Increment, in values, between best indices tensors.
- `filter_params`: The filter runtime parameters.

## Discussion

Discussion important: In Swift, the input data type and best values data type must be float, and the best indices data type must be int32. Use this function to find the maximum values and corresponding indices of a tensor along a specified axis. For example, given the following 4 x 4 row-major matrix: let source: [Float] = [1, 2, 3, 9,                        1, 6, 7, 1,                        9, 0, 1, 3,                        4, 5, 8, 1] The following code computes the top 2 elements of each column: let n = 4 let k = 2

var bestIndices = [Int32](repeating: -1,                           count: k * n) var bestValues = [Float](repeating: -1,                          count: k * n)

source.withUnsafeBufferPointer { srcPtr in     bestIndices.withUnsafeMutableBufferPointer { indicesPtr in         bestValues.withUnsafeMutableBufferPointer { valuesPtr in                          var srcDescriptor = BNNSNDArrayDescriptor(flags: BNNSNDArrayFlags(0),                                                        layout: BNNSDataLayoutRowMajorMatrix,                                                        size: (n, n, 0, 0, 0, 0, 0, 0),                                                        stride: (0, 0, 0, 0, 0, 0, 0, 0),                                                        data: UnsafeMutableRawPointer(mutating: srcPtr.baseAddress),                                                        data_type: .float,                                                        table_data: nil,                                                        table_data_type: .float,                                                        data_scale: 1,                                                        data_bias: 0)                          var indicesDescriptor = BNNSNDArrayDescriptor(flags: BNNSNDArrayFlags(0),                                                            layout: BNNSDataLayoutRowMajorMatrix,                                                            size: (n, k, 0, 0, 0, 0, 0, 0),                                                            stride: (0, 0, 0, 0, 0, 0, 0, 0),                                                            data: indicesPtr.baseAddress,                                                            data_type: .int32,                                                            table_data: nil,                                                            table_data_type: .int32,                                                            data_scale: 1,                                                            data_bias: 0)                          var valuesDescriptor = BNNSNDArrayDescriptor(flags: BNNSNDArrayFlags(0),                                                           layout: BNNSDataLayoutRowMajorMatrix,                                                           size: (n, k, 0, 0, 0, 0, 0, 0),                                                           stride: (0, 0, 0, 0, 0, 0, 0, 0),                                                           data: valuesPtr.baseAddress,                                                           data_type: .float,                                                           table_data: nil,                                                           table_data_type: .float,                                                           data_scale: 1,                                                           data_bias: 0)                          BNNSDirectApplyTopK(k,                                 0,                                 1,                                 &srcDescriptor, n * n,                                 &valuesDescriptor, k * n,                                 &indicesDescriptor, k * n,                                 nil)         }     } } On return, bestIndices and bestValues contain the following values, where 9 is the top value in the first column at index 2:                      |-- 1st --|     |-- 2nd --|           bestValues [9, 6, 8, 9,    4, 5, 7, 3]          bestIndices [2, 1, 3, 0,    3, 3, 1, 2]

## See Also

### Top-k layers

- [BNNSDirectApplyInTopK(_:_:_:_:_:_:_:_:_:_:)](accelerate/bnnsdirectapplyintopk(_:_:_:_:_:_:_:_:_:_:).md)