---
title: vDSP_vswmax
framework: Kernel
role: symbol
role_heading: Function
platforms: [macOS 10.10+]
path: kernel/1579990-vdsp_vswmax
---

# vDSP_vswmax

Finds the maximum value in a sliding window at each possible position in a single-precision input vector.

## Declaration

```occ
void vDSP_vswmax(const float *__A, vDSP_Stride __IA, float *__C, vDSP_Stride __IC, vDSP_Length __N, vDSP_Length __WindowLength);
```

## Parameters

- `__A`: The input vector.
- `__IA`: The input stride, which is the distance between the elements in the input vector in which this function accesses elements.
- `__C`: The output vector.
- `__IC`: The input stride, which is the distance between the elements in the output vector in which this function accesses elements.
- `__N`: The number of output values, which is the same as the number of window positions the operation uses.
- `__WindowLength`: The length of the window, which must be greater than zero.

## Discussion

Discussion Use this function to populate an output vector with the consecutive maximum values in a sliding window. The number of elements in the input vector must be one subtracted from the sum of the window length and the number of window positions.  The number of elements in the output vector must be the same as the number of elements in the input vector. This is because although the function only writes the number of window positions to the output vector, it may require additional elements for intermediate computation. The following code shows how to compute the maximum values in a five-element sliding window moved through a ten-element array. The output result contains six values. let input: [Float] = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

var output = [Float](repeating: .nan,                      count: input.count)

let windowLength = vDSP_Length(5) let outputCount = vDSP_Length(input.count) - windowLength + 1 let stride = vDSP_Stride(1)

vDSP_vswmax(input, stride,             &output, stride,             outputCount,             windowLength)

// Prints "[5.0, 6.0, 7.0, 8.0, 9.0, 10.0]" print(output[0 ..< Int(outputCount)]) The figure below illustrates the process the function uses to calculate the result:

Alternatively, the following code shows how to process input elements to generate a specific number of output elements. Use this technique, for example, to process streaming input data. The code below takes count elements from the input collection and returns the maximum values at six window positions: let input: [Float] = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10,                       11, 12, 13, 14, 15, 16, 17, 18, 19, 20,                       21, 22, 23, 24, 25, 26, 27, 28, 29, 30] let inputPageNumber = 1

let windowLength = vDSP_Length(5) let outputCount = vDSP_Length(6)

let count = Int(outputCount + windowLength - 1)

var output = [Float](repeating: .nan,                      count: count)

let stride = vDSP_Stride(1)

input.withUnsafeBufferPointer { srcPtr in     vDSP_vswmax((srcPtr.baseAddress!.advanced(by: count * inputPageNumber)),                 stride,                 &output, stride,                 outputCount,                 windowLength) }

// Prints "[15.0, 16.0, 17.0, 18.0, 19.0, 20.0]" print(output[0 ..< Int(outputCount)]) The figure below illustrates the process the function uses to calculate the result: