---
title: "BNNSBandPart(_:_:_:_:_:)"
framework: accelerate
role: symbol
role_heading: Function
path: "accelerate/bnnsbandpart(_:_:_:_:_:)"
---

# BNNSBandPart(_:_:_:_:_:)

Copies the specified subdiagonals and superdiagonals of a matrix, and sets other elements to zero.

## Declaration

```swift
func BNNSBandPart(_ num_lower: Int32, _ num_upper: Int32, _ input: UnsafePointer<BNNSNDArrayDescriptor>, _ output: UnsafeMutablePointer<BNNSNDArrayDescriptor>, _ filter_params: UnsafePointer<BNNSFilterParameters>?) -> Int32
```

## Parameters

- `num_lower`: The number of subdiagonals that the function copies. Set to a negative value to copy the entire lower triangle.
- `num_upper`: The number of superdiagonals that the function copies. Set to a negative value to copy the entire upper triangle.
- `input`: A pointer to the input descriptor.
- `output`: A pointer to the output descriptor.
- `filter_params`: The filter runtime parameters.

## Discussion

Discussion Use BNNSBandPart(_:_:_:_:_:) to copy a tensor’s main diagonal and zero or more upper and lower bands. You can specify that the function returns either the entire upper or lower triangle by passing a negative value to the corresponding num_* parameter. For example, given the following 8x8 matrix: let matrixValues: [Float] = [ 11, 12, 13, 14, 15, 16, 17, 18,                               21, 22, 23, 24, 25, 26, 27, 28,                               31, 32, 33, 34, 35, 36, 37, 38,                               41, 42, 43, 44, 45, 46, 47, 48,                               51, 52, 53, 54, 55, 56, 57, 58,                               61, 62, 63, 64, 65, 66, 67, 68,                               71, 72, 73, 74, 75, 76, 77, 78,                               81, 82, 83, 84, 85, 86, 87, 88 ]

var inputDescriptor = BNNSNDArrayDescriptor.allocate(      initializingFrom: matrixValues,      shape: .matrixRowMajor(8, 8)) The following code selects the upper triangle and sets other elements to zero: var outputDescriptor = BNNSNDArrayDescriptor.allocateUninitialized(     scalarType: Float.self,     shape: inputDescriptor.shape)

BNNSBandPart(0,  // num_lower              -1, // num_upper              &inputDescriptor,              &outputDescriptor, nil)

printMatrix(outputDescriptor) On return, outputDescriptor contains the following values: 11.0, 12.0, 13.0, 14.0, 15.0, 16.0, 17.0, 18.0  0.0, 22.0, 23.0, 24.0, 25.0, 26.0, 27.0, 28.0  0.0,  0.0, 33.0, 34.0, 35.0, 36.0, 37.0, 38.0  0.0,  0.0,  0.0, 44.0, 45.0, 46.0, 47.0, 48.0  0.0,  0.0,  0.0,  0.0, 55.0, 56.0, 57.0, 58.0  0.0,  0.0,  0.0,  0.0,  0.0, 66.0, 67.0, 68.0  0.0,  0.0,  0.0,  0.0,  0.0,  0.0, 77.0, 78.0  0.0,  0.0,  0.0,  0.0,  0.0,  0.0,  0.0, 88.0 The following code selects the 4 subdiagonals, 1 superdiagonal, and sets other elements to zero: var outputDescriptor = BNNSNDArrayDescriptor.allocateUninitialized(     scalarType: Float.self,     shape: inputDescriptor.shape)

BNNSBandPart(4, // num_lower              1, // num_upper              &inputDescriptor,              &outputDescriptor, nil)

printMatrix(outputDescriptor) On return, outputDescriptor contains the following values: 11.0, 12.0,  0.0,  0.0,  0.0,  0.0,  0.0,  0.0 21.0, 22.0, 23.0,  0.0,  0.0,  0.0,  0.0,  0.0 31.0, 32.0, 33.0, 34.0,  0.0,  0.0,  0.0,  0.0 41.0, 42.0, 43.0, 44.0, 45.0,  0.0,  0.0,  0.0 51.0, 52.0, 53.0, 54.0, 55.0, 56.0,  0.0,  0.0  0.0, 62.0, 63.0, 64.0, 65.0, 66.0, 67.0,  0.0  0.0,  0.0, 73.0, 74.0, 75.0, 76.0, 77.0, 78.0  0.0,  0.0,  0.0, 84.0, 85.0, 86.0, 87.0, 88.0

## See Also

### Errors

- [BNNS.Error](accelerate/bnns/error.md)
- [BNNSShuffle(_:_:_:_:)](accelerate/bnnsshuffle(_:_:_:_:).md)
- [BNNSShuffleType](accelerate/bnnsshuffletype.md)
- [BNNSTile(_:_:_:)](accelerate/bnnstile(_:_:_:).md)
- [BNNSTileBackward(_:_:_:)](accelerate/bnnstilebackward(_:_:_:).md)