# Wiki

Clone wiki# blaze / Bands

Bands provide views on a specific band of a dense or sparse matrix (e.g. the diagonal, the subdiagonal, ...). As such, bands act as a reference to a specific band. This reference is valid and can be used in every way any other vector can be used as long as the matrix containing the band is not resized or entirely destroyed. The band also acts as an alias to the band elements: Changes made to the elements (e.g. modifying values, inserting or erasing elements) are immediately visible in the matrix and changes made via the matrix are immediately visible in the band.

## Setup of Bands

A reference to a dense or sparse band can be created very conveniently via the `band()`

function. It can be included via the header files

```
#include <blaze/Blaze.h>
// or
#include <blaze/Math.h>
// or
#include <blaze/math/Band.h>
```

and forward declared via the header file

```
#include <blaze/Forward.h>
```

The band index must be in the range from `[min(0,1-M)..max(0,N-1)]`

, where `M`

is the total number of rows and `N`

is the total number of columns, and can be specified both at compile time or at runtime:

```
blaze::DynamicMatrix<double,blaze::rowMajor> A;
// ... Resizing and initialization
// Creating a reference to the 1st lower band of matrix A (compile time index)
auto band1 = band<-1L>( A );
// Creating a reference to the 2nd upper band of matrix A (runtime index)
auto band2 = band( A, 2L );
```

In addition, the `diagonal()`

function provides a convenient shortcut for the setup of a view on the diagonal of a dense or sparse matrix. It has the same effect as calling the `band()`

function with a compile time index of `0`

:

```
blaze::DynamicMatrix<double,blaze::rowMajor> A;
// ... Resizing and initialization
// Creating a reference to the diagonal of matrix A via the band() and diagonal() functions
auto diag1 = band<0L>( A );
auto diag2 = diagonal( A );
static_assert( blaze::IsSame< decltype(diag1), decltype(diag2) >::value, "Non-identical types detected" );
```

Both the `band()`

and the `diagonal()`

function return an expression representing the band view. The type of this expression depends on the given arguments, primarily the type of the matrix and the compile time arguments. If the type is required, it can be determined via the `decltype`

specifier:

```
using MatrixType = blaze::DynamicMatrix<int>;
using BandType = decltype( blaze::band<1L>( std::declval<MatrixType>() ) );
using DiagonalType = decltype( blaze::diagonal( std::declval<MatrixType>() ) );
```

This resulting view can be treated as any other vector, i.e. it can be assigned to, it can be copied from, and it can be used in arithmetic operations. By default, bands are considered column vectors, but this setting can be changed via the `BLAZE_DEFAULT_TRANSPOSE_FLAG`

switch (see Configuration Files#!default-vector-storage|Default Vector Storage). The reference can also be used on both sides of an assignment: The band can either be used as an alias to grant write access to a specific band of a matrix primitive on the left-hand side of an assignment or to grant read-access to a specific band of a matrix primitive or expression on the right-hand side of an assignment. The following example demonstrates this in detail:

```
blaze::DynamicVector<double,blaze::rowVector> x;
blaze::CompressedVector<double,blaze::rowVector> y;
blaze::DynamicMatrix<double,blaze::rowMajor> A, B;
blaze::CompressedMatrix<double,blaze::rowMajor> C, D;
// ... Resizing and initialization
// Setting the 2nd upper band of matrix A to x
auto band2 = band( A, 2L );
band2 = x;
// Setting the 3rd upper band of matrix B to y
band( B, 3L ) = y;
// Setting x to the 2nd lower band of the result of the matrix multiplication
x = band( A * B, -2L );
// Setting y to the 2nd upper band of the result of the sparse matrix multiplication
y = band( C * D, 2L );
```

**Warning**: It is the programmer's responsibility to ensure the band does not outlive the viewed matrix:

```
// Creating a band on a temporary matrix; results in a dangling reference!
auto band1 = band<1L>( DynamicMatrix<int>{ { 1, 2, 3 }, { 4, 5, 6 }, { 7, 8, 9 } } );
```

## Element Access

The elements of a band can be directly accessed with the subscript operator. The indices to access a band are zero-based:

```
blaze::DynamicMatrix<double,blaze::rowMajor> A;
// ... Resizing and initialization
// Creating a view on the 4th upper band of matrix A
auto band4 = band( A, 4L );
// Setting the 1st element of the dense band, which corresponds
// to the 1st element in the 4th upper band of matrix A
band4[1] = 2.0;
```

Alternatively, the elements of a band can be traversed via iterators. Just as with vectors, in case of non-const band, `begin()`

and `end()`

return an iterator, which allows to manipulate the elements, in case of constant bands an iterator to immutable elements is returned:

```
blaze::DynamicMatrix<int,blaze::rowMajor> A( 128UL, 256UL );
// ... Resizing and initialization
// Creating a reference to the 5th upper band of matrix A
auto band5 = band( A, 5L );
// Traversing the elements via iterators to non-const elements
for( auto it=band5.begin(); it!=band5.end(); ++it ) {
*it = ...; // OK; Write access to the dense band value
... = *it; // OK: Read access to the dense band value.
}
// Traversing the elements via iterators to const elements
for( auto it=band5.cbegin(); it!=band5.cend(); ++it ) {
*it = ...; // Compilation error: Assignment to the value via iterator-to-const is invalid.
... = *it; // OK: Read access to the dense band value.
}
```

```
blaze::CompressedMatrix<int,blaze::rowMajor> A( 128UL, 256UL );
// ... Resizing and initialization
// Creating a reference to the 5th band of matrix A
auto band5 = band( A, 5L );
// Traversing the elements via iterators to non-const elements
for( auto it=band5.begin(); it!=band5.end(); ++it ) {
it->value() = ...; // OK: Write access to the value of the non-zero element.
... = it->value(); // OK: Read access to the value of the non-zero element.
it->index() = ...; // Compilation error: The index of a non-zero element cannot be changed.
... = it->index(); // OK: Read access to the index of the sparse element.
}
// Traversing the elements via iterators to const elements
for( auto it=band5.cbegin(); it!=band5.cend(); ++it ) {
it->value() = ...; // Compilation error: Assignment to the value via iterator-to-const is invalid.
... = it->value(); // OK: Read access to the value of the non-zero element.
it->index() = ...; // Compilation error: The index of a non-zero element cannot be changed.
... = it->index(); // OK: Read access to the index of the sparse element.
}
```

## Element Insertion

Inserting/accessing elements in a sparse band can be done by several alternative functions. The following example demonstrates all options:

```
blaze::CompressedMatrix<double,blaze::rowMajor> A( 10UL, 100UL ); // Non-initialized 10x100 matrix
auto diag( band( A, 0L ) ); // Reference to the diagonal of A
// The subscript operator provides access to all possible elements of the sparse band,
// including the zero elements. In case the subscript operator is used to access an element
// that is currently not stored in the sparse band, the element is inserted into the band.
diag[42] = 2.0;
// The second operation for inserting elements is the set() function. In case the element
// is not contained in the band it is inserted into the band, if it is already contained in
// the band its value is modified.
diag.set( 45UL, -1.2 );
// An alternative for inserting elements into the band is the insert() function. However,
// it inserts the element only in case the element is not already contained in the band.
diag.insert( 50UL, 3.7 );
```

## Common Operations

A band view can be used like any other column vector. This means that with only a few exceptions all Vector Operations and Arithmetic Operations can be used. For instance, the current number of band elements can be obtained via the `size()`

function, the current capacity via the `capacity()`

function, and the number of non-zero elements via the `nonZeros()`

function. However, since bands are references to specific bands of a matrix, several operations are not possible, such as resizing and swapping. The following example shows this by means of a dense band view:

```
blaze::DynamicMatrix<int,blaze::rowMajor> A( 42UL, 42UL );
// ... Resizing and initialization
// Creating a reference to the 2nd upper band of matrix A
auto band2 = band( A, 2L );
band2.size(); // Returns the number of elements in the band
band2.capacity(); // Returns the capacity of the band
band2.nonZeros(); // Returns the number of non-zero elements contained in the band
band2.resize( 84UL ); // Compilation error: Cannot resize a single band of a matrix
auto band3 = band( A, 3L );
swap( band2, band3 ); // Compilation error: Swap operation not allowed
```

## Arithmetic Operations

Both dense and sparse bands can be used in all arithmetic operations that any other dense or sparse vector can be used in. The following example gives an impression of the use of dense bands within arithmetic operations. All operations (addition, subtraction, multiplication, scaling, ...) can be performed on all possible combinations of dense and sparse bands with fitting element types:

```
blaze::DynamicVector<double,blaze::columnVector> a( 2UL, 2.0 ), b;
blaze::CompressedVector<double,blaze::columnVector> c( 2UL );
c[1] = 3.0;
blaze::DynamicMatrix<double,blaze::rowMajor> A( 4UL, 2UL ); // Non-initialized 4x2 matrix
auto band1( band( A, 1L ) ); // Reference to the 1st upper band of A
auto diag ( band( A, 0L ) ); // Reference to the diagonal of A
band1[0] = 0.0; // Manual initialization of the 1st upper band of A
diag = 1.0; // Homogeneous initialization of the diagonal of A
band( A, -1L ) = a; // Dense vector initialization of the 1st lower band of A
band( A, -2L ) = c; // Sparse vector initialization of the 2nd lower band of A
b = diag + a; // Dense vector/dense vector addition
b = c + band( A, -1L ); // Sparse vector/dense vector addition
b = diag * band( A, -2L ); // Component-wise vector multiplication
band( A, -1L ) *= 2.0; // In-place scaling of the 1st upper band
b = band( A, -1L ) * 2.0; // Scaling of the 1st upper band
b = 2.0 * band( A, -1L ); // Scaling of the 1st upper band
band( A, -2L ) += a; // Addition assignment
band( A, -2L ) -= c; // Subtraction assignment
band( A, -2L ) *= band( A, 0L ); // Multiplication assignment
double scalar = trans( c ) * band( A, -1L ); // Scalar/dot/inner product between two vectors
A = band( A, -1L ) * trans( c ); // Outer product between two vectors
```

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