# mkl_dskymv

Computes matrix - vector product for a sparse matrix in the skyline storage format with one-based indexing.

## Syntax

Fortran:

call mkl_dskymv(transa, m, k, alpha, matdescra, val, pntr, x, beta, y)

C:

mkl_dskymv(&transa, &m, &k, &alpha, matdescra, val, pntr, x, &beta, y);

## Description

The mkl_dskymv routine performs a matrix-vector operation defined as
`y := alpha*A*x + beta*y`

or
`y := alpha*A'*x + beta*y,`

where:

alpha and beta are scalars,

x and y are vectors,

A is an m-by-k sparse matrix stored using the skyline storage scheme, A' is the transpose of A.

### Note

This routine supports only one-based indexing of the input arrays.

## Input Parameters

Parameter descriptions are common for all implemented interfaces with the exception of data types that refer here to the FORTRAN 77 standard types. Data types specific to the different interfaces are described in the section “Interfaces” below.

transa

  CHARACTER*1. Specifies the operation.

  If `transa = 'N'` or 'n', then `y := alpha*A*x + beta*y`

  If `transa = 'T'` or 't' or 'C' or 'c', then `y := alpha*A'*x + beta*y`,

m

  INTEGER. Number of rows of the matrix A.

k

  INTEGER. Number of columns of the matrix A.

alpha

  REAL*8. Specifies the scalar alpha.

matdescra

  CHARACTER. Array of six elements, specifies properties of the matrix used for operation. Only first four array elements are used, their possible values are given in the “Possible Values of the Parameter matdescra”.

  ### Note

  General matrices `(matdescra (1)='G')` is not supported.

val

  REAL*8. Array containing the set of elements of the matrix A in the skyline profile form.

  If `matdescrsa(2)= 'L'`, then val contains elements from the low triangle of the matrix A.

  If `matdescrsa(2)= 'U'`, then val contains elements from the upper triangle of the matrix A.

  Refer to values array description in Skyline Storage Scheme for more details.

pntr

  INTEGER. Array of length `(m+m)` for lower triangle, and `(k+k)` for upper triangle.

  It contains the indices specifying in the val the positions of the first element in each row (column) of the matrix A. Refer to pointers array description in Skyline Storage Scheme for more details.

x

  REAL*8.

  Array, DIMENSION at least k if `transa = 'N'` or 'n' and at least m otherwise. On entry, the array x must contain the vector x.

beta

  REAL*8. Specifies the scalar beta.

y

  REAL*8.

  Array, DIMENSION at least m if `transa = 'N'` or 'n' and at least k otherwise. On entry, the array y must contain the vector y.

## Output Parameters

y

  Overwritten by the updated vector y.

## Interfaces

## FORTRAN 77:

SUBROUTINE mkl_dskymv(transa, m, k, alpha, matdescra, val, pntr, x, beta, y)

CHARACTER*1 transa

CHARACTER matdescra(*)

INTEGER m, k

INTEGER pntr(*)

REAL*8 alpha, beta

REAL*8 val(*), x(*), y(*)

## C:

`void mkl_dskymv (char *transa, int *m, int *k, double *alpha, char *matdescra,`
`double *val, int *pntr, double *x, double *beta, double *y);`

Parent topic: Sparse BLAS Level 2 and Level 3 Routines.