# mkl_ddiagemv

Computes matrix - vector product of a sparse general matrix stored in the diagonal format with one-based indexing.

## Syntax

Fortran:

call mkl_ddiagemv(transa, m, val, lval, idiag, ndiag, x, y)

C:

mkl_ddiagemv(&transa, &m, val, &lval, idiag, &ndiag, x, y);

## Description

The mkl_ddiagemv routine performs a matrix-vector operation defined as
`y := A*x`

or
`y := A'*x,`

where:

x and y are vectors,

A is an m-by-m sparse square matrix in the diagonal storage format, 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 := A*x`

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

m

  INTEGER. Number of rows of the matrix A.

val

  REAL*8. Two-dimensional array of size lval by ndiag, contains non-zero diagonals of the matrix A. Refer to values array description in Diagonal Storage Scheme for more details.

lval

  INTEGER. Leading dimension of val, `lval≥m`. Refer to lval description in Diagonal Storage Scheme for more details.

idiag

  INTEGER. Array of length ndiag, contains the distances between main diagonal and each non-zero diagonals in the matrix A.

  Refer to distance array description in Diagonal Storage Scheme for more details.

ndiag

  INTEGER. Specifies the number of non-zero diagonals of the matrix A.

x

  REAL*8.

  Array, DIMENSION is m.

  On entry, the array x must contain the vector x.

## Output Parameters

y

  REAL*8.

  Array, DIMENSION at least m.

  On exit, the array y must contain the vector y.

## Interfaces

## FORTRAN 77:

SUBROUTINE mkl_ddiagemv(transa, m, val, lval, idiag, ndiag, x, y)

CHARACTER*1 transa

INTEGER m, lval, ndiag

INTEGER idiag(*)

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

## C:

`void mkl_ddiagemv(char *transa, int *m, double *val, int *lval,`
`int *idiag, int *ndiag, double *x, double *y);`

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