dgerf.f

      SUBROUTINE DGERF  ( M, N, ALPHA, X, INCX, Y, INCY, A, LDA )

*     .. Scalar Arguments ..
      DOUBLE PRECISION   ALPHA
      INTEGER            INCX, INCY, LDA, M, N
*     .. Array Arguments ..
      DOUBLE PRECISION   A( LDA, * ), X( * ), Y( * )
*     ..
*
*  Purpose
*  =======
*
*  DGER   performs the rank 1 operation
*
*     A := alpha*x*y' + A,
*
*  where alpha is a scalar, x is an m element vector, y is an n element
*  vector and A is an m by n matrix.
*
*  Parameters
*  ==========
*
*  M      - INTEGER.
*           On entry, M specifies the number of rows of the matrix A.
*           M must be at least zero.
*           Unchanged on exit.
*
*  N      - INTEGER.
*           On entry, N specifies the number of columns of the matrix A.
*           N must be at least zero.
*           Unchanged on exit.
*
*  ALPHA  - DOUBLE PRECISION.
*           On entry, ALPHA specifies the scalar alpha.
*           Unchanged on exit.
*
*  X      - DOUBLE PRECISION array of dimension at least
*           ( 1 + ( m - 1 )*abs( INCX ) ).
*           Before entry, the incremented array X must contain the m
*           element vector x.
*           Unchanged on exit.
*
*  INCX   - INTEGER.
*           On entry, INCX specifies the increment for the elements of
*           X. INCX must not be zero.
*           Unchanged on exit.
*
*  Y      - DOUBLE PRECISION array of dimension at least
*           ( 1 + ( n - 1 )*abs( INCY ) ).
*           Before entry, the incremented array Y must contain the n
*           element vector y.
*           Unchanged on exit.
*
*  INCY   - INTEGER.
*           On entry, INCY specifies the increment for the elements of
*           Y. INCY must not be zero.
*           Unchanged on exit.
*
*  A      - DOUBLE PRECISION array of DIMENSION ( LDA, n ).
*           Before entry, the leading m by n part of the array A must
*           contain the matrix of coefficients. On exit, A is
*           overwritten by the updated matrix.
*
*  LDA    - INTEGER.
*           On entry, LDA specifies the first dimension of A as declared
*           in the calling (sub) program. LDA must be at least
*           max( 1, m ).
*           Unchanged on exit.
*
*
*  Level 2 Blas routine.
*
*  -- Written on 22-October-1986.
*     Jack Dongarra, Argonne National Lab.
*     Jeremy Du Croz, Nag Central Office.
*     Sven Hammarling, Nag Central Office.
*     Richard Hanson, Sandia National Labs.
*
*
*     .. Parameters ..
      DOUBLE PRECISION   ZERO
      PARAMETER        ( ZERO = 0.0D+0 )
*     .. Local Scalars ..
      DOUBLE PRECISION   TEMP
      INTEGER            I, INFO, IX, J, JY, KX
*     .. External Subroutines ..
      EXTERNAL           XERBLA
*     .. Intrinsic Functions ..
      INTRINSIC          MAX
*     ..
*     .. Executable Statements ..
*
*     Test the input parameters.
*
      INFO = 0
      IF     ( M.LT.0 )THEN
         INFO = 1
      ELSE IF( N.LT.0 )THEN
         INFO = 2
      ELSE IF( INCX.EQ.0 )THEN
         INFO = 5
      ELSE IF( INCY.EQ.0 )THEN
         INFO = 7
      ELSE IF( LDA.LT.MAX( 1, M ) )THEN
         INFO = 9
      END IF
      IF( INFO.NE.0 )THEN
         CALL XERBLA( 'DGER  ', INFO )
         RETURN
      END IF
*
*     Quick return if possible.
*
      IF( ( M.EQ.0 ).OR.( N.EQ.0 ).OR.( ALPHA.EQ.ZERO ) )
     $   RETURN
*
*     Start the operations. In this version the elements of A are
*     accessed sequentially with one pass through A.
*
      IF( INCY.GT.0 )THEN
         JY = 1
      ELSE
         JY = 1 - ( N - 1 )*INCY
      END IF
      IF( INCX.EQ.1 )THEN
         DO 20, J = 1, N
            IF( Y( JY ).NE.ZERO )THEN
               TEMP = ALPHA*Y( JY )
               DO 10, I = 1, M
                  A( I, J ) = A( I, J ) + X( I )*TEMP
   10          CONTINUE
            END IF
            JY = JY + INCY
   20    CONTINUE
      ELSE
         IF( INCX.GT.0 )THEN
            KX = 1
         ELSE
            KX = 1 - ( M - 1 )*INCX
         END IF
         DO 40, J = 1, N
            IF( Y( JY ).NE.ZERO )THEN
               TEMP = ALPHA*Y( JY )
               IX   = KX
               DO 30, I = 1, M
                  A( I, J ) = A( I, J ) + X( IX )*TEMP
                  IX        = IX        + INCX
   30          CONTINUE
            END IF
            JY = JY + INCY
   40    CONTINUE
      END IF
*
      RETURN
*
*     End of DGER  .
*
      END