dger.f

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      SUBROUTINE dger  ( 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