sspr2.f

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      SUBROUTINE SSPR2(UPLO,N,ALPHA,X,INCX,Y,INCY,AP)
*     .. Scalar Arguments ..
      REAL ALPHA
      INTEGER INCX,INCY,N
      CHARACTER UPLO
*     ..
*     .. Array Arguments ..
      REAL AP(*),X(*),Y(*)
*     ..
*
*  Purpose
*  =======
*
*  SSPR2  performs the symmetric rank 2 operation
*
*     A := alpha*x*y**T + alpha*y*x**T + A,
*
*  where alpha is a scalar, x and y are n element vectors and A is an
*  n by n symmetric matrix, supplied in packed form.
*
*  Arguments
*  ==========
*
*  UPLO   - CHARACTER*1.
*           On entry, UPLO specifies whether the upper or lower
*           triangular part of the matrix A is supplied in the packed
*           array AP as follows:
*
*              UPLO = 'U' or 'u'   The upper triangular part of A is
*                                  supplied in AP.
*
*              UPLO = 'L' or 'l'   The lower triangular part of A is
*                                  supplied in AP.
*
*           Unchanged on exit.
*
*  N      - INTEGER.
*           On entry, N specifies the order of the matrix A.
*           N must be at least zero.
*           Unchanged on exit.
*
*  ALPHA  - REAL            .
*           On entry, ALPHA specifies the scalar alpha.
*           Unchanged on exit.
*
*  X      - REAL             array of dimension at least
*           ( 1 + ( n - 1 )*abs( INCX ) ).
*           Before entry, the incremented array X must contain the n
*           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      - REAL             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.
*
*  AP     - REAL             array of DIMENSION at least
*           ( ( n*( n + 1 ) )/2 ).
*           Before entry with  UPLO = 'U' or 'u', the array AP must
*           contain the upper triangular part of the symmetric matrix
*           packed sequentially, column by column, so that AP( 1 )
*           contains a( 1, 1 ), AP( 2 ) and AP( 3 ) contain a( 1, 2 )
*           and a( 2, 2 ) respectively, and so on. On exit, the array
*           AP is overwritten by the upper triangular part of the
*           updated matrix.
*           Before entry with UPLO = 'L' or 'l', the array AP must
*           contain the lower triangular part of the symmetric matrix
*           packed sequentially, column by column, so that AP( 1 )
*           contains a( 1, 1 ), AP( 2 ) and AP( 3 ) contain a( 2, 1 )
*           and a( 3, 1 ) respectively, and so on. On exit, the array
*           AP is overwritten by the lower triangular part of the
*           updated matrix.
*
*  Further Details
*  ===============
*
*  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 ..
      REAL ZERO
      PARAMETER (ZERO=0.0E+0)
*     ..
*     .. Local Scalars ..
      REAL TEMP1,TEMP2
      INTEGER I,INFO,IX,IY,J,JX,JY,K,KK,KX,KY
*     ..
*     .. External Functions ..
      LOGICAL LSAME
      EXTERNAL LSAME
*     ..
*     .. External Subroutines ..
      EXTERNAL XERBLA
*     ..
*
*     Test the input parameters.
*
      INFO = 0
      IF (.NOT.LSAME(UPLO,'U') .AND. .NOT.LSAME(UPLO,'L')) 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
      END IF
      IF (INFO.NE.0) THEN
          CALL XERBLA('SSPR2 ',INFO)
          RETURN
      END IF
*
*     Quick return if possible.
*
      IF ((N.EQ.0) .OR. (ALPHA.EQ.ZERO)) RETURN
*
*     Set up the start points in X and Y if the increments are not both
*     unity.
*
      IF ((INCX.NE.1) .OR. (INCY.NE.1)) THEN
          IF (INCX.GT.0) THEN
              KX = 1
          ELSE
              KX = 1 - (N-1)*INCX
          END IF
          IF (INCY.GT.0) THEN
              KY = 1
          ELSE
              KY = 1 - (N-1)*INCY
          END IF
          JX = KX
          JY = KY
      END IF
*
*     Start the operations. In this version the elements of the array AP
*     are accessed sequentially with one pass through AP.
*
      KK = 1
      IF (LSAME(UPLO,'U')) THEN
*
*        Form  A  when upper triangle is stored in AP.
*
          IF ((INCX.EQ.1) .AND. (INCY.EQ.1)) THEN
              DO 20 J = 1,N
                  IF ((X(J).NE.ZERO) .OR. (Y(J).NE.ZERO)) THEN
                      TEMP1 = ALPHA*Y(J)
                      TEMP2 = ALPHA*X(J)
                      K = KK
                      DO 10 I = 1,J
                          AP(K) = AP(K) + X(I)*TEMP1 + Y(I)*TEMP2
                          K = K + 1
   10                 CONTINUE
                  END IF
                  KK = KK + J
   20         CONTINUE
          ELSE
              DO 40 J = 1,N
                  IF ((X(JX).NE.ZERO) .OR. (Y(JY).NE.ZERO)) THEN
                      TEMP1 = ALPHA*Y(JY)
                      TEMP2 = ALPHA*X(JX)
                      IX = KX
                      IY = KY
                      DO 30 K = KK,KK + J - 1
                          AP(K) = AP(K) + X(IX)*TEMP1 + Y(IY)*TEMP2
                          IX = IX + INCX
                          IY = IY + INCY
   30                 CONTINUE
                  END IF
                  JX = JX + INCX
                  JY = JY + INCY
                  KK = KK + J
   40         CONTINUE
          END IF
      ELSE
*
*        Form  A  when lower triangle is stored in AP.
*
          IF ((INCX.EQ.1) .AND. (INCY.EQ.1)) THEN
              DO 60 J = 1,N
                  IF ((X(J).NE.ZERO) .OR. (Y(J).NE.ZERO)) THEN
                      TEMP1 = ALPHA*Y(J)
                      TEMP2 = ALPHA*X(J)
                      K = KK
                      DO 50 I = J,N
                          AP(K) = AP(K) + X(I)*TEMP1 + Y(I)*TEMP2
                          K = K + 1
   50                 CONTINUE
                  END IF
                  KK = KK + N - J + 1
   60         CONTINUE
          ELSE
              DO 80 J = 1,N
                  IF ((X(JX).NE.ZERO) .OR. (Y(JY).NE.ZERO)) THEN
                      TEMP1 = ALPHA*Y(JY)
                      TEMP2 = ALPHA*X(JX)
                      IX = JX
                      IY = JY
                      DO 70 K = KK,KK + N - J
                          AP(K) = AP(K) + X(IX)*TEMP1 + Y(IY)*TEMP2
                          IX = IX + INCX
                          IY = IY + INCY
   70                 CONTINUE
                  END IF
                  JX = JX + INCX
                  JY = JY + INCY
                  KK = KK + N - J + 1
   80         CONTINUE
          END IF
      END IF
*
      RETURN
*
*     End of SSPR2 .
*
      END