dd/d35/srscl_8f_source.html

00001       SUBROUTINE SRSCL( N, SA, SX, INCX )
00002 *
00003 *  -- LAPACK auxiliary routine (version 3.2) --
00004 *  -- LAPACK is a software package provided by Univ. of Tennessee,    --
00005 *  -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
00006 *     November 2006
00007 *
00008 *     .. Scalar Arguments ..
00009       INTEGER            INCX, N
00010       REAL               SA
00011 *     ..
00012 *     .. Array Arguments ..
00013       REAL               SX( * )
00014 *     ..
00015 *
00016 *  Purpose
00017 *  =======
00018 *
00019 *  SRSCL multiplies an n-element real vector x by the real scalar 1/a.
00020 *  This is done without overflow or underflow as long as
00021 *  the final result x/a does not overflow or underflow.
00022 *
00023 *  Arguments
00024 *  =========
00025 *
00026 *  N       (input) INTEGER
00027 *          The number of components of the vector x.
00028 *
00029 *  SA      (input) REAL
00030 *          The scalar a which is used to divide each component of x.
00031 *          SA must be >= 0, or the subroutine will divide by zero.
00032 *
00033 *  SX      (input/output) REAL array, dimension
00034 *                         (1+(N-1)*abs(INCX))
00035 *          The n-element vector x.
00036 *
00037 *  INCX    (input) INTEGER
00038 *          The increment between successive values of the vector SX.
00039 *          > 0:  SX(1) = X(1) and SX(1+(i-1)*INCX) = x(i),     1< i<= n
00040 *
00041 * =====================================================================
00042 *
00043 *     .. Parameters ..
00044       REAL               ONE, ZERO
00045       PARAMETER          ( ONE = 1.0E+0, ZERO = 0.0E+0 )
00046 *     ..
00047 *     .. Local Scalars ..
00048       LOGICAL            DONE
00049       REAL               BIGNUM, CDEN, CDEN1, CNUM, CNUM1, MUL, SMLNUM
00050 *     ..
00051 *     .. External Functions ..
00052       REAL               SLAMCH
00053       EXTERNAL           SLAMCH
00054 *     ..
00055 *     .. External Subroutines ..
00056       EXTERNAL           SLABAD, SSCAL
00057 *     ..
00058 *     .. Intrinsic Functions ..
00059       INTRINSIC          ABS
00060 *     ..
00061 *     .. Executable Statements ..
00062 *
00063 *     Quick return if possible
00064 *
00065       IF( N.LE.0 )
00066      $   RETURN
00067 *
00068 *     Get machine parameters
00069 *
00070       SMLNUM = SLAMCH( 'S' )
00071       BIGNUM = ONE / SMLNUM
00072       CALL SLABAD( SMLNUM, BIGNUM )
00073 *
00074 *     Initialize the denominator to SA and the numerator to 1.
00075 *
00076       CDEN = SA
00077       CNUM = ONE
00078 *
00079    10 CONTINUE
00080       CDEN1 = CDEN*SMLNUM
00081       CNUM1 = CNUM / BIGNUM
00082       IF( ABS( CDEN1 ).GT.ABS( CNUM ) .AND. CNUM.NE.ZERO ) THEN
00083 *
00084 *        Pre-multiply X by SMLNUM if CDEN is large compared to CNUM.
00085 *
00086          MUL = SMLNUM
00087          DONE = .FALSE.
00088          CDEN = CDEN1
00089       ELSE IF( ABS( CNUM1 ).GT.ABS( CDEN ) ) THEN
00090 *
00091 *        Pre-multiply X by BIGNUM if CDEN is small compared to CNUM.
00092 *
00093          MUL = BIGNUM
00094          DONE = .FALSE.
00095          CNUM = CNUM1
00096       ELSE
00097 *
00098 *        Multiply X by CNUM / CDEN and return.
00099 *
00100          MUL = CNUM / CDEN
00101          DONE = .TRUE.
00102       END IF
00103 *
00104 *     Scale the vector X by MUL
00105 *
00106       CALL SSCAL( N, MUL, SX, INCX )
00107 *
00108       IF( .NOT.DONE )
00109      $   GO TO 10
00110 *
00111       RETURN
00112 *
00113 *     End of SRSCL
00114 *
00115       END