Quellcode-Bibliothek

^© Kompilation durch diese Firma

[Weder Korrektheit noch Funktionsfähigkeit der Software werden zugesichert.]

Datei: snrm2.ftn77 Sprache: Fortran

Original von: ftn77^©

*DECK SNRM2
      REAL FUNCTION SNRM2 (N, SX, INCX)
C***BEGIN PROLOGUE  SNRM2
C***PURPOSE  Compute the Euclidean length (L2 norm) of a vector.
C***LIBRARY   SLATEC (BLAS)
C***CATEGORY  D1A3B
C***TYPE      SINGLE PRECISION (SNRM2-S, DNRM2-D, SCNRM2-C)
C***KEYWORDS  BLAS, EUCLIDEAN LENGTH, EUCLIDEAN NORM, L2,
C             LINEAR ALGEBRA, UNITARY, VECTOR
C***AUTHOR  Lawson, C. L., (JPL)
C           Hanson, R. J., (SNLA)
C           Kincaid, D. R., (U. of Texas)
C           Krogh, F. T., (JPL)
C***DESCRIPTION
C
C                B L A S  Subprogram
C    Description of Parameters
C
C     --Input--
C        N  number of elements in input vector(s)
C       SX  single precision vector with N elements
C     INCX  storage spacing between elements of SX
C
C     --Output--
C    SNRM2  single precision result (zero if N .LE. 0)
C
C     Euclidean norm of the N-vector stored in SX with storage
C     increment INCX .
C     If N .LE. 0, return with result = 0.
C     If N .GE. 1, then INCX must be .GE. 1
C
C     Four Phase Method using two built-in constants that are
C     hopefully applicable to all machines.
C         CUTLO = maximum of  SQRT(U/EPS)  over all known machines.
C         CUTHI = minimum of  SQRT(V)      over all known machines.
C     where
C         EPS = smallest no. such that EPS + 1. .GT. 1.
C         U   = smallest positive no.   (underflow limit)
C         V   = largest  no.            (overflow  limit)
C
C     Brief Outline of Algorithm.
C
C     Phase 1 scans zero components.
C     Move to phase 2 when a component is nonzero and .LE. CUTLO
C     Move to phase 3 when a component is .GT. CUTLO
C     Move to phase 4 when a component is .GE. CUTHI/M
C     where M = N for X() real and M = 2*N for complex.
C
C     Values for CUTLO and CUTHI.
C     From the environmental parameters listed in the IMSL converter
C     document the limiting values are as follows:
C     CUTLO, S.P.   U/EPS = 2**(-102) for  Honeywell.  Close seconds are
C                   Univac and DEC at 2**(-103)
C                   Thus CUTLO = 2**(-51) = 4.44089E-16
C     CUTHI, S.P.   V = 2**127 for Univac, Honeywell, and DEC.
C                   Thus CUTHI = 2**(63.5) = 1.30438E19
C     CUTLO, D.P.   U/EPS = 2**(-67) for Honeywell and DEC.
C                   Thus CUTLO = 2**(-33.5) = 8.23181D-11
C     CUTHI, D.P.   same as S.P.  CUTHI = 1.30438D19
C     DATA CUTLO, CUTHI /8.232D-11,  1.304D19/
C     DATA CUTLO, CUTHI /4.441E-16,  1.304E19/
C
C***REFERENCES  C. L. Lawson, R. J. Hanson, D. R. Kincaid and F. T.
C                 Krogh, Basic linear algebra subprograms for Fortran
C                 usage, Algorithm No. 539, Transactions on Mathematical
C                 Software 5, 3 (September 1979), pp. 308-323.
C***ROUTINES CALLED  (NONE)
C***REVISION HISTORY  (YYMMDD)
C   791001  DATE WRITTEN
C   890531  Changed all specific intrinsics to generic.  (WRB)
C   890831  Modified array declarations.  (WRB)
C   890831  REVISION DATE from Version 3.2
C   891214  Prologue converted to Version 4.0 format.  (BAB)
C   920501  Reformatted the REFERENCES section.  (WRB)
C***END PROLOGUE  SNRM2
      INTEGER NEXT
      REAL SX(*), CUTLO, CUTHI, HITEST, SUM, XMAX, ZERO, ONE
      SAVE CUTLO, CUTHI, ZERO, ONE
      DATA ZERO, ONE /0.0E0, 1.0E0/
C
      DATA CUTLO, CUTHI /4.441E-16,  1.304E19/
C***FIRST EXECUTABLE STATEMENT  SNRM2
      IF (N .GT. 0) GO TO 10
         SNRM2  = ZERO
         GO TO 300
C
   10 ASSIGN 30 TO NEXT
      SUM = ZERO
      NN = N * INCX
C
C                                                 BEGIN MAIN LOOP
C
      I = 1
   20    GO TO NEXT,(30, 50, 70, 110)
   30 IF (ABS(SX(I)) .GT. CUTLO) GO TO 85
      ASSIGN 50 TO NEXT
      XMAX = ZERO
C
C                        PHASE 1.  SUM IS ZERO
C
   50 IF (SX(I) .EQ. ZERO) GO TO 200
      IF (ABS(SX(I)) .GT. CUTLO) GO TO 85
C
C                                PREPARE FOR PHASE 2.
C
      ASSIGN 70 TO NEXT
      GO TO 105
C
C                                PREPARE FOR PHASE 4.
C
  100 I = J
      ASSIGN 110 TO NEXT
      SUM = (SUM / SX(I)) / SX(I)
  105 XMAX = ABS(SX(I))
      GO TO 115
C
C                   PHASE 2.  SUM IS SMALL.
C                             SCALE TO AVOID DESTRUCTIVE UNDERFLOW.
C
   70 IF (ABS(SX(I)) .GT. CUTLO) GO TO 75
C
C                     COMMON CODE FOR PHASES 2 AND 4.
C                     IN PHASE 4 SUM IS LARGE.  SCALE TO AVOID OVERFLOW.
C
  110 IF (ABS(SX(I)) .LE. XMAX) GO TO 115
         SUM = ONE + SUM * (XMAX / SX(I))**2
         XMAX = ABS(SX(I))
         GO TO 200
C
  115 SUM = SUM + (SX(I)/XMAX)**2
      GO TO 200
C
C                  PREPARE FOR PHASE 3.
C
   75 SUM = (SUM * XMAX) * XMAX
C
C     FOR REAL OR D.P. SET HITEST = CUTHI/N
C     FOR COMPLEX      SET HITEST = CUTHI/(2*N)
C
   85 HITEST = CUTHI / N
C
C                   PHASE 3.  SUM IS MID-RANGE.  NO SCALING.
C
      DO 95 J = I,NN,INCX
      IF (ABS(SX(J)) .GE. HITEST) GO TO 100
   95    SUM = SUM + SX(J)**2
      SNRM2 = SQRT( SUM )
      GO TO 300
C
  200 CONTINUE
      I = I + INCX
      IF (I .LE. NN) GO TO 20
C
C              END OF MAIN LOOP.
C
C              COMPUTE SQUARE ROOT AND ADJUST FOR SCALING.
C
      SNRM2 = XMAX * SQRT(SUM)
  300 CONTINUE
      RETURN
      END

¤ Dauer der Verarbeitung: 0.0 Sekunden (vorverarbeitet) ¤

Download des Quellennavigators
Download des sprechenden Kalenders
in der Quellcodebibliothek suchen

Haftungshinweis

Die Informationen auf dieser Webseite wurden nach bestem Wissen sorgfältig zusammengestellt. Es wird jedoch weder Vollständigkeit, noch Richtigkeit, noch Qualität der bereit gestellten Informationen zugesichert.

Bemerkung:

Die farbliche Syntaxdarstellung ist noch experimentell.