*DECK IDAMAX INTEGER FUNCTION IDAMAX (N, DX, INCX) C***BEGIN PROLOGUE IDAMAX C***PURPOSE Find the smallest index of that component of a vector C having the maximum magnitude. C***LIBRARY SLATEC (BLAS) C***CATEGORY D1A2 C***TYPE DOUBLE PRECISION (ISAMAX-S, IDAMAX-D, ICAMAX-C) C***KEYWORDS BLAS, LINEAR ALGEBRA, MAXIMUM COMPONENT, 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 DX double precision vector with N elements C INCX storage spacing between elements of DX C C --Output-- C IDAMAX smallest index (zero if N .LE. 0) C C Find smallest index of maximum magnitude of double precision DX. C IDAMAX = first I, I = 1 to N, to maximize ABS(DX(IX+(I-1)*INCX)), C where IX = 1 if INCX .GE. 0, else IX = 1+(1-N)*INCX. 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 890531 REVISION DATE from Version 3.2 C 891214 Prologue converted to Version 4.0 format. (BAB) C 900821 Modified to correct problem with a negative increment. C (WRB) C 920501 Reformatted the REFERENCES section. (WRB) C***END PROLOGUE IDAMAX DOUBLE PRECISION DX(*), DMAX, XMAG INTEGER I, INCX, IX, N C***FIRST EXECUTABLE STATEMENT IDAMAX IDAMAX = 0 IF (N .LE. 0) RETURN IDAMAX = 1 IF (N .EQ. 1) RETURN C IF (INCX .EQ. 1) GOTO 20 C C Code for increments not equal to 1. C IX = 1 IF (INCX .LT. 0) IX = (-N+1)*INCX + 1 DMAX = ABS(DX(IX)) IX = IX + INCX DO 10 I = 2,N XMAG = ABS(DX(IX)) IF (XMAG .GT. DMAX) THEN IDAMAX = I DMAX = XMAG ENDIF IX = IX + INCX 10 CONTINUE RETURN C C Code for increments equal to 1. C 20 DMAX = ABS(DX(1)) DO 30 I = 2,N XMAG = ABS(DX(I)) IF (XMAG .GT. DMAX) THEN IDAMAX = I DMAX = XMAG ENDIF 30 CONTINUE RETURN END