PCHDF Function

function PCHDF(K, X, S, IERR)

BEGIN PROLOGUE PCHDF SUBSIDIARY PURPOSE Computes divided differences for PCHCE and PCHSP LIBRARY SLATEC (PCHIP) TYPE SINGLE PRECISION (PCHDF-S, DPCHDF-D) AUTHOR Fritsch, F. N., (LLNL) **DESCRIPTION

  PCHDF:   PCHIP Finite Difference Formula

Uses a divided difference formulation to compute a K-point approx- imation to the derivative at X(K) based on the data in X and S.

Called by PCHCE and PCHSP to compute 3- and 4-point boundary derivative approximations.

On input: K is the order of the desired derivative approximation. K must be at least 3 (error return if not). X contains the K values of the independent variable. X need not be ordered, but the values MUST be distinct. (Not checked here.) S contains the associated slope values: S(I) = (F(I+1)-F(I))/(X(I+1)-X(I)), I=1(1)K-1. (Note that S need only be of length K-1.)

On return: S will be destroyed. IERR will be set to -1 if K.LT.2 . PCHDF will be set to the desired derivative approximation if IERR=0 or to zero if IERR=-1.

SEE ALSO PCHCE, PCHSP REFERENCES Carl de Boor, A Practical Guide to Splines, Springer- Verlag, New York, 1978, pp. 10-16. ROUTINES CALLED XERMSG REVISION HISTORY (YYMMDD) 820503 DATE WRITTEN 820805 Converted to SLATEC library version. 870813 Minor cosmetic changes. 890411 Added SAVE statements (Vers. 3.2). 890411 REVISION DATE from Version 3.2 891214 Prologue converted to Version 4.0 format. (BAB) 900315 CALLs to XERROR changed to CALLs to XERMSG. (THJ) 900328 Added TYPE section. (WRB) 910408 Updated AUTHOR and DATE WRITTEN sections in prologue. (WRB) 920429 Revised format and order of references. (WRB,FNF) 930503 Improved purpose. (FNF) **END PROLOGUE PCHDF

*End

DECLARE ARGUMENTS.

**FIRST EXECUTABLE STATEMENT PCHDF

Arguments

Return Value real