V3FIT
r8speval.f
1 C******************** START FILE SPEVAL.FOR ; GROUP TRKRLIB ************
2 C....................................................
3 CCCCCCCCCCCCCCC
4  REAL*8 FUNCTION r8speval(N, U, X, Y, B, C, D)
5  IMPLICIT NONE
6  INTEGER, PARAMETER :: R8=selected_real_kind(12,100)
7  INTEGER N
8  real*8 u, x(n), y(n), b(n), c(n), d(n)
9 C
10 CCCCCCCCCCCCCCC
11 CCCCCCCCCCCCCCC
12 C THIS SUBROUTINE EVALUATES THE DERIVATIVE OF THE CUBIC SPLINE FUNCTION
13 C
14 C
15 C WHERE X(I) .LT. U .LT. X(I+1), USING HORNER'S RULE
16 C
17 C IF U .LT. X(1) THEN I = 1 IS USED.
18 C IF U .GE. X(N) THEN I = N IS USED.
19 C
20 C INPUT..
21 C
22 C N = THE NUMBER OF DATA POINTS
23 C U = THE ABSCISSA AT WHICH THE SPLINE IS TO BE EVALUATED
24 C X,Y = THE ARRAYS OF DATA ABSCISSAS AND ORDINATES
25 C B,C,D = ARRAYS OF SPLINE COEFFICIENTS COMPUTED BY SPLINE
26 C
27 C IF U IS NOT IN THE SAME INTERVAL AS THE PREVIOUS CALL, THEN A
28 C BINARY SEARCH IS PERFORMED TO DETERMINE THE PROPER INTERVAL.
29 C
30  INTEGER I, J, K
31  real*8 dx
32  DATA i/1/
33  IF ( i .GE. n ) i = 1
34  IF ( u .LT. x(i) ) GO TO 10
35  IF ( u .LE. x(i+1) ) GO TO 30
36 C
37 C BINARY SEARCH
38 C
39  10 i = 1
40  j = n+1
41  20 k = (i+j)/2
42  IF ( u .LT. x(k) ) j = k
43  IF ( u .GE. x(k) ) i = k
44  IF ( j .GT. i+1 ) GO TO 20
45 C
46 C EVALUATE SPLINE
47 C
48  30 dx = u - x(i)
49  r8speval = b(i) + dx*(2._r8*c(i) + 3._r8*dx*d(i))
50  RETURN
51  END
52 C******************** END FILE SPEVAL.FOR ; GROUP TRKRLIB **************