V3FIT
r8seval2.f
1 C******************** START FILE SEVAL2.FOR ; GROUP TRKRLIB ************
2 C
3  REAL*8 FUNCTION r8seval2(N, U, X, Y, B, C, D, deriv)
4 !============
5 ! idecl: explicitize implicit REAL declarations:
6  IMPLICIT NONE
7  INTEGER, PARAMETER :: R8=selected_real_kind(12,100)
8  real*8 zslop
9 !============
10  INTEGER N
11  real*8 u, x(n), y(n), b(n), c(n), d(n), deriv
12 C
13 CCCCCCCCCCCCCCC
14 CCCCCCCCCCCCCCC
15 C THIS SUBROUTINE EVALUATES THE CUBIC SPLINE FUNCTION
16 C
17 C SEVAL2 = Y(I) + B(I)*(U-X(I)) + C(I)*(U-X(I))**2 + D(I)*(U-X(I))**3
18 C
19 C and the derivative
20 C
21 C deriv = B(i) + 2*C(i)*(U-X(i)) + 3*D(i)*(U-X(i))**2
22 C
23 C WHERE X(I) .LT. U .LT. X(I+1), USING HORNER'S RULE
24 C
25 C IF U .LT. X(1) THEN I = 1 IS USED.
26 C IF U .GE. X(N) THEN I = N IS USED.
27 C
28 C INPUT..
29 C
30 C N = THE NUMBER OF DATA POINTS
31 C U = THE ABSCISSA AT WHICH THE SPLINE IS TO BE EVALUATED
32 C X,Y = THE ARRAYS OF DATA ABSCISSAS AND ORDINATES
33 C B,C,D = ARRAYS OF SPLINE COEFFICIENTS COMPUTED BY SPLINE
34 C
35 C IF U IS NOT IN THE SAME INTERVAL AS THE PREVIOUS CALL, THEN A
36 C BINARY SEARCH IS PERFORMED TO DETERMINE THE PROPER INTERVAL.
37 C
38  INTEGER I, J, K, ILIN
39  real*8 dx
40  DATA i/1/
41  DATA ilin/0/
42 C
43  IF ( i .GE. n ) i = 1
44  IF ( u .LT. x(i) ) GO TO 10
45  IF ( u .LE. x(i+1) ) GO TO 30
46 C
47 C BINARY SEARCH
48 C
49  10 i = 1
50  j = n+1
51  20 k = (i+j)/2
52  IF ( u .LT. x(k) ) j = k
53  IF ( u .GE. x(k) ) i = k
54  IF ( j .GT. i+1 ) GO TO 20
55 C
56 C EVALUATE SPLINE
57 C
58  30 dx = u - x(i)
59  IF(ilin.EQ.0) THEN
60  r8seval2 = y(i) + dx*(b(i) + dx*(c(i) + dx*d(i)))
61  deriv = b(i) + dx*(2.0_r8*c(i) + dx*3.0_r8*d(i))
62  ELSE
63  IF(i.EQ.n) THEN
64  zslop=(y(n)-y(n-1))/(x(n)-x(n-1))
65  ELSE
66  zslop=(y(i+1)-y(i))/(x(i+1)-x(i))
67  ENDIF
68  r8seval2=y(i)+dx*zslop
69  deriv=zslop
70  ENDIF
71 C
72  RETURN
73  END
74 C******************** END FILE SEVAL2.FOR ; GROUP TRKRLIB **************
deriv
Definition: mapout_nc.f:94