NAG FL Interface
e01baf (dim1_spline)
1
Purpose
e01baf determines a cubic spline interpolant to a given set of data.
2
Specification
Fortran Interface
Integer, Intent (In) |
:: |
m, lck, lwrk |
Integer, Intent (Inout) |
:: |
ifail |
Real (Kind=nag_wp), Intent (In) |
:: |
x(m), y(m) |
Real (Kind=nag_wp), Intent (Out) |
:: |
lamda(lck), c(lck), wrk(lwrk) |
|
C Header Interface
#include <nag.h>
void |
e01baf_ (const Integer *m, const double x[], const double y[], double lamda[], double c[], const Integer *lck, double wrk[], const Integer *lwrk, Integer *ifail) |
|
C++ Header Interface
#include <nag.h> extern "C" {
void |
e01baf_ (const Integer &m, const double x[], const double y[], double lamda[], double c[], const Integer &lck, double wrk[], const Integer &lwrk, Integer &ifail) |
}
|
The routine may be called by the names e01baf or nagf_interp_dim1_spline.
3
Description
e01baf determines a cubic spline
, defined in the range
, which interpolates (passes exactly through) the set of data points
, for
, where
and
. Unlike some other spline interpolation algorithms, derivative end conditions are not imposed. The spline interpolant chosen has
interior knots
, which are set to the values of
respectively. This spline is represented in its B-spline form (see
Cox (1975)):
where
denotes the normalized B-spline of degree
,
defined upon the knots
, and
denotes its coefficient, whose value is to be determined by the routine.
The use of B-splines requires eight additional knots , ,
, ,
, ,
and to be specified; e01baf sets the first four of these to
and the last four to .
The algorithm for determining the coefficients is as described in
Cox (1975) except that
factorization is used instead of
decomposition. The implementation of the algorithm involves setting up appropriate information for the related
routine
e02baf followed by a call of that routine. (See
e02baf for further details.)
Values of the spline interpolant, or of its derivatives or definite integral, can subsequently be computed as detailed in
Section 9.
4
References
Cox M G (1975) An algorithm for spline interpolation J. Inst. Math. Appl. 15 95–108
Cox M G (1977) A survey of numerical methods for data and function approximation The State of the Art in Numerical Analysis (ed D A H Jacobs) 627–668 Academic Press
5
Arguments
-
1:
– Integer
Input
-
On entry: , the number of data points.
Constraint:
.
-
2:
– Real (Kind=nag_wp) array
Input
-
On entry: must be set to , the th data value of the independent variable , for .
Constraint:
, for .
-
3:
– Real (Kind=nag_wp) array
Input
-
On entry: must be set to , the th data value of the dependent variable , for .
-
4:
– Real (Kind=nag_wp) array
Output
-
On exit: the value of
, the th knot, for .
-
5:
– Real (Kind=nag_wp) array
Output
-
On exit: the coefficient
of the B-spline , for . The remaining elements of the array are not used.
-
6:
– Integer
Input
-
On entry: the dimension of the arrays
lamda and
c as declared in the (sub)program from which
e01baf is called.
Constraint:
.
-
7:
– Real (Kind=nag_wp) array
Workspace
-
8:
– Integer
Input
-
On entry: the dimension of the array
wrk as declared in the (sub)program from which
e01baf is called.
Constraint:
.
-
9:
– Integer
Input/Output
-
On entry:
ifail must be set to
,
. If you are unfamiliar with this argument you should refer to
Section 4 in the Introduction to the NAG Library FL Interface for details.
For environments where it might be inappropriate to halt program execution when an error is detected, the value
is recommended. If the output of error messages is undesirable, then the value
is recommended. Otherwise, if you are not familiar with this argument, the recommended value is
.
When the value is used it is essential to test the value of ifail on exit.
On exit:
unless the routine detects an error or a warning has been flagged (see
Section 6).
6
Error Indicators and Warnings
If on entry
or
, explanatory error messages are output on the current error message unit (as defined by
x04aaf).
Errors or warnings detected by the routine:
-
On entry, and .
Constraint: .
On entry,
lwrk is too small.
. Minimum possible dimension:
.
On entry, .
Constraint: .
-
On entry, , and .
Constraint: .
An unexpected error has been triggered by this routine. Please
contact
NAG.
See
Section 7 in the Introduction to the NAG Library FL Interface for further information.
Your licence key may have expired or may not have been installed correctly.
See
Section 8 in the Introduction to the NAG Library FL Interface for further information.
Dynamic memory allocation failed.
See
Section 9 in the Introduction to the NAG Library FL Interface for further information.
7
Accuracy
The rounding errors incurred are such that the computed spline is an exact interpolant for a slightly perturbed set of ordinates . The ratio of the root-mean-square value of the to that of the is no greater than a small multiple of the relative machine precision.
8
Parallelism and Performance
e01baf is not threaded in any implementation.
The time taken by e01baf is approximately proportional to .
All the
are used as knot positions except
and
. This choice of knots
(see
Cox (1977)) means that
is composed of
cubic arcs as follows. If
, there is just a single arc space spanning the whole interval
to
. If
, the first and last arcs span the intervals
to
and
to
respectively. Additionally if
, the
th arc, for
, spans the interval
to
.
After the call
Call e01baf (m, x, y, lamda, c, lck, wrk, lwrk, ifail)
the following operations may be carried out on the interpolant
.
The value of
at
can be provided in the real variable
s by the call
Call e02bbf (m+4, lamda, c, x, s, ifail)
(see
e02bbf).
The values of
and its first three derivatives at
can be provided in the real array
s
of dimension
, by the call
Call e02bcf (m+4, lamda, c, x, left, s, ifail)
(see
e02bcf).
Here
left
must specify whether the left- or right-hand value of the third derivative is required (see
e02bcf for details).
The value of the integral of
over the range
to
can be provided in the
real variable
dint
by
Call e02bdf (m+4, lamda, c, dint, ifail)
(see
e02bdf).
10
Example
This example sets up data from values of the exponential function in the interval to . e01baf is then called to compute a spline interpolant to these data.
The spline is evaluated by
e02bbf, at the data points and at points halfway between each adjacent pair of data points, and the spline values and the values of
are printed out.
10.1
Program Text
10.2
Program Data
None.
10.3
Program Results