NAG FL Interface
e02bbf (dim1_spline_eval)

Integer, Intent (In)	::	ncap7
Integer, Intent (Inout)	::	ifail
Real (Kind=nag_wp), Intent (In)	::	lamda(ncap7), c(ncap7), x
Real (Kind=nag_wp), Intent (Out)	::	s

C Header Interface

#include <nag.h>

void	e02bbf_ (const Integer ncap7, const double lamda[], const double c[], const double x, double s, Integer ifail)

The routine may be called by the names e02bbf or nagf_fit_dim1_spline_eval.

3 Description

e02bbf evaluates the cubic spline

s (x)

at a prescribed argument

x

from its augmented knot set

λ_{i}

, for

i = 1, 2, \dots, n + 7

, (see e02baf) and from the coefficients

c_{i}

, for

i = 1, 2, \dots, q

in its B-spline representation

s (x) = \sum_{i = 1}^{q} c_{i} N_{i} (x) .

Here

q = \bar{n} + 3

, where

\bar{n}

is the number of intervals of the spline, and

N_{i} (x)

denotes the normalized B-spline of degree

3

defined upon the knots

λ_{i}, λ_{i + 1}, \dots, λ_{i + 4}

. The prescribed argument

x

must satisfy

λ_{4} \leq x \leq λ_{\bar{n} + 4}

It is assumed that

λ_{j} \geq λ_{j - 1}

, for

j = 2, 3, \dots, \bar{n} + 7

, and

λ_{\bar{n} + 4} > λ_{4}

x

is a point at which

4

knots coincide,

s (x)

is discontinuous at

x

; in this case, s contains the value defined as

x

is approached from the right.

The method employed is that of evaluation by taking convex combinations due to de Boor (1972). For further details of the algorithm and its use see Cox (1972) and Cox and Hayes (1973).

It is expected that a common use of e02bbf will be the evaluation of the cubic spline approximations produced by e02baf. A generalization of e02bbf which also forms the derivative of

s (x)

is e02bcf. e02bcf takes about

50 %

longer than e02bbf.

4 References

Cox M G (1972) The numerical evaluation of B-splines J. Inst. Math. Appl. 10 134–149

Cox M G (1978) The numerical evaluation of a spline from its B-spline representation J. Inst. Math. Appl. 21 135–143

Cox M G and Hayes J G (1973) Curve fitting: a guide and suite of algorithms for the non-specialist user NPL Report NAC26 National Physical Laboratory

de Boor C (1972) On calculating with B-splines J. Approx. Theory 6 50–62

5 Arguments

1: $ncap7$ – Integer Input: On entry: $\bar{n} + 7$ , where $\bar{n}$ is the number of intervals (one greater than the number of interior knots, i.e., the knots strictly within the range $λ_{4}$ to $λ_{\bar{n} + 4}$ ) over which the spline is defined.

Constraint: $ncap7 \geq 8$ .
2: $lamda (ncap7)$ – Real (Kind=nag_wp) array Input: On entry: $lamda (j)$ must be set to the value of the $j$ th member of the complete set of knots, $λ_{j}$ , for $j = 1, 2, \dots, \bar{n} + 7$ .

Constraint: the $lamda (j)$ must be in nondecreasing order with $lamda (ncap7 - 3) > lamda (4)$ .
3: $c (ncap7)$ – Real (Kind=nag_wp) array Input: On entry: the coefficient $c_{i}$ of the B-spline $N_{i} (x)$ , for $i = 1, 2, \dots, \bar{n} + 3$ . The remaining elements of the array are not referenced.
4: $x$ – Real (Kind=nag_wp) Input: On entry: the argument $x$ at which the cubic spline is to be evaluated.

Constraint: $lamda (4) \leq x \leq lamda (ncap7 - 3)$ .
5: $s$ – Real (Kind=nag_wp) Output: On exit: the value of the spline, $s (x)$ .
6: $ifail$ – Integer Input/Output: On entry: ifail must be set to $0$ , $−1$ or $1$ to set behaviour on detection of an error; these values have no effect when no error is detected.
A value of $0$ causes the printing of an error message and program execution will be halted; otherwise program execution continues. A value of $−1$ means that an error message is printed while a value of $1$ means that it is not.

If halting is not appropriate, the value $−1$ or $1$ is recommended. If message printing is undesirable, then the value $1$ is recommended. Otherwise, the value $0$ is recommended. When the value $- 1$ or $1$ is used it is essential to test the value of ifail on exit.

On exit: $ifail = 0$ unless the routine detects an error or a warning has been flagged (see Section 6).

6 Error Indicators and Warnings

If on entry

ifail = 0

−1

, explanatory error messages are output on the current error message unit (as defined by x04aaf).

Errors or warnings detected by the routine:

$ifail = 1$: On entry, $x = ⟨ value ⟩$ , $ncap7 = ⟨ value ⟩$ and $lamda (ncap7 - 3) = ⟨ value ⟩$ .
Constraint: $x \leq lamda (ncap7 - 3)$ .

On entry, $x = ⟨ value ⟩$ and $lamda (4) = ⟨ value ⟩$ .
Constraint: $x \geq lamda (4)$ .

$ifail = 2$: On entry, $ncap7 = ⟨ value ⟩$ .
Constraint: $ncap7 \geq 8$ .

$ifail = - 99$: 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.

$ifail = - 399$: 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.

$ifail = - 999$: Dynamic memory allocation failed.
See Section 9 in the Introduction to the NAG Library FL Interface for further information.

7 Accuracy

The computed value of

s (x)

has negligible error in most practical situations. Specifically, this value has an absolute error bounded in modulus by

18 \times c_{\max} \times machine precision

, where

c_{\max}

is the largest in modulus of

c_{j}, c_{j + 1}, c_{j + 2}

and

c_{j + 3}

, and

j

is an integer such that

λ_{j + 3} \leq x \leq λ_{j + 4}

. If

c_{j}, c_{j + 1}, c_{j + 2}

and

c_{j + 3}

are all of the same sign, then the computed value of

s (x)

has a relative error not exceeding

20 \times machine precision

in modulus. For further details see Cox (1978).

8 Parallelism and Performance

e02bbf is not threaded in any implementation.

9 Further Comments

The time taken is approximately

c \times (1 + 0.1 \times \log (\bar{n} + 7))

seconds, where c is a machine-dependent constant.

Note: the routine does not test all the conditions on the knots given in the description of lamda in Section 5, since to do this would result in a computation time approximately linear in

\bar{n} + 7

instead of

\log (\bar{n} + 7)

. All the conditions are tested in e02baf, however.

10 Example

Evaluate at nine equally-spaced points in the interval

1.0 \leq x \leq 9.0

the cubic spline with (augmented) knots

1.0

1.0

1.0

1.0

3.0

6.0

8.0

9.0

9.0

9.0

9.0

and normalized cubic B-spline coefficients

1.0

2.0

4.0

7.0

6.0

4.0

3.0

The example program is written in a general form that will enable a cubic spline with

\bar{n}

intervals, in its normalized cubic B-spline form, to be evaluated at

m

equally-spaced points in the interval

lamda (4) \leq x \leq lamda (\bar{n} + 4)

. The program is self-starting in that any number of datasets may be supplied.

NAG FL Interfacee02bbf (dim1_​spline_​eval)

▸▿ Contents

1 Purpose

2 Specification

3 Description

4 References

5 Arguments

6 Error Indicators and Warnings

7 Accuracy

8 Parallelism and Performance

9 Further Comments

10 Example

10.1 Program Text

10.2 Program Data

10.3 Program Results

NAG FL Interface
e02bbf (dim1_spline_eval)