E01 Chapter Contents
E01 Chapter Introduction
NAG Library Manual

# NAG Library Routine DocumentE01TLF

Note:  before using this routine, please read the Users' Note for your implementation to check the interpretation of bold italicised terms and other implementation-dependent details.

## 1  Purpose

E01TLF evaluates the four-dimensional interpolating function generated by E01TKF and its first partial derivatives.

## 2  Specification

 SUBROUTINE E01TLF ( M, X, F, IQ, RQ, N, XE, Q, QX, IFAIL)
 INTEGER M, IQ(2*M+1), N, IFAIL REAL (KIND=nag_wp) X(4,M), F(M), RQ(15*M+9), XE(4,N), Q(N), QX(4,N)

## 3  Description

E01TLF takes as input the interpolant $Q\left(\mathbf{x}\right)$, $x\in {ℝ}^{4}$ of a set of scattered data points $\left({\mathbf{x}}_{\mathit{r}},{f}_{\mathit{r}}\right)$, for $\mathit{r}=1,2,\dots ,m$, as computed by E01TKF, and evaluates the interpolant and its first partial derivatives at the set of points ${\mathbf{x}}_{i}$, for $\mathit{i}=1,2,\dots ,n$.
E01TLF must only be called after a call to E01TKF.
E01TLF is derived from the new implementation of QS3GRD described by Renka (1988). It uses the modification for high-dimensional interpolation described by Berry and Minser (1999).
Berry M W, Minser K S (1999) Algorithm 798: high-dimensional interpolation using the modified Shepard method ACM Trans. Math. Software 25 353–366
Renka R J (1988) Algorithm 661: QSHEP3D: Quadratic Shepard method for trivariate interpolation of scattered data ACM Trans. Math. Software 14 151–152

## 5  Parameters

1:     M – INTEGERInput
On entry: must be the same value supplied for parameter M in the preceding call to E01TKF.
Constraint: ${\mathbf{M}}\ge 16$.
2:     X($4$,M) – REAL (KIND=nag_wp) arrayInput
Note: the coordinates of ${x}_{r}$ are stored in ${\mathbf{X}}\left(1,r\right)\dots {\mathbf{X}}\left(4,r\right)$.
On entry: must be the same array supplied as parameter X in the preceding call to E01TKF. It must remain unchanged between calls.
3:     F(M) – REAL (KIND=nag_wp) arrayInput
On entry: must be the same array supplied as parameter F in the preceding call to E01TKF. It must remain unchanged between calls.
4:     IQ($2×{\mathbf{M}}+1$) – INTEGER arrayInput
On entry: must be the same array returned as parameter IQ in the preceding call to E01TKF. It must remain unchanged between calls.
5:     RQ($15×{\mathbf{M}}+9$) – REAL (KIND=nag_wp) arrayInput
On entry: must be the same array returned as parameter RQ in the preceding call to E01TKF. It must remain unchanged between calls.
6:     N – INTEGERInput
On entry: $n$, the number of evaluation points.
Constraint: ${\mathbf{N}}\ge 1$.
7:     XE($4$,N) – REAL (KIND=nag_wp) arrayInput
On entry: ${\mathbf{XE}}\left(1:4,\mathit{i}\right)$ must be set to the evaluation point ${\mathbf{x}}_{\mathit{i}}$ , for $\mathit{i}=1,2,\dots ,n$.
8:     Q(N) – REAL (KIND=nag_wp) arrayOutput
On exit: ${\mathbf{Q}}\left(\mathit{i}\right)$ contains the value of the interpolant, at ${\mathbf{x}}_{\mathit{i}}$, for $\mathit{i}=1,2,\dots ,n$. If any of these evaluation points lie outside the region of definition of the interpolant the corresponding entries in Q are set to the largest machine representable number (see X02ALF), and E01TLF returns with ${\mathbf{IFAIL}}={\mathbf{3}}$.
9:     QX($4$,N) – REAL (KIND=nag_wp) arrayOutput
On exit: ${\mathbf{QX}}\left(j,i\right)$ contains the value of the partial derivatives with respect to ${\mathbf{x}}_{j}$ of the interpolant $Q\left(\mathbf{x}\right)$ at ${\mathbf{x}}_{\mathit{i}}$, for $\mathit{i}=1,2,\dots ,n$, and for each of the four partial derivatives $j=1,2,3,4$. If any of these evaluation points lie outside the region of definition of the interpolant, the corresponding entries in QX are set to the largest machine representable number (see X02ALF), and E01TLF returns with ${\mathbf{IFAIL}}={\mathbf{3}}$.
10:   IFAIL – INTEGERInput/Output
On entry: IFAIL must be set to $0$, $-1\text{​ or ​}1$. If you are unfamiliar with this parameter you should refer to Section 3.3 in the Essential Introduction for details.
For environments where it might be inappropriate to halt program execution when an error is detected, the value $-1\text{​ or ​}1$ is recommended. If the output of error messages is undesirable, then the value $1$ is recommended. Otherwise, if you are not familiar with this parameter, the recommended value is $0$. When the value $-\mathbf{1}\text{​ or ​}\mathbf{1}$ is used it is essential to test the value of IFAIL on exit.
On exit: ${\mathbf{IFAIL}}={\mathbf{0}}$ unless the routine detects an error or a warning has been flagged (see Section 6).

## 6  Error Indicators and Warnings

If on entry ${\mathbf{IFAIL}}={\mathbf{0}}$ or $-{\mathbf{1}}$, explanatory error messages are output on the current error message unit (as defined by X04AAF).
Errors or warnings detected by the routine:
${\mathbf{IFAIL}}=1$
 On entry, ${\mathbf{M}}<16$, or ${\mathbf{N}}<1$.
${\mathbf{IFAIL}}=2$
Values supplied in IQ or RQ appear to be invalid. Check that these arrays have not been corrupted between the calls to E01TKF and E01TLF.
${\mathbf{IFAIL}}=3$
At least one evaluation point lies outside the region of definition of the interpolant. At all such points the corresponding values in Q and QX have been set to the largest machine representable number (see X02ALF).

## 7  Accuracy

Computational errors should be negligible in most practical situations.

The time taken for a call to E01TLF will depend in general on the distribution of the data points. If the data points are approximately uniformly distributed, then the time taken should be only $\mathit{O}\left(n\right)$. At worst $\mathit{O}\left(mn\right)$ time will be required.

## 9  Example

This program evaluates the function
 $f x = 1.25 + cos5.4x4 cos6x1 cos6x2 6 + 6 3 x3 - 1 2$
at a set of $30$ randomly generated data points and calls E01TKF to construct an interpolating function $Q\left(\mathbf{x}\right)$. It then calls E01TLF to evaluate the interpolant at a set of random points.
To reduce the time taken by this example, the number of data points is limited to $30$. Increasing this value improves the interpolation accuracy at the expense of more time.

### 9.1  Program Text

Program Text (e01tlfe.f90)

### 9.2  Program Data

Program Data (e01tlfe.d)

### 9.3  Program Results

Program Results (e01tlfe.r)