QwtSpline

# QwtSpline

Section: Qwt User's Guide (3) Updated: 16 Nov 2004
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QwtSpline -

## Detailed Description

A class for spline interpolation.

The QwtSpline class is used for cubical spline interpolation. Two types of splines, natural and periodic, are supported.

Usage:.RS 4

1.
First call QwtSpline::recalc() to determine the spline coefficients for a tabulated function y(x).
2.
After the coefficients have been set up, the interpolated function value for an argument x can be determined by calling QwtSpline::value().

In order to save storage space, QwtSpline can be advised not to buffer the contents of x and y. This means that the arrays have to remain valid and unchanged for the interpolation to work properly. This can be achieved by calling QwtSpline::copyValues().

Example:.RS 4

```#include<qwt_spline.h>
#include<iostream.h>

QwtSpline s;
double x[30], y[30], xInter[300], yInter[300];
int i;

for(i=0;i<30;i++)               // fill up x[] and y[]
cin >> x[i] >> y[i];

if (s.recalc(x,y,30,0) == 0)    // build natural spline
{
for(i=0;i<300;i++)          // interpolate
{
xInter[i] = x[0] + double(i) * (x[29] - x[0]) / 299.0;
yInter[i] = s.value( xInter[i] );
}

do_something(xInter, yInter, 300);
}
else
cerr << 'Uhhh...;

```

Definition at line 63 of file qwt_spline.h.

### Public Member Functions

QwtSpline ()

~QwtSpline ()

double value (double x) const

int recalc (double *x, double *y, int n, int periodic=0)

int recalc (const QwtArray< double > &x, const QwtArray< double > &y, int periodic=0)

void copyValues (int tf=1)

## Constructor & Destructor Documentation

### QwtSpline::QwtSpline ()

CTOR.

Definition at line 31 of file qwt_spline.cpp.

### QwtSpline::~QwtSpline ()

DTOR.

Definition at line 68 of file qwt_spline.cpp.

## Member Function Documentation

### void QwtSpline::copyValues (int tf = 1)

Advise recalc() to buffer the tabulated function or switch off internal buffering.

By default, QwtSpline maintains an internal copy of the tabulated function given as *x and *y arguments of QwtSpline::recalc().

If QwtSpline::copyValues() is called with zero argument, subsequent calls to QwtSpline::recalc() will not buffer these values anymore and just store the pointers instead. The QwtSpline::value() function will then silently assume that these pointers remained valid and that the contents of the arrays have not been changed since the last QwtSpline::recalc().

If called with no or nonzero argument, any following QwtSpline::recalc() calls will use the internal buffer.

Parameters:

tf if nonzero, the function table will be buffered

Warning:

copyValues() resets all the contents of QwtSpline. A subsequent recalc() will be necessary.

Definition at line 61 of file qwt_spline.cpp.

### int QwtSpline::recalc (const QwtArray< double > & x, const QwtArray< double > & y, int periodic = 0)

re-calculate the spline coefficients

Depending on the value of periodic, this function will determine the coefficients for a natural or a periodic spline and store them internally. It also buffers the values of x and y, which are needed for the interpolation (See QwtSpline::value()).

Parameters:

x QwtArray<double> of points
y QwtArray<double> of points
periodic if non-zero, calculate periodic spline

Returns:

0
successful
Qwt::ErrMono<dd>Sequence of x values is not strictly monotone increasing
Qwt::ErrNoMem<dd>Couldn't allocate memory

Warning:

The sequence of x (but not y) values has to be strictly monotone increasing, which means x[0] < x[1] < .... < x[n-1]. If this is not the case, the function will return an error.

Definition at line 206 of file qwt_spline.cpp.

References recalc().

### int QwtSpline::recalc (double * x, double * y, int n, int periodic = 0)

re-calculate the spline coefficients

Depending on the value of periodic, this function will determine the coefficients for a natural or a periodic spline and store them internally. By default, it also buffers the values of x and y, which are needed for the interpolation (See QwtSpline::value()). In order to save memory, this last behaviour may be changed with the QwtSpline::copyValues() function.

Parameters:

x .br y points
n number of points
periodic if non-zero, calculate periodic spline

Returns:

0
successful
Qwt::ErrMono<dd>Sequence of x values is not strictly monotone increasing
Qwt::ErrNoMem<dd>Couldn't allocate memory

Warning:

The sequence of x (but not y) values has to be strictly monotone increasing, which means x[0] < x[1] < .... < x[n-1]. If this is not the case, the function will return an error.

Definition at line 127 of file qwt_spline.cpp.

Referenced by QwtCurve::drawSpline(), and recalc().

### double QwtSpline::value (double x) const

Calculate the interpolated function value corresponding to a given argument x. Definition at line 18 of file qwt_spline.cpp.

Referenced by QwtCurve::drawSpline().

## Author

Generated automatically by Doxygen for Qwt User's Guide from the source code.

## Index

NAME
SYNOPSIS
Detailed Description
Public Member Functions
Constructor & Destructor Documentation
QwtSpline::QwtSpline ()
QwtSpline::~QwtSpline ()
Member Function Documentation
void QwtSpline::copyValues (int tf = 1)
int QwtSpline::recalc (const QwtArray< double > & x, const QwtArray< double > & y, int periodic = 0)
int QwtSpline::recalc (double * x, double * y, int n, int periodic = 0)
double QwtSpline::value (double x) const
Author

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Time: 21:53:53 GMT, April 16, 2011