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QwtSpline Class Reference

A class for spline interpolation. More...

#include <qwt_spline.h>

List of all members.

Public Methods

 QwtSpline ()
 ~QwtSpline ()
double value (double x) const
int recalc (double *x, double *y, int n, int periodic=0)
void copyValues (int tf=1)


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:
  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:
#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...\n";


Constructor & Destructor Documentation

QwtSpline::QwtSpline  
 

CTOR.

QwtSpline::~QwtSpline  
 

DTOR.


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.

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 
y  points
n  number of points
periodic  if non-zero, calculate periodic spline
Returns:
0
successful
Qwt::ErrMono
Sequence of x values is not strictly monotone increasing
Qwt::ErrNoMem
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.

double QwtSpline::value double    x const
 

Calculate the interpolated function value corresponding to a given argument x.


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