xchebft.c

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/* Driver for routine chebft */

#include <stdio.h>
#include <math.h>
#define NRANSI
#include "nr.h"

#define NVAL 40
#define PIO2 1.5707963

float func(float x)
{
	return x*x*(x*x-2.0)*sin(x);
}

int main(void)
{
	float a=(-PIO2),b=PIO2,dum,f;
	float t0,t1,term,x,y,c[NVAL];
	int i,j,mval;

	chebft(a,b,c,NVAL,func);
	/* test result */
	for (;;) {
		printf("\nHow many terms in Chebyshev evaluation?\n");
		printf("Enter n between 6 and %2d. (n=0 to end).\n",NVAL);
		scanf("%d",&mval);
		if ((mval <= 0) || (mval > NVAL)) break;
		printf("\n%9s %14s %16s\n","x","actual","chebyshev fit");
		for (i = -8;i<=8;i++) {
			x=i*PIO2/10.0;
			y=(x-0.5*(b+a))/(0.5*(b-a));
			/* Evaluate Chebyshev polynomial without CHEBEV */
			t0=1.0;
			t1=y;
			f=c[1]*t1+c[0]*0.5;
			for (j=2;j<mval;j++) {
				dum=t1;
				t1=2.0*y*t1-t0;
				t0=dum;
				term=c[j]*t1;
				f += term;
			}
			printf("%12.6f %12.6f %12.6f\n",x,func(x),f);
		}
	}
	return 0;
}
#undef NRANSI