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#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#define EPSILON 1E-9
double
eval_poly(double x, double coeffs[], int order)
{
double res;
int i;
res = coeffs[0];
for ( i = 1; i <= order; i++ ) {
res = res * x + coeffs[i];
}
return res;
}
double
find_zero(double left, double right, double coeffs[], int order)
{
double c, lp, rp, cp, eps=EPSILON;
if(eval_poly(left, coeffs, order) * eval_poly(right, coeffs, order) >= 0) {
exit(1);
}
while(1) {
c = (left+right)/2;
lp = eval_poly(left, coeffs, order);
rp = eval_poly(right, coeffs, order);
cp = eval_poly(c, coeffs, order);
if(fabs(cp) < eps) {
break;
}
if(cp * lp < 0) {
right = c;
}
else if(cp * rp < 0) {
left = c;
}
else {
exit(1);
}
}
return c;
}
int
main(void)
{
double coeffs[] = { 7, 5, 3, 2, 1, 1 };
printf("eval_poly: %f\n", eval_poly(3, coeffs, 5));
printf("%f\n", find_zero(-100, 100, coeffs, 5));
return 0;
}
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