7-18 finding the single root of polynomial by dichotomy

Input format ：

Enter in 1 The order in the row gives the of the polynomial 4 A coefficient of a3​、a2​、a1​、a0​, In the 2 The end points of the interval are given in order in the row a and b. The problem is to ensure that the polynomial has a unique single root in a given interval .

Output format ：

Output the root of the polynomial in the interval in one line , Accurate to the decimal point 2 position .

sample input ：

3 -1 -3 1
-0.5 0.5

sample output ：

0.33

Code ： 

#include <stdio.h>

float f(float x);
float a,b,c,d;

int main(){
	float x, y;  
    scanf("%f %f %f %f\n%f %f", &a, &b, &c, &d, &x, &y);  
    float left, mid, right;  
    left = x;  
    right = y;  
    // Title pair “ Midpoint f The value is small but not necessarily the root ” There are requirements , So set the threshold to 0.001
	while (left <= right - 0.001 && f(left) * f(right) <= 0)  
    {  
        if(f(left)==0){
			printf("%.2f",left);  
			return 0;
		}
		if(f(right)==0){
			printf("%.2f",right);  
			return 0;
		}
		mid = (left + right) / 2;  
		if(f(mid)*f(left)>0){ // Same number  
			left=mid;
		}else{                // Different sign  
			right=mid;
		}
    }  
    printf("%.2f", mid);  
    return 0;  
}

float f(float x)  
{  
    float result;  
    result = a*x*x*x + b*x*x + c*x + d;  
    return result;  
}