c - EQUATION SOLVER absurd result despite correct algorithm -

#include<stdio.h> #include<math.h> #include<stdlib.h> void bisect(float *p,int n,int a); float value(float *p,int n,int a); int main() {     int a,i;     float *p;     printf("enter degree of polynomial\n");     scanf("%d",&a);     p=(float *) malloc(a*sizeof(float));     for(i=0;i<=a;i++)     {         printf("enter coefficient of x^%d\n",i);         scanf("%f",p+i);     }     printf("%f\n",value(p,-2,a));     printf("%f\n",value(p,1,a));     printf("%f\n",value(p,0,a));     for(i=-100;i<100;i++)     {         if(value(p,i,a)*value(p,i+1,a)==0.000)         {             printf("%d\n",value(p,i+1,a));             if(value(p,i,a)==0&&value(p,i+1,a)==0.00)             {                 printf("the roots %d,%d\n",i,i+1);             }             if(value(p,i+1,a)==0.0)             {                 printf("the real root %d\n",i+1);                 i++;                 continue;             }         }          if(value(p,i,a)*value(p,i+1,a)<0)         {             bisect(p,i,a);         }     }     return 0; } float value(float *p,int n,int a) {     float sum=0.0;     int i;     for(i=0;i<=a;i++)     {         sum=sum+*(p+i)*pow(n,i);     }     return sum; }  void bisect(float *p,int n,int a) {     float j,k,l;     int i;     j=n;k=n+1;l=(j+k)/2;     for(i=0;i<50;i++)     {         if(value(p,j,a)*value(p,l,a)==0){break;}         if(value(p,j,a)*value(p,l,a)<0)         {             j=j;k=l;l=(j+k)/2;         }         else if(value(p,l,a)*value(p,k,a)<0)         {             l=(l+k)/2;j=l;         }      }     printf("the root of equation %f\n",l); } 

i tried inserting print statements in main function, , found value function giving absurd results simple polynomials, roots correct polynomials wrong many. why roots correct if algorithm wrong?

there many problems in code:

1. in main method,

printf("%d\n",value(p,-2,a));

compiler should give warning:

warning: format '%d' expects argument of type 'int', argument 2 has type 'double'

use %f instead of %d value() returns float.

2. you not allocating enough space and casting return value of malloc. casting return value of malloc should avoided since malloc returns void * (which means needs no cast) , casting return value can conceal errors. can read more issue here.

p = (float *) malloc(a*sizeof(float));

to

p=malloc((a+1) * sizeof *p);

3. you comparing floating ponit number here:

   if(value(p,i,a)==0&&value(p,i+1,a)==0.00)

don't this, read what every computer scientist should know floating-point arithmetic reason. can use 1 of (e.g nearlyequal()) functions purpose.

4. in method bisect():

   j=j;k=l;l=(j+k)/2; // j = j, kidding?