Indexing with pointer C/C++ - c

Question

Hey I'm trying to write a program to carry out newtons method and find the roots of the equation exp(-x)-(x^2)+3. It works in so far as finding the root, but I also want it to print out the root after each iteration but I can't get it to work, Could anyone point out my mistake I think its something to do with my indexing?
Thanks a million :)
#include <stdio.h>
#include <math.h>
#include <malloc.h>
//Define Functions:
double evalf(double x)
{
double answer=exp(-x)-(x*x)+3;
return(answer);
}
double evalfprime(double x)
{
double answer=-exp(-x)-2*x;
return(answer);
}
double *newton(double initialrt,double accuracy,double *data)
{
double root[102];
data=root;
int maxit = 0;
root[0] = initialrt;
for (int i=1;i<102;i++)
{
*(data+i)=*(data+i-1)-evalf(*(data+i-1))/evalfprime(*(data+i-1));
if(fabs(*(data+i)-*(data+i-1))<accuracy)
{
maxit=i;
break;
}
maxit=i;
}
if((maxit+1==102)&&(fabs(*(data+maxit)-*(data+maxit-1))>accuracy))
{
printf("\nMax iteration reached, method terminated");
}
else
{
printf("\nMethod successful");
printf("\nNumber of iterations: %d\nRoot Estimate: %lf\n",maxit+1,*(data+maxit));
}
return(data);
}
int main()
{
double root,accuracy;
double *data=(double*)malloc(sizeof(double)*102);
printf("NEWTONS METHOD PROGRAMME:\nEquation: f(x)=exp(-x)-x^2+3=0\nMax No iterations=100\n\nEnter initial root estimate\n>> ");
scanf("%lf",&root);
_flushall();
printf("\nEnter accuracy required:\n>>");
scanf("%lf",&accuracy);
*data= *newton(root,accuracy,data);
printf("Iteration Root Error\n ");
printf("%d %lf \n", 0,*(data));
for(int i=1;i<102;i++)
{
printf("%d %5.5lf %5.5lf\n", i,*(data+i),*(data+i)-*(data+i-1));
if(*(data+i*sizeof(double))-*(data+i*sizeof(double)-1)==0)
{
break;
}
}
getchar();
getchar();
free(data);
return(0);
}

Answer 1

No offenses, but your question is highly seductive to downvoting. Unrelated question title, ridiculous coding style (I mean tabulation).
Also within your newton function there's no actual need to store all the intermediate results, the Newton-Raphson should not use extra memory (i.e. it's O(1)).
Just add printf within your newton inside the iteration loop. Is this a problem?

Answer 2

In newton, you are returning the address of a local variable that doesn't exist anymore after the function returns. Accessing it afterwards is undefined behaviour.
In main you have
if(*(data+i*sizeof(double))-*(data+i*sizeof(double)-1)==0)
data is a double*, so data + i addresses the i-th double from the start. By multiplying the offset with sizeof(double), you access beyond the end of the array if i > number_of_elements/sizeof(double), yet more undefined behaviour.
And, thanks to JeremyP for finding it, in main you call newton
*data= *newton(root,accuracy,data);
which dereferences the pointer returned by newton (undefined behaviour, but at that point likely to do what you want) and stores that value in the first slot of the data allocated in main. Sot that probably gives you the initial element of the root array from newton, but doesn't change anything else in the memory block allocated to data in main.

Answer 3

void newton(double initialrt,double accuracy,double *data)
{
double root[102];
data=root;
// at this moment the values in the original argument data[]
// are no longer accessible to this function.
int maxit = 0;
root[0] = initialrt;
for (int i=1; i < 102; i++)
{
data[i] = data[i-1]
- evalf(data[i-1]) / evalfprime( data[i-1] );
if ( fabs(data[i] - data[i-1]) < accuracy )
{
maxit=i;
break;
}
maxit=i;
}
if ( maxit+1 == 102 && fabs(data[maxit] - data[maxit-1] ) > accuracy )
{
printf("\nMax iteration reached, method terminated");
}
else
{
printf("\nMethod successful");
printf("\nNumber of iterations: %d\nRoot Estimate: %lf\n",maxit+1
,data[maxit);
}
return;
}
(this is merely a stylistic comment, since the real issues have already been addressed)
pointer dereferencing and array indexing are equivalent in C. Your *(data+i) = ... is equivalent to data[i] = ...
the return value is useless, the function could return instead something usefull (eg accuracy)
return is not a function. No need to return(something); just return something; will do.
you return a pointer to the local array root, which is out of scope when the caller sees it.
whitespace makes a difference in reading experience
UPDATE: on second thought, I think in the inner loop the OP intended something like:
root[i] = root[i-1]
- evalf(data[i-1]) / evalfprime( data[i-1] );
if ( fabs(data[i] - data[i-1]) < accuracy )
{
maxit=i;
break;
}
maxit=i;