Solve segmentation fault error in C - c

Question

I am trying to store the sum 1/k^2 in an array using this code:
int main()
{
int i,n, e;
double terms[n];
double x, y;
printf(" Introduce n\n");
scanf("%d", &n);
y=1;
i=0;
while(e<n)
{
x=1/((y)*(y));
printf("x is %.16lf\n",x);
terms[i]=x;
printf("terms is %.16lf\n", terms[i]);
y++;
i++;
e++;
}
}
And I get the "segmentation fault" error. Why is this happening? How can I avoid it?
I am programming in C

Answer 1

n is garbage in double terms[n]; that causes undefined behaviour. Enable compiler warning by compiling with gcc -Wall and observe. Never ignore warnings.
Initialize n before declaring terms[n] OR
printf(" Introduce n\n");
scanf("%d", &n);
double terms[n];
Also e is uninitialized, initialize it.
e = 0 ;
while (e < n){
/* code */
}

Answer 2

When declaring an array with n elements (double terms[n];), the allocation of said array is done in the compilation stage. Because you left your n variable un-initialized, it has indeterminate value (random from the user perspective), so you don't know what is the size of said array.
Scanning an int into n later, does not help at all as it is done in the run-time (and also, it is done AFTER the array declaration. It could have worked if you have used malloc for the allocation AFTER the scanf).
Anyway, you currently have an array with "random" size, and accessing it is Undefined Behavior -> segfault

Answer 3

The crash comes from undefined size of the terms[n] array. You are crossing array memory boundaries.
A few variables are not initialized:
double terms[n]; // n is not initialized
while (e<n){ // e is not initialized
You have many choices to properly build terms array (marked in the code as well):
1) Decide upfront on the specific size of the array. (This is not flexible approach!)
2) Allocate array dynamically when you read n.
3) Declare terms[n] after reading n.
Test program:
#include <stdio.h>
#include <stdlib.h>
int main(){
int i;
int n;
int e = 0;
// 1. One of the 3 choices - thus is a rigid one
// double terms[TERMS_SIZE]; // make TERMS_SIZE big enough, read n has to be less than TERMS_SIZE
double x, y;
printf(" Introduce n\n");
scanf("%d", &n);
// 2.
// double *terms = malloc ( n* sizeof sizeof(double));
// or
// 3.
double terms[n];
y=1;
i=0;
while (e<n){
x=1/((y)*(y));
printf("x is %.16lf\n",x);
terms[i]=x;
printf("terms is %.16lf\n",terms[i]);
y++;
i++;
e++;
}
// If 2. used free the memory
// free(terms);
return 0;
}
Output:
4
Introduce n
x is 1.0000000000000000
terms is 1.0000000000000000
x is 0.2500000000000000
terms is 0.2500000000000000
x is 0.1111111111111111
terms is 0.1111111111111111
x is 0.0625000000000000
terms is 0.0625000000000000