Fibonacci Sequence C program error - c

Question

I am trying to write a program which takes the first 2 numbers of the Fibonacci sequence as input and also the value of n. Then the program gives the output of the nth digit of the Fibonacci sequence.
#include <stdio.h>
#include <stdlib.h>
int main () {
int n, i;
int s[n - 1];
int a, b;
printf("Enter two first two numbers:");
scanf("%d %d", &a, &b);
printf("Enter the value of n(3-100):");
scanf("%d", &n);
for (i = 2; i <= n - 1; i++) {
s[i] = s[i - 1] + s[i - 2];
}
printf("The nth digit is %d", s[n - 1]);
return(0);
}
I am getting the answer number which is followed by some additional arbitrary numbers

Answer 1

Actually to implement your code there is no need of an array s[] .
This can be simply implemented as :-
#include <stdio.h>
#include <stdlib.h>
int main()
{
int n, i;
int a, b;
printf("Enter two first two numbers:");
scanf("%d%d", &a, &b); // not scanf("%d %d", &a, &b);
printf("Enter the value of n(3-100):");
scanf("%d", &n);
for (i = 1; i < n; i++)
{
b += a;
a = b - a;
}
printf("The nth digit is %d\n", a);
return (0);
}
Output:
Enter two first two numbers:0 1
Enter the value of n(3-100):5
The nth digit is 3 // 0 1 1 2 3

Answer 2

Here you define an array of unknown size, being lucky that n does not happen to be 0 or 1 or negative.
int s[n-1];
Here you ignore the return value of scanf, which you really should check to verify success of scanning.
scanf("%d %d",&a,&b);
scanf("%d",&n);
Even assuming a meaningfully defined array, you set a loop up to produce indexes beyond the array here:
for (i=2 ; i<=n-1 ; i++)
And then you write beyond the array (during the last iteration in the loop) here:
s[i]=
With this code all bets are off, you have guaranteed undefined behaviour and therefor any explanation of what exactly goes wrong is futile.

Answer 3

A few things. As mentioned, you are trying to use n before it has been given a value. Also, you should use malloc() when using a variable to determine the array size.
Next, if you are computing the nth sum, then you need the array to have n elements, not n-1
Third, you read in the two starting values, a and b, but you never use them to initialize the first two elements of your array.
And finally, you need to fix your loop indexing. (actually, your indexing is ok once you change the array to have n elements instead of n-1 elements, however, it is certainly preferred to use i < n rather than i <= n-1)
int main() {
int n, i;
int a, b;
printf("Enter two first two numbers:");
scanf("%d %d", &a, &b);
printf("Enter the value of n(3-100):");
scanf("%d", &n);
int *s = malloc(n * sizeof(*s));
s[0] = a;
s[1] = b;
for (i = 2; i < n; i++) {
s[i] = s[i - 1] + s[i - 2];
}
printf("The nth digit is %d", s[n - 1]);
return(0);
}