Imprecise average at run time - c

Question

I am trying to solve a problem and I've run into a bit of an issue.
I have to find the running average of a series of numbers.
Example:
input 4 2 7
output 4 3 4.3333
Now here's the problem although I get the answer, it is not the precise answer.
Accepted Output: accuracy difference shown in the image
290.6666666667
385.4000000000
487.8333333333
477.4285714286
496.4444444444
...
523.8571166992
506.0454406738
495.3043518066
I can't find whats wrong. Some help would be highly appreciated.
#include<stdio.h>
main(){
int n;
printf("set:");
scanf("%d",&n);
float arr[n+1],resarr[n+1];
float sum=0;
for(int i=1; i<=n; i++){
scanf("%f",&arr[i]);
sum=arr[i]+sum;
float res= sum/(float)i;
resarr[i]=res;
}
int i=1;
while(i<=n) {
printf("%0.10f\n",resarr[i]);
i++;
}
return 0;
}

Answer 1

Here
for(int i=1; i<=n; i++){ }
you are trying to access out of bound array elements, this certainly causes undefined behavior as let's assume if n is 5 then you are accessing arr[5] also which doesn't exist.
C doesn't perform array boundary condition check, its programmer responsibility to not to access out of bound elements else it causes UB.
In C array index starts from 0 not from 1. So better start rotating loop from 0 to n. For e.g
for(int i=0; i<n; i++) {
scanf("%f",&arr[i]);
/* some code */
}

Answer 2

Code fails to achieve the desired accuracy as it is using float rather than double. #Some programmer dude
Typical float is precise to 1 part in 223. For printing to 0.0000000001, better to use double which is typically precise to 1 part in 253.
#include<stdio.h>
int main(void) {
//float arr[n + 1], resarr[n + 1];
//float sum = 0;
double arr[n + 1], resarr[n + 1];
double sum = 0;
...
// scanf("%f", &arr[i]);
scanf("%lf", &arr[i]);
...
// float res = sum / (float) i;
double res = sum / i; // cast not needed as `sum` is `double`
...
}
Iterating from 1 is not idiomatic in C. More common to iterate starting at 0.
size_t is best for array sizing and indexing. int may be too narrow. Of course with small arrays, it makes scant difference.
#include<stdio.h>
int main(void) {
printf("set:");
size_t n;
scanf("%zu", &n);
double arr[n], resarr[n];
double sum = 0;
for (size_t i = 0; i < n; i++) {
scanf("%lf", &arr[i]);
sum = arr[i] + sum;
double res = sum / (i+1);
resarr[i] = res;
}
for (size_t i = 0; i < n; i++) {
printf("%0.10f\n", resarr[i]);
}
return 0;
}
More robust code would check the input from the user it insure it is valid, allocate rather than use a VLA if n is allowed to be large, flush output before reading, etc.
Note that array arr[] is not needed, just a single double for the input and sum.