removing a range of elements from an array in c - c

Question

I need to remove a range of elements from an array but I can't figure out how. I tried this for loop where start is the start of the range and end is the end of the range.
int main(void)
{
int n, n2, i, start, end, index;
int a1[n];
int a2[n2];
printf("Enter the length of the array#1: ");
scanf("%d", &n);
printf("Enter the elements of the array#1: ");
for (i = 0; i < n; i++){
scanf("%d", &a1[i]);}
printf("Enter the length of the array#2: ");
scanf("%d", &n2);
printf("Enter the elements of the array#2: ");
for (i = 0; i < n2; i++){
scanf("%d", &a2[i]);}
printf("Enter the start and end indexof array #1 to be removed: ");
scanf("%d %d", &start, &end);
int a3[(end-start)+1];
printf("Enter the position(index)of the array #2 to be added before: ");
scanf("%d", &index);
for (i=0;i < (n - end - 1);i++){
a1[start + i] = a1[end + i + 1];
a1[end + i + 1] = 0;
}
printf("\n");
printf("array1: ");
for (i=0;i < (n);i++){
printf("%d", a1[i]);
printf(" ");
}

Answer 1

You have:
Used n and n2 to give size of the array without initializing those variables first. Though you have not initialized your array but if you did, it would've failed due to VLA.
Not checked the input you are receiving from the user whether the range provided is feasible? What if end - start is greater than the length of the arrays.
Used 0 as a value to replace after that element has moved to a new location/index, what if the number being moved is 0 itself?
For the last point, use some value which has a very less probability of being entered by user like INT_MAX & you'll be needing limits.h header + after that it'll be good to make your for loop look like this:
for(index=0;((index<sizeArr)&&((arr[index]!=INT_MAX)));++index)
This loop will print start - end lesser values from the original array.

Answer 2

Since you want to determine the array size at runtime (via input from the user), you should be using dynamic allocation.
I'm not positive exactly what you are trying to accomplish with this script, but here's an example of what moving to dynamic allocation would look like:
int main(void)
{
int n, n2, i, start, end, index;
int *a1;
int *a2;
printf("Enter the length of the array#1: ");
scanf("%d", &n);
a1 = malloc(sizeof(int)*n);
printf("Enter the elements of the array#1: ");
for (i = 0; i < n; i++){
scanf("%d", &a1[i]);
}
printf("Enter the length of the array#2: ");
scanf("%d", &n2);
a2 = malloc(sizeof(int)*n2);
printf("Enter the elements of the array#2: ");
for (i = 0; i < n2; i++){
scanf("%d", &a2[i]);
}
You can access the values pointed to by a1 using normal array notation, but you should add some bounds checking to ensure the user does not specify an array index outside the bounds of n/n2.