Doubt related to Two Pointer Approach - Pair Sum Problem

Is it necessary to define the elements of sorted array in the code itself? Can’t we provide the inputs from console?
When I am trying to write the code for that , it’s not working. Please let me know, where am I going wrong?

#include
using namespace std;
int main()
{

int n;
cin>>n;
int a[1000];
for(int i=0;i<n;i++)
{
	cin>>a[i];
}
int s;
cin>>s;

int i=0;
int j=sizeof(a)/sizeof(int)-1;

while(i<j)
{
	int c_sum=a[i]+a[j];
	if(c_sum>s)
	{
		j--;
	}
	else if(c_sum<s)
	{
		i++;
	}
	else if(c_sum==s)
	{
		cout<<a[i]<<" and"<<a[j]<<endl;
		i++;
		j--;
	}
}
return 0;

}