Infinite run time - c++

Question

For the code below in Ubuntu 16.04
For custom testcase
4 3
1 2 3 7
My runtime is infinite and there is no output. (No output means that 'a' is also not printed on console). However when I comment the function call foo
// int temp = foo(ab, 0, n-1);
I get output
a##
What can be the reason of infinite run time and no output that I am missing?
#include<bits/stdc++.h>
using namespace std;
int a[1000000];
int foo(int x, int f, int l)
{
int m = (f+l)/2;
while(f<=l)
{
if(a[m]==x)
return 1;
else if(a[m]<x)
f = m+1;
else if(a[m]>x)
l = m-1;
}
return 0;
}
int main()
{
int n, x;
cin >> n;
cin >> x;
int i=0;
for (i = 0; i < n; i++)
{
cin >> a[i];
}
cout<<'a';
sort(a,a+n);
for(i=0; i<n-1; i++)
{
if(a[i]==a[i+1])
{
cout << 0;
return 0;
}
}
cout<<'#';
int ab = a[i];
int temp = foo(ab, 0, n-1);
cout<<'#';
return 0;
}

Answer 1

The reason you're not getting any output (not even the 'a') is that stdout is buffered. Typically it is line-buffered, meaning it will only be printed when a new line is written.
You can force whatever's in the buffer to print out immediately by doing std::cout << std::flush;, or for example in your case, std::cout << 'a' << std::flush; would work.

Answer 2

The reason of infinite loop is that your f and l values are updated at most once. initially m points to the middle of array (i.e. (n-1)/2) and from that moment,
-if the element a[m] is less than x, you infinitely assign m+1 i.e. (n-1)/2 + 1to f
-if the element a[m] is larger than x, you infinitely assign (n-1)/2 - 1 to l
In either case, f never gets larger than l and l never gets smaller than f (unless array has a single element or it has two elements and x<a[0])
Try updating the function in the following way:
int foo(int x, int f, int l)
{
int m = (f+l)/2;
while(f<=l)
{
if(a[m]==x)
return 1;
else if(a[m]<x)
{
f = m+1; m = f;
}
else if(a[m]>x)
{
l = m-1; m = l;
}
}
return 0;
}