Unexpected behaviour of Priority_queue size - c++

Question

I was writing the code for the problem. Median of the stream of integers when I encountered an issue. Note that this issue is not the algorithmic but rather ambiguous behavior of the priority_queue size.
#include <bits/stdc++.h>
using namespace std;
priority_queue<double> small;
priority_queue<double, vector<double>, greater<double> > large;
void rebalance()
{
cout << "Initial size\n";
cout << "small " << small.size() << " large " << large.size() << endl;
if (small.size() - large.size()>1)
{
large.push(small.top());
small.pop();
}
else if (large.size() - small.size()>1)
{
cout << "Unexpectedly goes here\n";
cout << "garbage size difference " << large.size() - small.size() << endl;
small.push(large.top());
large.pop();
}
}
void addNum(int num) {
if (small.size() == 0 || num<small.top())
{
small.push(num);
}
else
{
large.push(num);
}
rebalance();
}
double findMedian() {
if (small.size() == large.size())
{
double ans = (small.top() + large.top()) / 2.0;
return ans;
}
else if (small.size()>large.size())
{
return (double)small.top();
}
else
{
return (double)large.top();
}
}
int main()
{
std::ios_base::sync_with_stdio(false);
int num = 5;
addNum(num);
cout << findMedian() << endl;
return 0;
}
The output for this code is
Initial size
small 1 large 0
Unexpectedly goes here
garbage size difference 18446744073709551615
fish: “./a.out” terminated by signal SIGSEGV (Address boundary error)
In the rebalance function the initial size of small is 1 and large is 0 which suggest that the loop should neither enter the if condition nor the else if condition but the loop enters the else if condition with a garbage value in size.why does this happen? Moreover I tried saving the small and large size in an integer variable and then comparing them in conditionals,which lead to acceptance of the code. Hence the algorithm handles the correctness.
What leads to this garbage value?

Answer 1

In
else if(large.size()-small.size()>1)
size() returns an unsigned number. A unsigned number can never be negative so if would be a negative number it wraps around to the largest number it could be and then goes backwards from there. Since large has a size of 0 and small has a size of 1 then 0 - 1 gives you 18446744073709551615. I believe what you are trying to do should be expressed as
if(small.size() < large.size())
{
small.push(large.top());
large.pop():
}
else if(large.size() < small.size())
{
large.push(small.top());
small.pop();
}