[SOLVED] Comparing int64 and float value does not work inside loop

Comparing int64 and float value does not work inside loop

The following loop would run only when off which is uint64_t is less than the value returned by ceil which is double. However I don't see the loop being executed.

#include <bits/stdc++.h>
using namespace std;

int main()
{
    uint64_t offset = 1164226701485, size = 945, div = 10000;
    for (uint64_t off = offset / div; off < ceil((float)(offset + size) / div); off++)
    {
        cout<<off;
    }
                            
    return 0;
}

SO I tried printing those values which seems correct to me and the loop should have been executed atleast once

cout.precision(17);
cout<< offset / div<<" "<< ceil((float)(offset + size) / div);

Output:

116422670 116422672

I am not really sure what's happening here, how can I make the loop to execute?

Solution

Problem is this expression:

off < ceil((float)(offset + size) / div)

note that on one side of this comparation you have uint64_t on other float.

Before compare can be done common type has to be selected. It is a float.

So your value on the left: 116422670 is converted to float. Since float has only 6 significant digits, result is rounded to nearest value which can be represented in float.

It happens to be: 116422672f.

Demo: https://godbolt.org/z/xhEnMKE8e

It is hard to spot rounding issue when watching decimal value so take a look on this: https://godbolt.org/z/TTEqvx1vG