[c++] C++: How to round a double to an int?

I have a double (call it x), meant to be 55 but in actuality stored as 54.999999999999943157 which I just realised.

So when I do

double x = 54.999999999999943157;
int y = (int) x;

y = 54 instead of 55!

This puzzled me for a long time. How do I get it to correctly round?

Casting is not a mathematical operation and doesn't behave as such. Try

int y = (int)round(x);

It is worth noting that what you're doing isn't rounding, it's casting. Casting using (int) x truncates the decimal value of x. As in your example, if x = 3.9995, the .9995 gets truncated and x = 3.

As proposed by many others, one solution is to add 0.5 to x, and then cast.

Casting to an int truncates the value. Adding 0.5 causes it to do proper rounding.

int y = (int)(x + 0.5);

#include <iostream>
#include <cmath>
using namespace std;

int main()
{
    double x=54.999999999999943157;
    int y=ceil(x);//The ceil() function returns the smallest integer no less than x
    return 0;
}