Problems with C++ in casting and rounding

The reason for this behaviour is that most floating point numbers can't be represented exactly.

2.09 is such a number. When converting the string to a single precision (float) the stored value is 2.089999914. So the result of the multiplication is 208.9999914. Because casting to int is just truncating, the result is 208.

Even using double precision (double) would not help because that can't store the exact value 2.09 too.

The solution is to use round() as suggested by CPallini or adding 0.5:

float floatResult = userNumber * n;
change = static_cast<int>(floatResult < 0 ? floatResult - 0.5f : floatResult + 0.5f);</int>

But note that the above should only be used when you know that the result will fit in an integer (does not generate an overflow).

if you use setprecision, it shows what has happened. The decimal has been truncated. Use ceil() in math.h to round up.

#include <iostream>
#include <iomanip>
#include <math.h>
using namespace std;

int main ( )
{

float userNumber;
float h,k;
int change = 0;
int n = 100;

cout << "Enter cash: ";

cin >> userNumber;

cout << "\nuserNumber value: " << userNumber << "\n";
h=userNumber*n;

cout << setprecision(10) << h;

k=ceil(userNumber*n);

change = k;

cout << "\nChange after userNumber * 100: " << change << " userNumber: " << userNumber << "\n\n";
}