Q:

C++ round a double up to 2 decimal places

I am having trouble rounding a GPA double to 2 decimal places. (ex of a GPA needed to be rounded: 3.67924) I am currently using ceil to round up, but it currently outputs it as a whole number (368)
here is what I have right now
if (cin >> gpa) {
    if (gpa >= 0 && gpa <= 5) {
           // valid number

           gpa = ceil(gpa * 100);

           break;
    } else {
           cout << "Please enter a valid GPA (0.00 - 5.00)" << endl;
           cout << "GPA: ";

    }
}

using the above code with 3.67924 would output 368 (which is what I want, but just without the period between the whole number and the decimals). How can I fix this?

A:

To round a double up to 2 decimal places, you can use:
#include <iostream>
#include <cmath>

int main() {
    double value = 0.123;
    value = std::ceil(value * 100.0) / 100.0;
    std::cout << value << std::endl; // prints 0.13
    return 0;
}

To round up to n decimal places, you can use:
double round_up(double value, int decimal_places) {
    const double multiplier = std::pow(10.0, decimal_places);
    return std::ceil(value * multiplier) / multiplier;
}

This method won't be particularly fast, if performance becomes an issue you may need another solution.

A:

If it is just a matter of writing to screen then to round the number use 
std::cout.precision(3);
std::cout << gpa << std::endl;

see
floating points are not exactly represented so by internally rounding the value and then using that in your calculations you are increasing the inexactness.

A:

You can't round doubles to two decimal places. Doubles don't have decimal places. They have binary places, and they aren't commensurable with decimal places.
If you want decimal places, you must use a decimal radix, e.g. when formatting for output with printf("%.2f", ...).

