How to properly set precision for doubles in C++

Question

I'm working on a project where I need to do some math and give the user output with dollars in it, so I would like to have my console tell the user an answer like $20.15 instead of $20.153. I used the set precision function as such: cout << setprecision(2);, but rather than have the numbers become what I want them to be, they are converted into scientific notation.

I'm outputting a lot of numbers, so having a function like setprecision would be best for me for ease of use.

How do I properly have the numbers displayed with only two decimal places and not have the console give me numbers in scientific notation?

Thanks

Nathan

EDIT:

Here is the part of my code I'm having problems with:

int main() {

cout << setprecision(2);

if (totalCostHybrid < totalCostNonHybrid) {
            cout << "Hybrid car: " << endl;
            cout << "Total cost: " << totalCostHybrid << endl;
            cout << "Total gallons used: " << milesPerYear / hybridEffic << endl;
            cout << "Total gas cost: " << gasCostHybrid << endl;
            cout << "Non-hybrid car: " << endl;
            cout << "Total cost: " << totalCostNonHybrid << endl;
            cout << "Total gallons used: " << milesPerYear / nonHybridEffic << endl;
            cout << "Total gas cost: " << gasCostNonHybrid << endl;
            cout << "Hybrid is cheaper!" << endl;
}

Obviously there's more to it, but this is what I need help with.

Answer 1

To fix that, you should use fixed floating-point notation for cout. You can find more info here.

Try addind cout << fixed to your code, like the code below. To set the precision to 2, you can use the precision property.

cout << fixed;
cout.precision(2);

Here is the complete code:

using namespace std;

int main() {

    cout << fixed;
    cout.precision(2);

    if (totalCostHybrid < totalCostNonHybrid) {
            cout << "Hybrid car: " << endl;
            cout << "Total cost: " << totalCostHybrid << endl;
            cout << "Total gallons used: " << milesPerYear / hybridEffic << endl;
            cout << "Total gas cost: " << gasCostHybrid << endl;
            cout << "Non-hybrid car: " << endl;
            cout << "Total cost: " << totalCostNonHybrid << endl;
            cout << "Total gallons used: " << milesPerYear / nonHybridEffic << endl;
            cout << "Total gas cost: " << gasCostNonHybrid << endl;
            cout << "Hybrid is cheaper!" << endl;
     }
}

Answer 2

Iostreams are a pain for formatting floating-point values. But why are you using floating-point to represent currency values? You should store integer pennies (or tenth-pennies) because, though you're not measuring in whole numbers of dollars, your values are actually fixed-point. And you really don't need the trouble that floating-point brings. And then you can stream the whole and "fractional" parts of your value separately (use / and %!), as integers, with a '.' in the middle.

In the meantime, try std::fixed.

Answer 3

Cheat and watch purists go crazy...

    double time;  //Only want two decimal places.
    double timeCon = time * 100.0;  //Pull out the two decimals I want.
    int timeCut = timeCon;  //Cut all decimal values.
    double timeRevert = timeCut / 100.0;  //Laugh.
    cout << timeRevert << endl;  //Watch heads explode.