C ++，输出小数点后一位

Question

I'm new in C++ and would like to get some help. I don't understand why I'm getting an output of only one digit after the decimal on the sum below. I have tried to solve this with no success.

int main()
{
    double alt, t; 
    t = 4.5;
    // function for calculating the altitude over time.
    alt = (-0.12)*pow(t, 4) +(12.0)*pow(t, 3) -(380.0)*pow(t, 2) +(4100.0)*t +220.0;
    cout << alt << endl;
    return 0;
}

Answer 1

The default behaviour of cout is to print six significant digits of floating points. You can change that with:

cout.precision(10);
cout << alt << endl;

which gives the output:

12019.2925

which seems to be the correct solution.

You should not try to set the precision to anything higher than roughly 15, because that is about the precision limit of the double type (typically). You can use the numeric_limits<double>::digits10 from <limits> to make sure what precision you actually have.