C++ Walkthrough cout.setf(ios::fixed); and cout.precision();

Question

/* Problem 38 */
        #include <iostream>
        using namespace std;
        class abc {
        double n;
          public:
        abc() { n = 67.5; cout << "1\n"; }
        abc(double num) { set(num); cout << "2\n"; }
        double get() const { cout<<"3\n"; return n; }
        virtual void set(double num) {
            if (num < 10)
            n = 10;
            else if (num > 100)
            n = 100;
            else
            n = num;
            cout << "4\n";
        }
        };
        class def: public abc {
        double m;
          public:
        def() { m = 6.2; cout << "5\n"; }
        def(double num1, double num2): abc(num1) {
            set(num2 - abc::get()); cout << "6\n"; }
        double get() const {
            cout << "7\n"; return m + abc::get(); }
        void set(double num) {
            if (num < 10 || 100 < num)
            m = num;
            else
            m = 55;
            cout << "8\n";
        }
        };
        void do_it(abc &var, double num)
        {   cout << var.get() << '\n';
        var.set(num);
        cout << var.get() << '\n';
        }
        int main()
        {   abc x(45);
        def y(2, 340);
        cout.setf(ios::fixed);
        cout.precision(3);
        do_it(x, 200);
        do_it(y, 253);
        cout << x.get() << '\n';
        cout << y.get() << '\n';
        return 0;
        }

With the above code I just wanted to know what below two lines will really do in the above code

cout.setf(ios::fixed); cout.precision(3);

Please do not just give me answer some explanation would be so appreciated because I'm doing a walkthrough to prepare for my final exam tomorrow.

I searched and some source says it is to set flags but really I don't get what is the concept of it and how it works

Answer 1

cout.setf(ios::fixed)

makes cout print floats with a fixed number of decimals and

cout.precision(3)

sets this number to be three.

For example, if you got a

double f = 2.5;

then

cout << f;

will print

2.500

Answer 2

Great documentation about how to format your output : Output formatting

It's always usefull when you're trying to do a command-line UI.

Answer 3

#include<iostream>

using namespace std;

int main(){
    float large = 2000000000;
    cout << large;
    return 0;
}

The output will be in scientific notation as:

2e+009

In order to get the value as it is you should use cout.setf(ios::fixed)

#include<iostream>

    using namespace std;

    int main(){
        cout.setf(ios::fixed);
        float large = 2000000000;
        cout << large;
        return 0;
    }

The output will be as it is with default precision which is 6 for float.

2000000000.000000

You can set the precision in the above code as:

cout.precision(7);

.....

Answer 4

It's similar to including the manipulation library:
include :

<iomanip>

And then using the following functions

cout << fixed << showpoint;
cout << setprecision(3);