2.3 Some Tips on Formatting Real Number Output

When program output contains values of type "float", "double" or "long double", we may wish to restrict the precision with which these values are displayed on the screen, or specify whether the value should be displayed in fixed or floating point form. The following example program uses the library identifier "sqrt" to refer to the square root function, a standard definition of which is given in the header file cmath (or in the old header style math.h). #include
#include
using namespace std;

int main()
{
float number;

cout << "Type in a real number.\n";
cin >> number;
cout.setf(ios::fixed); // LINE 10
cout.precision(2);
cout << "The square root of " << number << " is approximately ";
cout << sqrt(number) << ".\n";

return 0;
}
Program 2.3.1
This produces the output Type in a real number.
200
The square root of 200.00 is approximately 14.14.
whereas replacing line 10 with "cout.setf(ios::scientific)" produces the output: Type in a real number.
200
The square root of 2.00e+02 is approximately 1.41e+01.
We can also include tabbing in the output using a statement such as "cout.width(20)". This specifies that the next item output will have a width of at least 20 characters (with blank space appropriately added if necessary). This is useful in generating tables. However the C++ compiler has a default setting for this member function which makes it right justified. In order to produce output left-justified in a field we need to use some fancy input and output manipulation. The functions and operators which do the manipulation are to be found in the library file iomanip (old header style iomanip.h) and to do left justification we need to set a flag to a different value (i.e. left) using the setiosflags operator: #include
#include
#include
using namespace std;

int main()
{
int number;

cout << setiosflags ( ios :: left );
cout.width(20);
cout << "Number" << "Square Root\n\n";

cout.setf(ios::fixed);
cout.precision(2);
for (number = 1 ; number <= 10 ; number = number + 1) {
cout.width(20);
cout << number << sqrt( (double) number) << "\n";
}

return 0;
}
Program 2.3.2
This program produces the output Number Square Root

1 1.00
2 1.41
3 1.73
4 2.00
5 2.24
6 2.45
7 2.65
8 2.83
9 3.00
10 3.16
(In fact, the above programs work because "cout" is an identifier for an object belonging to the class "stream", and "setf(...)", "precision(...)" and "width(...)" are member functions of "stream". Don't worry too much about this for now - you will learn more about objects, classes and member functions later in the object-oriented part of the course.)