2.2 Completing the basics  (Page 4/6)

where the new_type portion is the keyword representing the type to which you want to cast the variable.

Example:

int iNum = 100;

float fNum;

fNum = float (inum);

If you do not explicitly cast a variable of one data type to another data type, then C++ will try to automatically perform the cast for you.

Program input using the cin object

So far, our programs have been limited in the sense that all their data must be defined within the program source code.

We will now learn how to write programs which enable data to be entered via the keyboard, while the program is running.

Such programs can be made to operate upon different data every time they run, making them much more flexible and useful.

Standard input stream

The cin object reads in information from the keyboard via the standard input stream.

The extraction operator (>>) retrieves information from the input stream.

When the statement cin>>num1; is encountered, the computer stops program execution and accepts data from the keyboard. The user responds by typing an integer (or float) and then pressing the Enter key (sometimes called the Return key) to send the number to the computer. When a data item is typed, the cin object stores the integer (or float) into the variable listed after the>>operator.

The cin and cout stream objects facilitate interaction between the user and the computer. Because this interaction resembles a dialogue, it is often called conversational computing or interactive computing.

Example

#include<iostream.h>

int main()

{

int integer1, integer2, sum; // declaration

cout<<"Enter first integer\n"; // prompt

cin>>integer1; // read an integer

cout<<"Enter second integer\n"; // prompt

cin>>integer2; // read an integer

sum = integer1 + integer2;

cout<<"Sum is "<<sum<<endl;

return 0; // indicate that program ended successfully

}

The output of the above program:

Enter the first integer

45

Enter the second integer

72

Sum is 117

Example

#include<iostream.h>

int main()

{

int integer1, integer2, sum; // declaration

cout<<"Enter two integers\n"; // prompt

cin>>integer1>>integer2; // read two integers

sum = integer1 + integer2;

cout<<"Sum is "<<sum<<endl;

return 0;

}

The output of the above program:

Enter two integers

45 72

Sum is 117

Symbolic constants

C++ introduces the concept of a named constant that is just like a variable, except that its value cannot be changed. The qualifier const tells the compiler that a name represents a constant. Any data type, built-in or user-defined, may be defined as const. If you define something as const and then attempt to modify it, the compiler will generate an error.

To define a constant in a program, we use const declaration qualifier.

Example:

const float PI = 3.1416;

const double SALESTAX = 0.05;

const int MAXNUM = 100;

Once declared, a constant can be used in any C++ statement in place of the number it represents.

Example

// this program calculates the circumference of a circle

// given its radius

#include<iostream.h>

int main()

{

const float PI = 3.1416

float radius, circumference;

radius = 2.0;

circumference = 2.0 * PI * radius;