Variables

 

 Variables

Sample program

// Definition and use of variables
#include <iostream>
using namespace std;
   
int gVar1;                 // Global variables,
int gVar2 = 2;             // explicit initialization
   
int main()
{
   char ch('A');  // Local variable being initialized
                  // or:  char ch = 'A';
   
   cout << "Value of gVar1:    " << gVar1  << endl;
   cout << "Value of gVar2:    " << gVar2  << endl;
   cout << "Character in ch:   " << ch     << endl;
   
   int sum, number = 3; // Local variables with
                        // and without initialization
   sum = number + 5;
   cout << "Value of sum:      " << sum  << endl;
   
   return 0;
} 

HINT

Both strings and all other values of fundamental types can be output with cout. Integers are printed in decimal format by default.

Screen output

Value of gVar1:  0 
Value of gVar2:  2 
Character in ch: A 
Value of sum:    8 

Data such as numbers, characters, or even complete records are stored in variables to enable their processing by a program. Variables are also referred to as objects, particularly if they belong to a class.

Defining Variables

A variable must be defined before you can use it in a program. When you define a variable the type is specified and an appropriate amount of memory reserved. This memory space is addressed by reference to the name of the variable. A simple definition has the following syntax:

Syntax:

typ name1 [name2 ... ]; 

This defines the names of the variables in the list name1 [, name2 ...] as variables of the type type. The parentheses [ ... ] in the syntax description indicate that this part is optional and can be omitted. Thus, one or more variables can be stated within a single definition.

Examples:

char c;
int i, counter;
double x, y, size; 

In a program, variables can be defined either within the program's functions or outside of them. This has the following effect:

• a variable defined outside of each function is global, i.e. it can be used by all functions
• a variable defined within a function is local, i.e. it can be used only in that function.

Local variables are normally defined immediately after the first brace—for example at the beginning of a function. However, they can be defined wherever a statement is permitted. This means that variables can be defined immediately before they are used by the program.

Initialization

A variable can be initialized, i.e. a value can be assigned to the variable, during its definition. Initialization is achieved by placing the following immediately after the name of the variable:

• an equals sign ( = ) and an initial value for the variable or
• round brackets containing the value of the variable.

Examples:

char c = 'a';
float x(1.875); 

Any global variables not explicitly initialized default to zero. In contrast, the initial value for any local variables that you fail to initialize will have an undefined initial value.