A variable is a named location in memory. Variables are used to give a value a name so we can refer to it later.
The name of a variable is an identifier token. Here are rules for declaring a variable:
- Identifiers may contain numbers, letters, and underscores(_).
- Variable name can not start with a number.
- Keywords can't be used as variable name.
The following table lists the keywords in C++.
| alignas | alignof | and | and_eq |
| asm | bitor | bool | bitand |
| catch | char16_t | class | compl |
| constexpr | const_cast | decltype | nullptr |
| auto | double | int | struct |
| break | else | long | switch |
| case | enum | register | typedef |
| char | extern | return | union |
| const | float | short | unsigned |
| continue | for | signed | void |
| default | goto | sizeof | volatile |
| do | if | static | while |
Table: Keywords in C++
All keywords in C++ are here.
Variable Declaration and Initialization
You must declare all variables before they can be used. The basic form of a variable declaration is shown here:
data_type variable [ = value][, variable [= value] …] ;
Here data_type is one of C++'s data types and variable is the name of the variable.
To declare more than one variable of the specified type, you can use a comma-separated list. Following are valid examples of variable declaration in C++:
int a, b, c; // Declares three ints, a, b and c.
There are three different ways to initalize variables in C++.
1. C-like initialization
Syntax:
type identifier = initial_value;
Example
int a = 10, b = 10; //Declares variables a & b and initializes them with value of 10 each
And here is an example of initialization
byte B = 22; // initializes a byte type variable B. double pi = 3.14159; // declares and assigns a value of PI. char a = 'a'; // the char variable a is initialized with value 'a'
2. Constructor initialization
Syntax:
type identifier (initial_value);
Example:
int a(10); //declares an int variable a and initializes with value 10
3. Uniform initialization
Syntax:
type identifier {initial_value};
Example:
int a{10}; //declare a variable of type int called a and initialize it to a value of 10
Expressions
An expression is a statement that has a value – for instance, a number, a string, the sum of two numbers, etc.
Expressions combine variables and constants to produce new values. For example, 4+2, x-1, and "Hello, world!\n" are all expressions.
And, a statement is a unit of code that does something. Statements are a basic building block of a program.
Note: Not every statement is an expression. For instance, it makes no sense to talk about the value of an #include statement.
Here, is example program to illustrate all points mentioned above.
// initialization of variables, statement and expression example
#include <iostream>
using namespace std;
int main ()
{
int a = 10; // initial value: 10
int b(30); // initial value: 30
int c{20}; // initial value: 20
int result; // initial value undefined
a = a + b; // statement but a+b is expression
result = a - c; // statement
cout << "Result: "<< result;
return 0;
}
The output of the program is:
Result: 20
Basic Input
Just as cout << is the syntax for outputting values, cin >> is the syntax for inputting values.
The following program illustrates basic input.
#include <iostream>
#include <string>
using namespace std;
int main ()
{
int my_num;
cout<<"Enter an integer: ";
cin >> my_num;
cout<<"You entered: "<<my_num <<endl;
string my_string;
cout<<"Enter a string: ";
cin >> my_string;
cout<<"You entered: "<<my_string;
return 0;
}
The output of the program is:
Enter an integer: 12 You entered: 12 Enter a string: We love you all. You entered : We love you all
So, cin object can input any type of data.