The standard input device is usually the keyboard. Handling the standard input in C is done by applying the overloaded operator of extraction ( ) on the cin stream. The operator must be followed by the variable that will store the data that is going to be extracted from the stream. This way, unlike the others, can only be used to add one to a variable rather than adding a number of your choice. This method is using something called the increment operator. The increment operator is shown in C by writing on either the left or right side of a variable - as you might expect, it increments that variable. Variable Scope in C - A scope is a region of the program and broadly speaking there are three places, where variables can be declared −. The collection of input and generation of output is known under the general term, input/output, or I/O for short, and is a core function of computers. Interestingly, the C programming language doesn't have I/O abilities built into it.
The most fundamental of all concepts in C++ is the variable — a variable is like a small box. You can store things in the box for later use, particularly numbers. The concept of a variable is borrowed from mathematics. A statement such as
stores the value 1 in the variable x. From that point forward, the mathematician can use the variable x in place of the constant 1 — until he changes the value of x to something else.
Variables work the same way in C++. You can make the assignment
From that point forward in the execution of the program, the value of x is 1 until the program changes the value to something else. References to x are replaced by the value 1.
A mathematician might write something like the following:
Any reader who’s had algebra realizes right off that the mathematician has introduced the variables x and y. But C++ isn’t that smart. (Computers may be fast, but they’re stupid.)
You have to announce each variable to C++ before you can use it. You have to say something soothing like this:
These lines of code declare that a variable x exists, is of type int, and has the value 10; and that a variable y of type int also exists with the value 5. You can declare variables (almost) anywhere you want in your program — as long as you declare the variable before you use it.
A variable provides us with named storage that our programs can manipulate. Each variable in C++ has a specific type, which determines the size and layout of the variable's memory; the range of values that can be stored within that memory; and the set of operations that can be applied to the variable.
The name of a variable can be composed of letters, digits, and the underscore character. It must begin with either a letter or an underscore. Upper and lowercase letters are distinct because C++ is case-sensitive −
There are following basic types of variable in C++ as explained in last chapter −
|Sr.No||Type & Description|
Stores either value true or false.
Typically a single octet (one byte). This is an integer type.
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The most natural size of integer for the machine.
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A single-precision floating point value.
A double-precision floating point value.
Represents the absence of type.
A wide character type.
C++ also allows to define various other types of variables, which we will cover in subsequent chapters like Enumeration, Pointer, Array, Reference, Data structures, and Classes.
Following section will cover how to define, declare and use various types of variables.
A variable definition tells the compiler where and how much storage to create for the variable. A variable definition specifies a data type, and contains a list of one or more variables of that type as follows −
Here, type must be a valid C++ data type including char, w_char, int, float, double, bool or any user-defined object, etc., and variable_list may consist of one or more identifier names separated by commas. Some valid declarations are shown here −
The line int i, j, k; both declares and defines the variables i, j and k; which instructs the compiler to create variables named i, j and k of type int.
Variables can be initialized (assigned an initial value) in their declaration. The initializer consists of an equal sign followed by a constant expression as follows −
Some examples are −
For definition without an initializer: variables with static storage duration are implicitly initialized with NULL (all bytes have the value 0); the initial value of all other variables is undefined.
A variable declaration provides assurance to the compiler that there is one variable existing with the given type and name so that compiler proceed for further compilation without needing complete detail about the variable. A variable declaration has its meaning at the time of compilation only, compiler needs actual variable definition at the time of linking of the program.
A variable declaration is useful when you are using multiple files and you define your variable in one of the files which will be available at the time of linking of the program. You will use extern keyword to declare a variable at any place. Though you can declare a variable multiple times in your C++ program, but it can be defined only once in a file, a function or a block of code.
Try the following example where a variable has been declared at the top, but it has been defined inside the main function −
When the above code is compiled and executed, it produces the following result −
Same concept applies on function declaration where you provide a function name at the time of its declaration and its actual definition can be given anywhere else. For example −
There are two kinds of expressions in C++ −
lvalue − Expressions that refer to a memory location is called 'lvalue' expression. An lvalue may appear as either the left-hand or right-hand side of an assignment.
rvalue − The term rvalue refers to a data value that is stored at some address in memory. An rvalue is an expression that cannot have a value assigned to it which means an rvalue may appear on the right- but not left-hand side of an assignment.
Variables are lvalues and so may appear on the left-hand side of an assignment. Numeric literals are rvalues and so may not be assigned and can not appear on the left-hand side. Following is a valid statement −
But the following is not a valid statement and would generate compile-time error −