When we declare a variable name and its type, the compiler allocates a block of memory for placing its value. In fact, for the computer, this allocated block of memory is the variable and it recognizes it by its name. The sizes of memory blocks allocated for different types of data may vary on different computers depending on the hardware, the operating system, and the compiler used. The names of variables or identifiers should be carefully selected. The general guidelines are as follows:
(1) A valid identifier is a single sequence of characters, that is, it should be only one word consisting of one or more characters. For example, Count, number, and Age are all valid identifiers. Similarly, x, y, z, A, or Care all valid names. But New Delhi and Delhi 16 are invalid identifiers/names.
(2) It should start with a letter or with an underscore symbol ( _) but not with a digit.
_tini, Rectangle_length, Delhi_24, or A55 are valid identifiers, but 4bars and 55Delhi are invalid.
(3) There should be no marked character in the identifier.
(4) The length of an identifier should be less than 32 characters. Although there is no limit to the
length of an identifier, some compliers support names/identifiers of up to the first 32 characters only.
(5) It should not have any other symbol except alphabets, digits, and underscore symbol.
For example, Newdelhi_2 is valid but Newdelhi-2 or Newdelhi (2) are invalid identifiers.
(6) It must not be a keyword of C language.
Thus, float or double, and int are invalid identifiers, whereas Double, Int, and INT are valid identifiers because the case of letters has been altered. If one or more characters in the name are in uppercase, it is a safe bet against a keyword being used as name of a variable.
Where to Declare Variables in a Program?
In case of C-90, all of the variables that are used in a program must be declared at the beginning of the program or at the beginning of block of statements in which they are to be defined. This condition is not present in C-99. In C-99, a variable may be declared anywhere in the program, but it should be declared and initialized, that is, a value should be assigned to it before it is actually used. However, all the compilers have not been upgraded to C-99, so if you are using an older version of a compiler, then all of the variable and constants have to be declared at the beginning of a program. Furthermore, any variable or constant must be initialized before it is used in the program, that is, a value should be assigned to it. Initialization is done by assigning a value at the time of declaration. Value may be assigned later in the program or during the execution of the program. The assignment operator (=) is used for the purpose as illustrated in the example below.
A number of variables of the same type may be declared in same line with type appearing only in the beginning as shown in the following example:
int n = 6, m =2, p; p = m * n; float x = 6.85; char kh = 'D'; char Name  ; Name = "Krishna";
In the first line, three integer variables n, m, and p are declared. The variables n and m are initialized, while the variable p is assigned value equal to product of n and m in the next line. This would be executed when program runs. In the third line, a float variable x is declared and initialized with a value 6.85. In the fourth line, a variable of type char and with name kh is declared. It is initialized with value ‘D’. Remember, whenever a value is assigned in terms of a character, it must be enclosed between single quotes (‘ ‘). Similarly, when a value is assigned in terms of a string of characters, as done in the last line above, the value must be enclosed between double quotes (” “).