Declaration of Variables

In C, variable names are identifiers that you use to name variables. Here are the rules of construction for variable names in C:

Syntax:

Variable names must begin with a letter (a-z, A-Z) or an underscore (_). After the first character, variable names can also contain digits (0-9).

            int my_variable;

Length:

The length of a variable name can vary, but only the first 31 characters are significant. However, it's good practice to keep variable names meaningful and not too long for readability.

 int a_very_long_variable_name; // valid, but not recommended

Case Sensitivity:

C is case-sensitive, so uppercase and lowercase letters are considered different.

int my_variable;
            int My_Variable; // different variable from my_variable

Keywords:

Variable names cannot be the same as C keywords (reserved words).

            int if; // invalid, 'if' is a keyword
            

Variable names cannot contain spaces or special characters (except underscore _).

int my variable; // invalid, contains a space
                int my_variable!; // invalid, contains a special character (!)
            

Names:

Certain names may be reserved by the implementation, so it's best to avoid using them.

int int; // invalid, 'int' is a restricted name

Global vs. Local Scope:

Variable names declared in different scopes (e.g., global vs. local to a function) can have the same name without conflict.

                            int global_variable;
                         
                            void my_function() {
                                int local_variable;
                                // Both 'global_variable' and 'local_variable' are valid names
                            }

Q2. Which of the following is not a valid declaration in C?
                            1. short int x;
                           2. signed short x; 
                           3. short x; 
                           4. unsigned short x;