Variables in C: Definition, Rules, and Types (With Examples)

Variables in C: Definition, Rules, Examples & Types

In C programming, a variable is simply a name given to a memory location where data is stored. You can think of it like a box where you keep some value (number, character, etc.) and give that box a label (name).

Example: If we say int age = 18;, here:

• int → data type (integer)
• age → variable name
• 18 → value stored in that variable

Variable Declaration

datatype variable_name;
// or for multiple variables:
datatype variable1, variable2, variable3;
  

Examples:

• int marks; → a variable for storing marks
• float salary, bonus; → two variables of type float

Rules for Naming Variables

There are some simple rules you must follow while naming variables in C:

• Variable names can have 1 to 255 characters (recommended: 1–31 characters).
• They must start with a letter or underscore (_).
• After the first letter, you can use letters and numbers (no spaces or special characters).
• Case-sensitive → Age and age are different.
• You cannot use keywords (like int, while, if) as variable names.
• Use meaningful names: e.g., totalMarks is better than x.

Valid vs Invalid Variable Names

Valid	Invalid
studentMarks	3students
grade_on_test	grade#1
_value	total marks (space not allowed)

Types of Variables in C

There are mainly 5 types of variables in C:

1. Local Variable: Declared inside a function or block. Only accessible there.
   Example: A variable inside main().
2. Global Variable: Declared outside all functions. Accessible throughout the program.
   Example: A variable declared before main().
3. Static Variable: Retains its value even after the function ends (declared using static keyword).
4. External Variable: Declared with extern keyword. Can be shared across multiple files.
5. Automatic Variable: By default, variables inside a function are automatic. Declared using auto (rarely used explicitly).

Comparison of Variables

Type	Scope	Lifetime
Local	Inside function/block	Till function ends
Global	Whole program	Till program ends
Static	Inside function	Till program ends (retains value)

Tips for Writing Good Variable Names

• Always use meaningful names (e.g., totalMarks instead of a).
• Follow one style (camelCase or underscore_style).
• Do not use very long names.
• Avoid using confusing names like O (letter O) or l (small L).

Conclusion

Variables are the foundation of any program. If you master how to declare, name, and use them correctly, you will write cleaner and more efficient C programs.