C Variables

Summary: in this tutorial, you will learn about C variables that allow you to manipulate data in your program.

Introduction to C variables

A program consists of data and a set of instructions that process the data. The data can be numbers and characters. To store data in the program, you use variables.

A variable allows you to store data during the excution of the program.

Declaring variables

Before using a variable, you need to declare it. A variable declartion serves two purposes:

• Define the name of the variable.
• Define the type of data that the variable can store.

For example, the following statement declares a variable:

int age;
Code language: C++ (cpp)

The keyword int tells C that this variable will hold an integer value. The variable name is age. The semicolon (;) ends the statement.

The general form of a variable declaration is:

type variable_name;
Code language: C++ (cpp)

The type can be any valid type in C.

When you declare a variable:

• C reserves a space in the memory to hold the value of the variable. The amount of memory depends on the type of value that the variable will store.
• C also allocates a memory space that associate with the variable name and a unique address.

A variable can be declared at any point in the program before it is used. It’s a good practice to declare a variable at the place that’s closest to its first use.

The variable name in C needs to follow these rules:

• The variable name can contain letters, digits, and the underscore ( _) . The first character of the variable name must be a letter or underscore ( _). However, you should avoid using the underscore ( _) as the first letter because it can clash with standard system variables.
• According to ANSI C, the variable name should have a maximum number of 31 characers.
• Also, variable names must not be the same as reserved words or keywords in C.

The following table shows the keywords in C:

If you have multiple variable with the same type, you can declare them in a single statement. For example:

int age, min_age, max_age;
Code language: C++ (cpp)

Initializing Variables

To make it more convenient, C allows you to initialize a variable when you declare it. For example

int age = 1;
char ch = 'a';
Code language: C++ (cpp)

It’s a good practice to place initialized variables on a separate line and add a descriptive comment to explain why the variable is initialized to a specific value. For example:

int speed = 50; // minimum speed limit on highway
Code language: C++ (cpp)

Assigning values to variables

To assign a variable a value, you can use the assignment operator (=). For example:

int age = 1;
int speed_limit;
age = 2;
speed_limit = 50; // 50 miles per hour
Code language: C++ (cpp)

How it works.

• First, declare two variables age and speed_limit.
• Second, assign 2 to the age variable and 50 to the speed_limit variable.

Besides a value, you can assign a the value of one variable to another. For example:

int revenue = 100, cost = 90;
int profit;
profit = revenue - cost; // 10
Code language: C++ (cpp)

Summary

• A variable stores data during the execution of a program.
• Do declare a variable before using it.
• A variable declaration includes type of data that the variable will hold and the variable name.
• A variable can be assigned an initial value during declaration.
• Use the assignment operator (=) to assign a value to a variable.