C Variables

Variables are containers for storing data values, like numbers and characters.

In C, there are different types of variables (defined with different keywords), for example:

  • int - stores integers (whole numbers), without decimals, such as 123 or -123
  • float - stores floating point numbers, with decimals, such as 19.99 or -19.99
  • char - stores single characters, such as 'a' or 'B'. Characters are surrounded by single quotes

Declaring (Creating) Variables

To create a variable, specify the type and assign it a value:

Syntax

type variableName = value;

Where type is one of C types (such as int), and variableName is the name of the variable (such as x or myName). The equal sign is used to assign a value to the variable.

So, to create a variable that should store a number, look at the following example:

Example

Create a variable called myNum of type int and assign the value 15 to it:

int myNum = 15;

You can also declare a variable without assigning the value, and assign the value later:

Example

// Declare a variable
int myNum;

// Assign a value to the variable
myNum = 15;

Output Variables

You learned from the output chapter that you can output values/print text with the printf() function:

Example

printf("Hello World!");

In many other programming languages (like Python, Java, and C++), you would normally use a print function to display the value of a variable. However, this is not possible in C:

Example

int myNum = 15;
printf(myNum);  // Nothing happens

To output variables in C, you must get familiar with something called "format specifiers"

Format Specifiers

Format specifiers are used together with the printf() function to tell the compiler what type of data the variable is storing. It is basically a placeholder for the variable value.

A format specifier starts with a percentage sign %, followed by a character.

For example, to output the value of an int variable, use the format specifier %d surrounded by double quotes (""), inside the printf() function:

Example

int myNum = 15;
printf("%d", myNum);  // Outputs 15

To print other types, use %c for char and %f for float:

Example

// Create variables
int myNum = 15;            // Integer (whole number)
float myFloatNum = 5.99;   // Floating point number
char myLetter = 'D';       // Character

// Print variables
printf("%d\n", myNum);
printf("%f\n", myFloatNum);
printf("%c\n", myLetter);

To combine both text and a variable, separate them with a comma inside the printf() function:

Example

int myNum = 15;
printf("My favorite number is: %d", myNum);

To print different types in a single printf() function, you can use the following:

Example

int myNum = 15;
char myLetter = 'D';
printf("My number is %d and my letter is %c", myNum, myLetter);

You will learn more about Data Types in a later chapter.

Print Values Without Variables

You can also just print a value without storing it in a variable, as long as you use the correct format specifier:

Example

printf("My favorite number is: %d", 15);
printf("My favorite letter is: %c", 'D');
 
However, it is more sustainable to use variables as they are saved for later and can be re-used whenever.

Change Variable Values

If you assign a new value to an existing variable, it will overwrite the previous value:

Example

int myNum = 15;  // myNum is 15
myNum = 10;  // Now myNum is 10

You can also assign the value of one variable to another:

Example

int myNum = 15;

int myOtherNum = 23;

// Assign the value of myOtherNum (23) to myNum
myNum = myOtherNum;

// myNum is now 23, instead of 15
printf("%d", myNum);

Or copy values to empty variables:

Example

// Create a variable and assign the value 15 to it
int myNum = 15;

// Declare a variable without assigning it a value
int myOtherNum;

// Assign the value of myNum to myOtherNum
myOtherNum = myNum;

// myOtherNum now has 15 as a value
printf("%d", myOtherNum);

Add Variables Together

To add a variable to another variable, you can use the + operator:

Example

int x = 5;
int y = 6;
int sum = x + y;
printf("%d", sum);

Declare Multiple Variables

To declare more than one variable of the same type, use a comma-separated list:

Example

int x = 5, y = 6, z = 50;
printf("%d", x + y + z);

You can also assign the same value to multiple variables of the same type:

Example

int x, y, z;
x = y = z = 50;
printf("%d", x + y + z);

C Variable Names (Identifiers)

All C variables must be identified with unique names.

These unique names are called identifiers.

Identifiers can be short names (like x and y) or more descriptive names (age, sum, totalVolume).

Note: It is recommended to use descriptive names in order to create understandable and maintainable code:

Example

// Good variable name
int minutesPerHour = 60;

// OK, but not so easy to understand what m actually is
int m = 60;

The general rules for naming variables are:

  • Names can contain letters, digits and underscores
  • Names must begin with a letter or an underscore (_)
  • Names are case-sensitive (myVar and myvar are different variables)
  • Names cannot contain whitespaces or special characters like !, #, %, etc.
  • Reserved words (such as int) cannot be used as names

Real-Life Example

Often in our examples, we simplify variable names to match their data type (myInt or myNum for int types, myChar for char types, and so on). This is done to avoid confusion.

However, for a practical example of using variables, we have created a program that stores different data about a college student:

Example

// Student data
int studentID = 15;
int studentAge = 23;
float studentFee = 75.25;
char studentGrade = 'B';

// Print variables
printf("Student id: %d\n", studentID);
printf("Student age: %d\n", studentAge);
printf("Student fee: %f\n", studentFee);
printf("Student grade: %c", studentGrade);

Calculate the Area of a Rectangle

In this real-life example, we create a program to calculate the area of a rectangle (by multiplying the length and width):

Example

// Create integer variables
int length = 4;
int width = 6;
int area;

// Calculate the area of a rectangle
area = length * width;

// Print the variables
printf("Length is: %d\n", length);
printf("Width is: %d\n", width);
printf("Area of the rectangle is: %d", area);