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 as123or-123float- stores floating point numbers, with decimals, such as19.99or-19.99char- stores single characters, such as'a'or'B'. Char values 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:
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:
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,
you must use the format
specifier %d or %i
surrounded by double quotes,
inside the printf() function:
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);
Try it Yourself »
To combine both text and a variable, separate them with a comma inside the
printf() function:
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);
Try it Yourself »
You will learn more about Data Types in the next chapter.
Change Variable Values
Note: If you assign a new value to an existing variable, it will overwrite the previous value:
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);
Try it Yourself »
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);
Try it Yourself »
Add Variables Together
To add a variable to another variable, you can use the +
operator:
Declare Multiple Variables
To declare more than one variable of the same type, use a comma-separated list:
You can also assign the same value to multiple variables of the same type:
C Variable Names
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
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 (
myVarandmyvarare 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 etc). This is done to avoid confusion.
However, if you want a real-life example on how variables can be used, take a look at the following, where we have made a program that stores different data of 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);
Try it Yourself »
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);
Try it Yourself »