Variables in C

A variable is a named storage location that holds a value. For example:

int age = 20;

Here, age is a variable that stores the value 20.

Variables allow us to:

• Store information.
• Use stored values later.
• Perform calculations.
• Change values during program execution.
• Make programs easier to understand.

A variable can be declared by specifying its data type followed by its name.

int age;

This declares an integer variable named age.

Initialization means assigning an initial value to a variable when it is declared.

int age = 20;

Here, age is initialized with the value 20.

A variable can first be declared and then assigned a value.

int age;

age = 20;

Both declaration and assignment are separate statements in this example.

The data type tells the compiler what kind of value a variable is intended to store.

int age = 20;
float price = 99.50;
char grade = 'A';

Different data types are suitable for different kinds of values.

The int type is commonly used for integer values.

int age = 25;
int marks = 85;

These variables store whole-number values.

The float type can store floating-point values.

float price = 99.50f;

The f suffix can be used to indicate a float literal.

The char type is used to store a single character.

char grade = 'A';

A character constant is normally written inside single quotation marks.

The double type is used for floating-point values with greater precision than float.

double salary = 25000.75;

Common rules for variable names include:

• Use letters, digits, and underscores.
• The name must not start with a digit.
• Do not use a C keyword as a variable name.
• C variable names are case-sensitive.

Examples of valid variable names:

age
studentName
marks1
_total
course_fee

Examples of invalid variable names:

1age
student-name
total fee
int

These names violate one or more C identifier rules.

C treats uppercase and lowercase letters as different.

int age = 20;
int Age = 30;

Here, age and Age are two different variables.

A variable's value can be changed after declaration.

int age = 20;

age = 25;

After the assignment, age contains 25.

Variables can be displayed using printf().

#include <stdio.h>

int main()
{
    int age = 20;

    printf("%d", age);

    return 0;
}

The %d conversion specifier is commonly used with an int value in printf().

int marks = 85;

printf("%d", marks);

A floating-point value can be printed with %f.

float price = 99.50f;

printf("%f", price);

The %c conversion specifier is used to print a character.

char grade = 'A';

printf("%c", grade);

Multiple variables can be declared in a program.

int age = 20;
float height = 5.8f;
char grade = 'A';

Each variable can store a different type of value.

Variables of the same data type can be declared together.

int a = 10, b = 20, c = 30;

This creates three integer variables.

Variables are frequently used in mathematical calculations.

int a = 10;
int b = 20;

int sum = a + b;

printf("%d", sum);

The value of sum becomes 30.

The assignment operator = is used to assign a value to a variable.

int marks;

marks = 90;

The value on the right side is assigned to the variable on the left side.

One variable can receive the value of another variable.

int a = 10;
int b;

b = a;

After the assignment, b contains the value 10.

A variable declared inside a function or block is generally a local variable. Its scope is limited to the block where it is declared.

int main()
{
    int age = 20;

    printf("%d", age);

    return 0;
}

A variable declared outside all functions has file scope and is commonly called a global variable.

#include <stdio.h>

int count = 10;

int main()
{
    printf("%d", count);

    return 0;
}

Use meaningful names that describe the stored value.

int studentAge;
float courseFee;
int totalMarks;

Meaningful names make programs easier to read.

#include <stdio.h>

int main()
{
    int age = 20;
    int marks = 85;
    float fee = 5000.0f;
    char grade = 'A';

    printf("Age = %d\n", age);
    printf("Marks = %d\n", marks);
    printf("Fee = %.2f\n", fee);
    printf("Grade = %c\n", grade);

    return 0;
}

Create a program that stores and displays a student's name, age, marks, and grade.

#include <stdio.h>

int main()
{
    int age = 18;
    int marks = 90;
    char grade = 'A';

    printf("Age = %d\n", age);
    printf("Marks = %d\n", marks);
    printf("Grade = %c\n", grade);

    return 0;
}

Practice by changing the variable values and observing the output.