C Variables

C variables are named containers that store data in memory. In C every variable has a type, and you must declare that type before you use the variable.

Declaring and Initializing Variables

The syntax is type name = value;. Here int stores whole numbers, double stores decimals and char stores a single character.

#include <stdio.h>

int main(void) {
    int age = 25;
    double price = 9.99;
    char grade = 'A';

    printf("Age: %d\n", age);
    printf("Price: %.2f\n", price);
    printf("Grade: %c\n", grade);
    return 0;
}

Age: 25
Price: 9.99
Grade: A

Format Specifiers

printf cannot guess a variable's type, so you tell it with a format specifier inside the string.

• %d or %i for int
• %f for double (use %.2f for two decimals)
• %c for char
• %s for strings
• %zu for size_t values such as sizeof results
• %p for pointer addresses

Changing Variable Values

Assigning a new value replaces the old one. You can also declare several variables of the same type in one line.

#include <stdio.h>

int main(void) {
    int x = 5, y = 10, z;
    z = x + y;
    x = 100;   // overwrite x
    printf("x=%d y=%d z=%d\n", x, y, z);
    return 0;
}

x=100 y=10 z=15

C Variable Naming Rules

• Names can contain letters, digits and underscores.
• A name must begin with a letter or an underscore, never a digit.
• Names are case-sensitive: total and Total are different variables.
• Reserved keywords like int, return and while cannot be used as names.
• Prefer descriptive names like itemCount or item_count over single letters.

Uninitialized Variables

A local variable that is declared but not given a value contains garbage, whatever bits were already in that memory. Reading it is undefined behavior, so always initialize variables before use.

Frequently asked questions

What happens if I use an uninitialized variable in C?

Its value is unpredictable and reading it is undefined behavior. Your program might print random numbers or behave differently each run. Always give variables a starting value.

What is the difference between declaration and initialization in C?

Declaration tells the compiler a variable's name and type (int x;). Initialization gives it its first value at the same time (int x = 5;).

Why does printf need %d?

printf receives raw values and uses the format specifier to know how to interpret and display each one. Using the wrong specifier gives wrong output or undefined behavior.