Variables in C: Declaration, Initialization, Assignment and Scope Basics

A variable is a named piece of memory that holds a value. In C, every variable has a type, decided when you declare it, which fixes what kind of value it can hold and how much memory it uses.

Declaring and initialising#

#include <stdio.h>

int main(void) {
    int age = 21;            /* declare and initialise */
    double price = 49.99;
    char grade = 'A';

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

Age: 21
Price: 49.99
Grade: A

The pattern is type name = value;. The format specifier in printf must match the type: %d for int, %f for double, %c for char.

Assignment#

After declaring a variable, you can change its value with =:

int score = 10;
score = 15;
score = score + 5;
printf("%d\n", score);

20

score = score + 5 reads the current value (15), adds 5, and stores the result back.

Uninitialised variables#

A local variable declared without a value contains whatever happened to be in that memory. Reading it is a bug, and its value is unpredictable:

int total;           /* not initialised: holds garbage */
total = total + 5;   /* bug: the result is unpredictable */

Always initialise variables when you declare them, even if only to 0. With -Wall, GCC warns about many uses of uninitialised variables.

Several variables at once#

int x = 1, y = 2, z = 3;
printf("%d\n", x + y + z);

6

Be careful: int a, b = 0; initialises only b. One declaration per line avoids this trap.

Where to declare#

Since C99, you can declare variables anywhere in a block, close to where they are used, including in a for loop header. A variable exists only inside the block where it is declared (its scope):

int outer = 1;
{
    int inner = 2;
    printf("%d %d\n", outer, inner);
}
/* inner does not exist here */
printf("%d\n", outer);

1 2
1

Scope and storage classes are covered in Scope and Storage Classes.

Naming variables#

• Use descriptive names: total_price, not tp.
• snake_case is the most common style in C.
• Names are case-sensitive and cannot start with a digit or be keywords (Syntax).

Practice#

#include <stdio.h>

int main(void) {
    int quantity = 3;
    double unit_price = 149.50;

    printf("Total: %.2f\n", quantity * unit_price);
    quantity = 5;
    printf("Total: %.2f\n", quantity * unit_price);
    return 0;
}

Total: 448.50
Total: 747.50

Key takeaway#