Variables & Data Types in C

A variable in C is a named box in memory that holds a value, and its data type decides what kind of value fits in that box and how much memory it uses. Before you can use a variable in C, you must declare it with a type — for example int age; reserves space for a whole number called age.

What is a variable?

Declaring a variable tells the compiler its name and type. You can declare and assign in one line:

int score = 90;        /* declare and initialize together */
float price;           /* declare now... */
price = 49.5f;         /* ...assign later */

A variable name must start with a letter or underscore, and can contain letters, digits, and underscores. C is case-sensitive: Score and score are different variables.

The core data types

C's built-in types fall into a few families. The exact sizes depend on the platform, but typical values on a 64-bit system are shown below.

You can also add modifiers like short, long, and unsigned to change range, e.g. unsigned int (no negatives) or long long (a wider integer).

A complete example

#include <stdio.h>

int main(void) {
    int   age = 19;          /* whole number */
    float height = 5.8f;     /* the f suffix marks a float literal */
    double pi = 3.14159;     /* more precise decimal */
    char  grade = 'A';       /* single character in single quotes */

    /* Each format specifier must MATCH its variable's type */
    printf("Age: %d\n", age);
    printf("Height: %.1f feet\n", height);  /* .1 = one decimal place */
    printf("Pi: %lf\n", pi);
    printf("Grade: %c\n", grade);

    return 0;
}

Notice 'A' uses single quotes (a char), while text like "Pi:" uses double quotes (a string). Mixing them up is a classic beginner error.

Constants and the const keyword

If a value should never change, mark it const:

const double GRAVITY = 9.8;  /* the compiler stops you reassigning it */

This documents intent and lets the compiler catch accidental edits.

Choosing the right type

• Counting things? Use int.
• Money or measurements with decimals? Prefer double over float for accuracy.
• A single letter or symbol? Use char.
• Storing text? You need an array of char — see Strings in C.

Common mistakes

• Wrong format specifier: printing a double with %d (or an int with %f) gives garbage. Match the specifier to the type.
• Forgetting to initialize: an uninitialized local variable holds a random value, not zero. Always assign before reading.
• Single vs double quotes: 'A' is a char; "A" is a string (two bytes including the null terminator).
• Integer division surprise: 5 / 2 is 2, not 2.5, because both operands are integers. Use 5.0 / 2 to get 2.5.
• Float precision: 0.1 + 0.2 is not exactly 0.3. Never compare floats with ==.

FAQ

float or double? Use double by default; it has more precision. Use float only when memory is tight.

Can a variable change type? No. In C a variable's type is fixed at declaration.

Keep exploring at the hub C Programming, then Operators in C and If-Else in C.

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