Chapter 2 — Variables & Data Types

What you'll learn

• 1Declare variables of `int`, `float`, `double`, `char`, `_Bool`.
• 2Read sizes with `sizeof` and pick types accordingly.
• 3Use `const` for read-only values.
• 4Read printf format specifiers per type.

Core Concepts

1) Primitive types

2) `sizeof`

printf("%zu\n", sizeof(int));    // typically 4
printf("%zu\n", sizeof(double)); // typically 8

`sizeof` is a compile-time operator; it returns `size_t`, print with `%zu`.

3) `const`

const double PI = 3.14159265;
PI = 3.14; // ❌ compile error

Use `const` when a value should never change after init.

4) Common pitfalls of integer width

`int` is **usually** 32-bit, but not guaranteed. For big values use `long long` or `<stdint.h>` (`int64_t`, `uint32_t`).

Examples

Example 1 — `ex01_variables.c`: declare and print various types

int age = 20;
double height = 1.75;
char grade = 'A';
printf("age=%d height=%.2f grade=%c\n", age, height, grade);

**Output**

age=20 height=1.75 grade=A

Key: each type uses its own format specifier.

Example 2 — `ex02_types.c`: integer overflow

int max = 2147483647;
printf("max = %d\n", max);
printf("max + 1 = %d\n", max + 1);

**Output**

max = 2147483647
max + 1 = -2147483648

Key: signed overflow wraps to the negative end — undefined behavior in standard C; gcc happens to wrap.

Example 3 — `ex03_sizeof.c`: type sizes on this machine

printf("int    = %zu bytes\n", sizeof(int));
printf("long   = %zu bytes\n", sizeof(long));
printf("double = %zu bytes\n", sizeof(double));

**Output**

int    = 4 bytes
long   = 8 bytes
double = 8 bytes

Key: don't hard-code sizes; ask `sizeof`.

Example 4 — `ex04_const.c`: const stops accidental writes

const double PI = 3.14159;
double r = 5.0;
printf("area = %.2f\n", PI * r * r);

**Output**

area = 78.54

Key: `const` makes intent explicit and the compiler your ally.

Common mistakes

1. **Wrong format specifier** — `%d` for double prints garbage.
2. **Uninitialized variables** — reading them is undefined behavior; always init.
3. **Comparing float for equality** — use a small epsilon instead.
4. **Confusing `char` and `string`** — `'A'` is a char; `"A"` is a 2-byte string.
5. **Assuming int sizes** — for portability use `int32_t` etc.

Recap

• Choose the smallest type that fits — but don't be stingy with `int` for counters.
• `sizeof` tells the truth about the current platform.
• `const` documents intent and prevents bugs.

Try it

cd src
gcc -std=c11 -Wall -o ex01 ex01_variables.c && ./ex01
gcc -std=c11 -Wall -o ex02 ex02_types.c && ./ex02
gcc -std=c11 -Wall -o ex03 ex03_sizeof.c && ./ex03
gcc -std=c11 -Wall -o ex04 ex04_const.c && ./ex04

💻 Examples

Compilable, runnable examples — see the output yourself.

#include <stdio.h>

int main(void) {
    int age = 20;
    int year;

    year = 2026;

    printf("age: %d\n", age);
    printf("year: %d\n", year);

    age = age + 1;
    printf("next year age: %d\n", age);

    return 0;
}

age=20 height=1.75 grade=A

#include <stdio.h>

int main(void) {
    int    score = 95;
    float  ratio = 0.75f;
    double price = 19999.99;
    char   grade = 'A';

    printf(" (%%d): %d\n", score);
    printf(" (%%f): %f\n", ratio);
    printf(" (%%.2f): %.2f\n", ratio);
    printf(" (%%lf): %lf\n", price);
    printf(" (%%c): %c\n", grade);
    printf(" ASCII (%%d): %d\n", grade);

    return 0;
}

max = 2147483647
max + 1 = -2147483648

#include <stdio.h>

int main(void) {
    printf("char   = %zu byte\n", sizeof(char));
    printf("short  = %zu byte\n", sizeof(short));
    printf("int    = %zu byte\n", sizeof(int));
    printf("long   = %zu byte\n", sizeof(long));
    printf("float  = %zu byte\n", sizeof(float));
    printf("double = %zu byte\n", sizeof(double));

    int a = 0;
    printf(" a : %zu byte\n", sizeof(a));

    return 0;
}

int    = 4 bytes
long   = 8 bytes
double = 8 bytes

#include <stdio.h>

#define PI 3.14159
#define MAX_USERS 100

int main(void) {
    const int DAYS_IN_WEEK = 7;
    double radius = 5.0;

    printf("pi(): %f\n", PI);
    printf(" (): %d\n", MAX_USERS);
    printf("(const): %d\n", DAYS_IN_WEEK);
    printf("radius %.1f  : %f\n", radius, PI * radius * radius);

    /* DAYS_IN_WEEK = 8;  //   - const   */

    return 0;
}

area = 78.54

📝 Exercises

Try them yourself first, then open the solution to compare.

Height: 175.5 cm
Weight: 68.2 kg

#include <stdio.h>

int main(void) {
    float height = 175.5f;
    float weight = 68.2f;

    printf("Height: %.1f cm\n", height);
    printf("Weight: %.1f kg\n", weight);

    return 0;
}

Height: 175.5 cm
Weight: 68.2 kg

Volume: 64
Surface area: 96

#include <stdio.h>

int main(void) {
    int side = 4;
    int volume = side * side * side;
    int surface = 6 * side * side;

    printf("Volume: %d\n", volume);
    printf("Surface area: %d\n", surface);

    return 0;
}

Volume: 64
Surface area: 96

#include <stdio.h>

#define LIGHT_SPEED 299792458   /* m/s */

int main(void) {
    long meters = LIGHT_SPEED;
    long km = meters / 1000;

    printf("Light travels ~%ld km in one second.\n", km);
    return 0;
}