Variables and Data Types

In the last lesson, we printed a fixed message. To write useful programs, we need a way to store and manipulate data that can change. This is where variables come in.

WHAT IS A VARIABLE? A VARIABLE is a named storage location in the computer’s memory. Think of it as a labeled box where you can store a piece of information. You give it a name, and you can change the contents of the box (the value) as your program runs.

WHAT IS A DATA TYPE? A DATA TYPE specifies the type of data that a variable can hold. C is a “statically-typed” language, which means you must explicitly tell the compiler the data type of every variable you create. This helps the compiler allocate the correct amount of memory and ensures you use the variable correctly.

The process of creating a variable is called DECLARATION. The process of giving a variable its first value is called INITIALIZATION.

An INTEGER is a whole number (no decimal point). The int data type is used to store them.

Below, we DECLARE a variable named player_score of type int. This tells the compiler: “Reserve a spot in memory for an integer, and I will call it player_score.”

Now we INITIALIZE the variable by assigning it a value using the assignment operator =.

To print the value of a variable, we use printf with a FORMAT SPECIFIER. A FORMAT SPECIFIER is a placeholder in the string that tells printf what type of data to expect and how to format it.

%d is the format specifier for a signed decimal integer (int). The variable player_score is passed as a second argument to printf, and its value will replace %d in the output.

To store numbers with decimal points, we use FLOATING-POINT types. The most common one is double, which stands for “double-precision”. It can store numbers with much greater precision than its counterpart, float. For this course, we will prefer double for all decimal numbers.

%f is the format specifier for a double (and float).

By default, %f might print many extra zeros. You can control the number of decimal places by modifying the format specifier. For example, %.2f tells printf to print only 2 digits after the decimal point.

The char data type is used to store a single CHARACTER. This can be a letter, a number, or a symbol.

A CHARACTER LITERAL is enclosed in SINGLE quotes (e.g., ‘A’). This is different from a string literal, which uses double quotes.

%c is the format specifier for a char.

You can print multiple variables in a single printf call. Just make sure the format specifiers and the variables are listed in the same order.

Full Source

/**
 * @file 2_variables_and_data_types.c
 * @brief Part 1, Lesson 2: Variables and Data Types
 * @author dunamismax
 * @date 06-15-2025
 *
 * This file introduces the fundamental concepts of variables and data types.
 * We learn how to store information in memory and print it to the console.
 */

/*
 * =====================================================================================
 * |                                   - LESSON START -                                  |
 * =====================================================================================
 *
 * In the last lesson, we printed a fixed message. To write useful programs,
 * we need a way to store and manipulate data that can change. This is where
 * variables come in.
 *
 * WHAT IS A VARIABLE?
 * A VARIABLE is a named storage location in the computer's memory. Think of it
 * as a labeled box where you can store a piece of information. You give it a
 * name, and you can change the contents of the box (the value) as your
 * program runs.
 *
 * WHAT IS A DATA TYPE?
 * A DATA TYPE specifies the type of data that a variable can hold. C is a
 * "statically-typed" language, which means you must explicitly tell the
 * compiler the data type of every variable you create. This helps the compiler
 * allocate the correct amount of memory and ensures you use the variable correctly.
 *
 * The process of creating a variable is called DECLARATION.
 * The process of giving a variable its first value is called INITIALIZATION.
 */

#include <stdio.h>

int main(void)
{
    // The main function body is where we'll declare and use our variables.

    // --- Part 1: The Integer (int) ---

    /*
     * An INTEGER is a whole number (no decimal point). The `int` data type is
     * used to store them.
     *
     * Below, we DECLARE a variable named `player_score` of type `int`.
     * This tells the compiler: "Reserve a spot in memory for an integer,
     * and I will call it `player_score`."
     */
    int player_score; // Declaration

    /*
     * Now we INITIALIZE the variable by assigning it a value using the
     * assignment operator `=`.
     */
    player_score = 1250; // Initialization

    /*
     * To print the value of a variable, we use `printf` with a FORMAT SPECIFIER.
     * A FORMAT SPECIFIER is a placeholder in the string that tells `printf`
     * what type of data to expect and how to format it.
     *
     * `%d` is the format specifier for a signed decimal integer (`int`).
     * The variable `player_score` is passed as a second argument to `printf`,
     * and its value will replace `%d` in the output.
     */
    printf("Player score: %d\n", player_score);

    // You can also declare and initialize a variable in a single statement.
    int number_of_lives = 3;
    printf("Number of lives: %d\n", number_of_lives);
    printf("\n"); // Print a blank line for separation

    // --- Part 2: The Double-Precision Floating-Point Number (double) ---

    /*
     * To store numbers with decimal points, we use FLOATING-POINT types.
     * The most common one is `double`, which stands for "double-precision".
     * It can store numbers with much greater precision than its counterpart, `float`.
     * For this course, we will prefer `double` for all decimal numbers.
     *
     * `%f` is the format specifier for a `double` (and `float`).
     */
    double item_price = 19.99; // Declaration and initialization
    printf("The price of the magic sword is: $%f\n", item_price);

    /*
     * By default, `%f` might print many extra zeros. You can control the
     * number of decimal places by modifying the format specifier. For example,
     * `%.2f` tells `printf` to print only 2 digits after the decimal point.
     */
    printf("The price, formatted neatly, is: $%.2f\n", item_price);
    printf("\n");

    // --- Part 3: The Character (char) ---

    /*
     * The `char` data type is used to store a single CHARACTER. This can be
     * a letter, a number, or a symbol.
     *
     * A CHARACTER LITERAL is enclosed in SINGLE quotes (e.g., 'A'). This is
     * different from a string literal, which uses double quotes.
     *
     * `%c` is the format specifier for a `char`.
     */
    char player_grade = 'A'; // Declaration and initialization
    printf("Player's performance grade: %c\n", player_grade);

    // --- Part 4: Putting It All Together ---

    /*
     * You can print multiple variables in a single `printf` call. Just make
     * sure the format specifiers and the variables are listed in the same order.
     */
    printf("Summary: Player with %d lives and a grade of '%c' has a score of %d.\n",
           number_of_lives, player_grade, player_score);

    return 0; // Signal successful execution
}

/*
 * =====================================================================================
 * |                                    - LESSON END -                                   |
 * =====================================================================================
 *
 * In this lesson, you learned about three fundamental data types:
 * - `int` for whole numbers.
 * - `double` for numbers with decimal points.
 * - `char` for single characters.
 *
 * You also learned how to DECLARE variables, INITIALIZE them with values, and
 * print them using `printf` with FORMAT SPECIFIERS (`%d`, `%f`, `%c`).
 *
 * HOW TO COMPILE AND RUN THIS CODE:
 *
 * 1. Open a terminal or command prompt.
 * 2. Navigate to the directory where you saved this file.
 * 3. Use the GCC compiler to create an executable file:
 *    `gcc -Wall -Wextra -std=c11 -o 2_variables_and_data_types 2_variables_and_data_types.c`
 * 4. Run the executable:
 *    - On Linux/macOS:   `./2_variables_and_data_types`
 *    - On Windows:       `2_variables_and_data_types.exe`
 */

How to Compile and Run

cc -Wall -Wextra -std=c11 -o 2_variables_and_data_types 2_variables_and_data_types.c
./2_variables_and_data_types