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User Input

So far, our programs have been one-way streets: they run and print things without any input from the outside world. To build truly useful tools, we need our programs to be interactive.

THE scanf FUNCTION The counterpart to printf (for printing) is scanf (for scanning/reading). The scanf function reads formatted input from the standard input, which is usually your keyboard.

Like printf, it uses FORMAT SPECIFIERS to know what type of data to expect. However, it has one very important new requirement we will explore.

To read input, we first need a variable to store that input in. We DECLARE an integer variable user_age. Notice we do not initialize it, because its value will be provided by the user.

It’s good practice to print a “prompt” to the user, so they know what you’re asking for.

Now, we call scanf. This is the most important part of the lesson.

  • "%d": This is the format specifier. It tells scanf to look for and read an integer from the keyboard input.

  • &user_age: This is new! The & symbol is the ADDRESS-OF OPERATOR. It gets the memory address of the user_age variable.

WHY DO WE NEED &? You must tell scanf where in memory to store the data it reads. Simply providing user_age would only give scanf the variable’s current value (which is garbage at this point). By providing &user_age, you are giving it the location of the box, so it can put the user’s input directly into it.

The pattern is the same for other data types. We just need to change the variable type and the format specifier.

IMPORTANT: For scanf, the format specifier for a double is %lf (which stands for “long float”). This is DIFFERENT from printf, which uses %f for a double. This is a common point of confusion!

scanf can sometimes behave in unexpected ways because of how it handles whitespace (like spaces, tabs, and newlines).

When you typed your GPA and pressed Enter, the \n (newline character) was left behind in the INPUT BUFFER. A subsequent scanf call for a character (%c) would read that leftover newline instead of waiting for you to type a new character!

THE FIX: To fix this, we put a single space before the %c in the format string: " %c". That leading space tells scanf to “skip any and all leading whitespace characters before reading the character”. It’s a crucial trick.

Full Source

/**
 * @file 3_user_input.c
 * @brief Part 1, Lesson 3: User Input
 * @author dunamismax
 * @date 06-15-2025
 *
 * This lesson demonstrates how to make programs interactive by reading
 * input from the user's keyboard.
 */

/*
 * =====================================================================================
 * |                                   - LESSON START -                                  |
 * =====================================================================================
 *
 * So far, our programs have been one-way streets: they run and print things
 * without any input from the outside world. To build truly useful tools,
 * we need our programs to be interactive.
 *
 * THE `scanf` FUNCTION
 * The counterpart to `printf` (for printing) is `scanf` (for scanning/reading).
 * The `scanf` function reads formatted input from the standard input, which is
 * usually your keyboard.
 *
 * Like `printf`, it uses FORMAT SPECIFIERS to know what type of data to expect.
 * However, it has one very important new requirement we will explore.
 */

#include <stdio.h>

int main(void)
{
    // --- Part 1: Reading an Integer ---

    /*
     * To read input, we first need a variable to store that input in.
     * We DECLARE an integer variable `user_age`. Notice we do not initialize it,
     * because its value will be provided by the user.
     */
    int user_age;

    /*
     * It's good practice to print a "prompt" to the user, so they know
     * what you're asking for.
     */
    printf("Please enter your age (as a whole number): ");

    /*
     * Now, we call `scanf`. This is the most important part of the lesson.
     *
     * - `"%d"`: This is the format specifier. It tells `scanf` to look for
     *   and read an integer from the keyboard input.
     *
     * - `&user_age`: This is new! The `&` symbol is the ADDRESS-OF OPERATOR.
     *   It gets the memory address of the `user_age` variable.
     *
     * WHY DO WE NEED `&`?
     * You must tell `scanf` *where* in memory to store the data it reads.
     * Simply providing `user_age` would only give `scanf` the variable's
     * *current value* (which is garbage at this point). By providing `&user_age`,
     * you are giving it the *location* of the box, so it can put the user's
     * input directly into it.
     */
    scanf("%d", &user_age);

    // Let's print the value to confirm we received it correctly.
    printf("Thank you. You entered %d.\n", user_age);
    printf("\n"); // Add a blank line for spacing.

    // --- Part 2: Reading a Double ---

    /*
     * The pattern is the same for other data types. We just need to change
     * the variable type and the format specifier.
     *
     * IMPORTANT: For `scanf`, the format specifier for a `double` is `%lf`
     * (which stands for "long float"). This is DIFFERENT from `printf`,
     * which uses `%f` for a `double`. This is a common point of confusion!
     */
    double user_gpa;
    printf("Please enter your GPA (e.g., 3.8): ");
    scanf("%lf", &user_gpa); // Use %lf for reading a double!

    printf("Your GPA is %.2f.\n", user_gpa); // Use %f for printing a double.
    printf("\n");

    // --- Part 3: A Common `scanf` Pitfall and Reading a Character ---

    /*
     * `scanf` can sometimes behave in unexpected ways because of how it handles
     * whitespace (like spaces, tabs, and newlines).
     *
     * When you typed your GPA and pressed Enter, the `\n` (newline character)
     * was left behind in the INPUT BUFFER. A subsequent `scanf` call for a
     * character (`%c`) would read that leftover newline instead of waiting for
     * you to type a new character!
     *
     * THE FIX: To fix this, we put a single space before the `%c` in the format
     * string: `" %c"`. That leading space tells `scanf` to "skip any and all
     * leading whitespace characters before reading the character". It's a crucial trick.
     */
    char favorite_letter;
    printf("Please enter your favorite letter: ");
    scanf(" %c", &favorite_letter); // The space before %c is very important!

    printf("Your favorite letter is '%c'.\n", favorite_letter);

    return 0;
}

/*
 * =====================================================================================
 * |                                    - LESSON END -                                   |
 * =====================================================================================
 *
 * In this lesson, you learned the basics of making your programs interactive.
 *
 * Key Takeaways:
 * - The `scanf()` function reads formatted input from the keyboard.
 * - `scanf()` requires the ADDRESS-OF OPERATOR `&` before the variable name to know
 *   *where* to store the input.
 * - Format specifiers for `scanf()` can sometimes be different from `printf()`.
 *   - For `double`: Use `%lf` with `scanf`, but `%f` with `printf`.
 * - `scanf()` can leave newline characters in the input buffer, which can cause
 *   problems. The `" %c"` trick (with a leading space) is used to solve this
 *    when reading characters.
 *
 * 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 3_user_input 3_user_input.c`
 * 4. Run the executable:
 *    - On Linux/macOS:   `./3_user_input`
 *    - On Windows:       `3_user_input.exe`
 */

How to Compile and Run

cc -Wall -Wextra -std=c11 -o 3_user_input 3_user_input.c
./3_user_input