Inputs

This section focuses on handling user input in C using scanf() and fgets(), two of the most common functions for reading keyboard input. Other input functions exist, but we’ll focus on these two; you can explore others (like getchar()) if you’re curious.

What is Input in C?

In C programming, input refers to data provided by the user, typically through the keyboard, during program execution. The <stdio.h> header file provides functions like scanf() and fgets() to capture this input.

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1. Using scanf() for Input

The scanf() function reads formatted data, such as numbers, single words, or characters, from the user. It uses format specifiers to define the type of data to read.

Syntax

scanf("format_specifier", &variable);

• Format specifiers:
• %d: Integer
• %f: Float
• %c: Single character
• %s: String (stops at a space)

• %lf: Double

• The & (address-of operator) tells scanf() where to store the input in memory. For strings (%s), & is not needed.

Example: Reading an Integer

#include <stdio.h>

int main() {
    int age;
    printf("Enter your age: ");
    scanf("%d", &age); // Reads an integer
    printf("You are %d years old.\n", age);
    return 0;
}

Explanation:

• printf() prompts the user to enter their age.
• scanf("%d", &age) reads an integer and stores it in age.
• The program prints the entered age.

Example: Reading Multiple Inputs

Note: We will study array in later section of the course. For reference, array also points to memory location so no need to include & when getting input as a string or array.

#include <stdio.h>

int main() {
    char name[50];
    int age;
    float height;

    printf("Enter your name, age, and height (in meters): ");
    scanf("%s %d %f", name, &age, &height); // Reads string, int, and float

    printf("Name: %s, Age: %d, Height: %.2f meters\n", name, age, height);
    return 0;
}

Explanation:

• %s reads a string (a single word, as it stops at a space).
• %d and %f read an integer and float, respectively.
• Inputs are separated by spaces when typed (e.g., John 25 1.75).

Note: scanf() with %s stops at a space. For example, entering "John Doe" will only store "John". Use fgets() for full sentences.

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2. Using fgets() for String Input

The fgets() function reads a full line of text, including spaces, making it ideal for strings. It’s safer than other string input methods because you can limit the input size to avoid buffer overflow.

Syntax

fgets(variable, size, stdin);

• variable: The array to store the string.
• size: Maximum number of characters to read (including the null terminator \0).
• stdin: The input source (keyboard).

Example: Reading a Sentence

#include <stdio.h>

int main() {
    char sentence[100];
    printf("Enter a sentence: ");
    fgets(sentence, 100, stdin); // Reads up to 99 characters
    printf("You entered: %s", sentence);
    return 0;
}

Explanation:

• fgets() reads a line of text, including spaces, until the user presses Enter or the size limit (100) is reached.
• The newline character (\n) is included in the string.

Example: Removing Newline from fgets()

#include <stdio.h>
#include <string.h>

int main() {
    char sentence[100];
    printf("Enter a sentence: ");
    fgets(sentence, 100, stdin);

    // Remove newline if present
    if (sentence[strlen(sentence) - 1] == '\n') {
        sentence[strlen(sentence) - 1] = '\0';
    }

    printf("You entered: %s\n", sentence);
    return 0;
}

Explanation:

• strlen(sentence) gets the string length.
• If the last character is \n, it’s replaced with \0 (null terminator) to remove the newline.

Note: When using scanf() followed by fgets(), the newline character left in the input buffer by scanf() can cause fgets() to read an empty line. To fix this, clear the input buffer

Summary

• Use scanf() for numbers, single characters, or single words.
• Use fgets() for full lines of text or strings with spaces.
• Clear the input buffer when mixing scanf() and fgets().
• Other input functions like getchar() or non-standard ones exist if you want to explore more.