A Complete Guide - C Programming Input and Output Functions printf, scanf

Part 1: Understanding printf Function

The printf function is used to format and print data to the output device (usually the screen). The general syntax of printf is as follows:

printf("format string", value1, value2, ...);

Example 1: Printing a Simple Message

This example shows how to print a simple text message to the console.

#include <stdio.h>

int main() {
    // Print a simple message to the user.
    printf("Hello, World!\n");
    return 0;
}

Output:

Hello, World!

Explanation:

• #include <stdio.h>: This includes the standard input-output library which contains definitions of the C I/O functions such as printf.
• main() is the entry point of a C program.
• printf("Hello, World!\n");: Prints the text "Hello, World!" followed by a newline character (\n) to the console.

Example 2: Including Variables in printf

Let's see how to include variables within the output.

#include <stdio.h>

int main() {
    int number = 42;
    float pi = 3.14159;
    char letter = 'A';
    
    // Print variable values with appropriate format specifiers
    printf("Number: %d\n", number);       // %d for integers
    printf("Pi: %.2f\n", pi);            // %.2f for floats with 2 decimal places
    printf("Letter: %c\n", letter);      // %c for characters
    
    return 0;
}

Output:

Number: 42
Pi: 3.14
Letter: A

Explanation:

• %d: Format specifier for integer types.
• %.2f: Format specifier for floating-point numbers with 2 decimal places.
• %c: Format specifier for single character types.

Part 2: Understanding scanf Function

The scanf function is used to read formatted data from the standard input (usually the keyboard). Its syntax looks like this:

scanf("format string", &value1, &value2, ...);

Example 1: Reading an Integer from User Input

This example reads an integer from the user and prints it back.

#include <stdio.h>

int main() {
    int number;
    
    // Prompt the user for input
    printf("Enter an integer: ");
    
    // Read the integer from user input
    scanf("%d", &number);
    
    // Display the entered integer
    printf("You entered: %d\n", number);
    return 0;
}

Sample Interaction:

Enter an integer: 56
You entered: 56

Explanation:

• The program prompts the user to enter an integer.
• scanf reads the input and stores it in the number variable after matching the %d format specifier.
• It uses the address-of operator (&) to store the value in memory location associated with number.

Example 2: Reading Multiple Variable Types

This example demonstrates reading multiple different kinds of variables from user input.

#include <stdio.h>

int main() {
    int num;
    float floatNum;
    char ch;
    
    // Prompt the user for input
    printf("Enter an integer, a float, and a character separated by spaces: ");
    
    // Read the input values of different types
    scanf("%d %f %c", &num, &floatNum, &ch);
    
    // Display the entered values
    printf("Integer entered: %d\n", num);
    printf("Float entered: %.2f\n", floatNum);
    printf("Character entered: %c\n", ch);
    
    return 0;
}

Sample Interaction:

Enter an integer, a float, and a character separated by spaces: 12 2.718 e
Integer entered: 12
Float entered: 2.72
Character entered: e

Explanation:

• The program asks the user to enter three inputs: an integer, a float, and a character.
• scanf matches the format specifiers (%d, %f, %c) and stores the values at the addresses provided (&num, &floatNum, &ch).
• Each value is subsequently printed using printf, formatted appropriately with their respective specifiers.

Additional Tips:

1. Whitespace Handling:

   • scanf interprets whitespace (spaces, tabs, newlines) as delimiters for different input fields. Ensure inputs are separated correctly when you are entering multiple values.

2. Format Specifiers:

   • There are different format specifiers for various data types like:
     • %d or %i : Signed or unsigned integer.
     • %u : Unsigned integer.
     • %f : Floating-point number.
     • %s : String (without spaces).
     • %c : Single character.
     • %% : Literal percent sign.

3. Input Buffer:

   • After using scanf, leftover newline characters might remain in the input buffer. If your next input operation is scanf with %c, it may cause unexpected behavior. A common way to clear the buffer is by reading till the newline character, using scanf("%*[^\n]"); scanf("%*c"); or getchar();.

4. Error Checking:

   • scanf returns the number of items successfully matched and assigned. Always check this return value to handle errors and malformed user inputs.

Practice Problems:

1. Modify Example 2 to read two integers and one floating-point number, then calculate and print their sum.
2. Create a program that asks the user for their name and age, then welcomes them and mentions their age.
3. Write a program that reads three integers, determines the maximum among them, and prints it.