User Input and Output

# CHAPTER 6

User Input and Output #

1. Introduction #

2. Learning Objectives #

• Use printf() to output formatted data.

• Use scanf() to read user input.

• Understand format specifiers deeply.

• Master escape sequences.

• Address common input buffer issues.

3. Output with printf() #

#include <stdio.h>

int main() {
    int age = 20;
    float gpa = 3.85;
    
    // Combining text and format specifiers
    printf("I am %d years old.\n", age);
    printf("My GPA is %.1f\n", gpa); // %.1f rounds to 1 decimal place
    
    return 0;
}

4. Input with scanf() #

#include <stdio.h>

int main() {
    int age;
    
    printf("Enter your age: ");
    scanf("%d", &age); // The '&' is CRITICAL!
    
    printf("You are %d years old.\n", age);
    return 0;
}

Why the & (Ampersand)? scanf needs to know *where in memory* to store the data you typed. The & symbol means "address of". So &age means "store this input at the memory address of the variable age."

5. Format Specifiers Reference #

6. Escape Sequences #

• \n : New line

• \t : Tab (horizontal spacing)

• \\ : Prints a backslash

• \" : Prints a double quote

printf("She said, \"C is awesome!\"\n");
printf("Column1\tColumn2\n");

7. Reading Multiple Inputs #

int day, month, year;
printf("Enter your birthdate (DD MM YYYY): ");
scanf("%d %d %d", &day, &month, &year);
printf("You were born on %d/%d/%d\n", day, month, year);

8. Mini Project: Student Information System #

#include <stdio.h>

int main() {
    char initial;
    int age;
    float gpa;

    printf("--- STUDENT INFO REGISTRATION ---\n");
    
    printf("Enter first name initial: ");
    scanf("%c", &initial);
    
    printf("Enter age: ");
    scanf("%d", &age);
    
    printf("Enter current GPA: ");
    scanf("%f", &gpa);
    
    printf("\n--- REGISTRATION SUCCESSFUL ---\n");
    printf("Initial: %c\n", initial);
    printf("Age: %d\n", age);
    printf("GPA: %.2f\n", gpa);

    return 0;
}

9. Memory-Level Explanation #

1. 1. The program pauses and waits for the OS to send keyboard data (stdin).

1. 2. You type 25 and hit Enter. The buffer holds 25\n.

1. 3. scanf reads the 25, converts it to binary, and writes it directly to the memory address provided by &age.

1. 4. The \n remains in the buffer (which can cause bugs for future scanf calls!).

10. Common Mistakes #

• Forgetting the & in scanf: scanf("%d", age); will cause the program to crash (Segmentation Fault) because it tries to write to memory address 20 (or whatever value age held).

• Using & with strings: When reading strings (%s), you don't need the & (we'll explain why in Chapter 11).

• The Input Buffer Issue: If you read an integer, then try to read a character, scanf("%c") might accidentally read the "Enter" key (\n) left behind by the integer input. Add a space before %c: scanf(" %c", &var); to fix it.

11. Exercises #

1. 1. Write a program that asks for a temperature in Celsius and prints it in Fahrenheit. (F = C * 9/5 + 32).

1. 2. Write a program that uses \t to print a neatly aligned grocery receipt.

12. MCQ Quiz with Answers #

13. Interview Questions #

• Q: Why do we pass the address (&) of a variable to scanf instead of the variable itself?

• Q: What is the input buffer, and why does reading a %c after a %d sometimes fail?

• Q: How can you prevent buffer overflows when reading strings using scanf?

14. Summary #

15. Next Chapter Recommendation #