User Input and Output in C++

# CHAPTER 6

User Input and Output #

1. Introduction #

2. Learning Objectives #

• Use cout to output data.

• Use cin to read user input.

• Use getline() to read strings with spaces.

• Format output safely.

• Address common input buffer issues.

3. Output with cout #

#include <iostream>
using namespace std;

int main() {
    int age = 20;
    float gpa = 3.85;
    
    // Combining text and variables
    cout << "I am " << age << " years old." << endl;
    cout << "My GPA is " << gpa << endl;
    
    return 0;
}

4. Input with cin #

#include <iostream>
using namespace std;

int main() {
    int age;
    
    cout << "Enter your age: ";
    cin >> age; 
    
    cout << "You are " << age << " years old." << endl;
    return 0;
}

*Note: Unlike C's scanf, you do NOT need the & operator when using cin.*

5. Reading Strings (The cin vs getline() problem) #

string name;
cout << "Enter full name: ";
cin >> name; // If you type "John Doe", name will only store "John"

The Solution: getline() To read a full line of text including spaces, use getline(cin, variable).

#include <iostream>
#include <string> // Required for strings
using namespace std;

int main() {
    string fullName;
    cout << "Enter full name: ";
    getline(cin, fullName); 
    cout << "Hello, " << fullName << "!" << endl;
    return 0;
}

6. The Input Buffer Issue (cin.ignore()) #

Why? When you type 25 and hit ENTER, the buffer stores 25\n. cin >> reads the 25 but leaves the \n in the buffer. getline() sees the \n and thinks you pressed ENTER on an empty string!

Fix: Use cin.ignore() to clear the buffer.

int age;
string name;

cout << "Age: ";
cin >> age;

cin.ignore(); // Clears the leftover '\n' from the buffer!

cout << "Name: ";
getline(cin, name);

7. Mini Project: Student Profile Generator #

#include <iostream>
#include <string>
using namespace std;

int main() {
    string name;
    int age;
    float gpa;

    cout << "--- STUDENT INFO REGISTRATION ---" << endl;
    
    cout << "Enter age: ";
    cin >> age;
    
    cout << "Enter current GPA: ";
    cin >> gpa;
    
    cin.ignore(); // Clear buffer before getline
    
    cout << "Enter full name: ";
    getline(cin, name);
    
    cout << "\n--- REGISTRATION SUCCESSFUL ---" << endl;
    cout << "Name: " << name << endl;
    cout << "Age: " << age << endl;
    cout << "GPA: " << gpa << endl;

    return 0;
}

8. Memory-Level Explanation #

9. Common Mistakes #

• Wrong arrows: cin << age; is illegal. The arrows point towards where the data is going. From cin *into* age (cin >> age).

• Skipping cin.ignore(): Causes the program to skip string inputs if they follow an integer or float input.

10. 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 asks for the user's favorite quote (including spaces) and prints it with quotation marks around it.

11. MCQ Quiz with Answers #

12. Interview Questions #

• Q: Explain why cin >> str stops at a space, but getline() does not.

• Q: How do you check if cin failed (e.g., user entered a string instead of an integer) and how do you recover? (Hint: cin.fail(), cin.clear()).

13. Summary #

14. Next Chapter Recommendation #