Basic Input/Output

Learn how to make your programs interactive by reading user input and displaying output in the console.

A Simple Example

#include <iostream>
#include <string>

int main() {
    // Output - prompt the user
    std::cout << "Enter your name: ";

    // Input - read a single word
    std::string name;
    std::cin >> name;

    std::cout << "Enter your age: ";
    int age;
    std::cin >> age;

    // Display results
    std::cout << "Hello, " << name << "!" << "\n";
    std::cout << "You are " << age << " years old." << "\n";

    return 0;
}

Breaking It Down

std::cout - Output Stream

• What it does: Sends data to the console (standard output)
• Use << operator: The insertion operator sends data to cout
• Chainable: You can chain multiple << operators together
• Use '\n' for newlines: Prefer '\n' over std::endl as it's faster and cleaner

std::cin - Input Stream

• What it does: Reads data from the console (standard input)
• Use >> operator: The extraction operator reads data into variables
• Stops at whitespace: std::cin >> name only reads one word
• Remember: Use std::getline(std::cin, name) to read full lines with spaces

The Direction of << and >>

• << points where data flows: cout << data sends data OUT
• >> points where data flows: cin >> variable pulls data INTO variable
• Visual mnemonic: The arrows literally show data direction
• Remember: Think of them as data flow arrows, not just syntax

std::getline() for Full Lines

• What it does: Reads an entire line including spaces
• Syntax: std::getline(std::cin, variable)
• Use for: Names, addresses, sentences, any multi-word input
• Remember: Required for reading strings with spaces

Why This Matters

• Real programs interact with users. Whether you're building a calculator, game, or data processor, you need to accept input and display results clearly.
• Input/output is how your code communicates with the world - it's the bridge between your algorithms and the user experience.

Critical Insight

Think of std::cin and std::cout as opposite ends of a pipe. The << operator points where data flows: cout << data sends data out, cin >> variable pulls data into a variable.

The arrows show the direction of data flow - it's not just syntax, it's a visual representation of what's happening!

std::cout << "Output";  // Data flows OUT to screen
std::cin >> input;      // Data flows IN to variable

Best Practices

Always prompt before input: Tell users what to enter with clear messages like "Enter your age: ".

Use std::getline() for text with spaces: For names, addresses, or sentences, use std::getline(std::cin, variable) instead of std::cin >>.

Validate input types: Remember that entering text when a number is expected causes input failure.

Prefer '\n' over std::endl: Use '\n' for newlines - it's faster because std::endl unnecessarily flushes the buffer.

Common Mistakes

Reading names with spaces: std::cin >> name only reads the first word. Use std::getline(std::cin, name) for full names.

Not prompting the user: Always tell users what to enter. Silent programs are confusing.

Type mismatch: Reading text into an int variable causes input failure and unpredictable behavior.

Mixing cin >> and getline: After using cin >>, there's often a leftover newline that getline reads. Use cin.ignore() to clear it.

Debug Challenge

This program tries to read a full name but has a bug. Click the highlighted line to fix it:

1 #include <iostream>

2 #include <string>

3

4 int main() {

5 std::cout << "Enter your full name: ";

6 std::string name;

7 std::cin >> name;

8 std::cout << "Hello, " << name << "\n";

9 return 0;

10 }

std::cin >> name; std::getline(std::cin, name); std::cin.get(name); std::readline(name);

Quick Quiz

1. What happens when you use std::cin >> age and the user types "twenty"?

Input fails and age gets 0 or garbage

It stores 20

Compiler error

std::cin expects a number for an int variable. Text input fails and leaves the variable in an invalid state.

2. What does the << operator do with std::cout?

Sends data to the output stream

Multiplies numbers

Reads user input

The insertion operator << inserts data into the output stream (console)

3. How do you read a full name with spaces using std::cin?

std::getline(std::cin, name);

std::cin >> name;

std::cin.read(name);

std::cin >> stops at whitespace. Use std::getline() to read entire lines including spaces

Check Answers Try Again

Practice Playground

Time to try out what you just learned! Play with the example code below, experiment by making changes and running the code to deepen your understanding.

Run Code Reset

Output:

Error:

Lesson Progress

• Fix This Code
• Quick Quiz
• Practice Playground - run once

Complete Lesson