> For the complete documentation index, see [llms.txt](https://alham-rizvi.gitbook.io/alhamrizvi/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://alham-rizvi.gitbook.io/alhamrizvi/c++-notes/08.-user-input.md).

# 08. User Input

`std::cin` ("console in") reads input the user types in the terminal. It's the counterpart to `std::cout`.

```cpp
#include <iostream>

int main() {
    int age;

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

    std::cout << "You are " << age << " years old\n";

    return 0;
}
```

`>>` here is the **extraction operator** — it pulls data *out of* the input stream and *into* your variable. (Notice it points the opposite direction of `<<`, which pushes data *out* to the console.)

***

### Reading multiple values at once

```cpp
#include <iostream>

int main() {
    int a, b;

    std::cout << "Enter two numbers: ";
    std::cin >> a >> b; // chain multiple reads together

    std::cout << "Sum: " << a + b << '\n';

    return 0;
}
```

The user can type `5 10` (space-separated) and hit Enter — `cin` splits on whitespace automatically.

***

### ⚠️ The classic `cin` + strings trap

`std::cin >> someString` only reads **one word** — it stops at the first space. To read a full line (like a full name), use `std::getline`.

```cpp
#include <iostream>
#include <string>

int main() {
    std::string fullName;

    std::cout << "Enter your full name: ";
    std::getline(std::cin, fullName); // reads the WHOLE line, spaces included

    std::cout << "Hello, " << fullName << "!\n";

    return 0;
}
```

**Compare the two:**

```cpp
std::string name;
std::cin >> name;          // "Cristiano Ronaldo" -> only stores "Cristiano"
std::getline(std::cin, name); // "Cristiano Ronaldo" -> stores the whole thing
```

***

### More cool examples

#### 1. Simple age checker

```cpp
#include <iostream>

int main() {
    int age;

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

    if (age >= 18) {
        std::cout << "You're an adult\n";
    } else {
        std::cout << "You're a minor\n";
    }

    return 0;
}
```

#### 2. Mini calculator using input

```cpp
#include <iostream>

int main() {
    double num1, num2;
    char op;

    std::cout << "Enter calculation (e.g. 5 + 3): ";
    std::cin >> num1 >> op >> num2;

    double result = 0;

    if (op == '+') result = num1 + num2;
    else if (op == '-') result = num1 - num2;
    else if (op == '*') result = num1 * num2;
    else if (op == '/') result = num1 / num2;

    std::cout << "Result: " << result << '\n';

    return 0;
}
```

#### 3. Reading name + age together (mixing `cin >>` and `getline`)

```cpp
#include <iostream>
#include <string>

int main() {
    std::string name;
    int age;

    std::cout << "Enter your name: ";
    std::getline(std::cin, name);

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

    std::cout << name << " is " << age << " years old\n";

    return 0;
}
```

#### 4. A tiny login-style prompt loop

```cpp
#include <iostream>
#include <string>

int main() {
    std::string password;

    std::cout << "Enter password: ";
    std::cin >> password;

    if (password == "1234") {
        std::cout << "Access granted\n";
    } else {
        std::cout << "Access denied\n";
    }

    return 0;
}
```

#### 5. Validating input isn't garbage

```cpp
#include <iostream>

int main() {
    int number;

    std::cout << "Enter a number: ";

    if (std::cin >> number) {
        std::cout << "You entered: " << number << '\n';
    } else {
        std::cout << "That wasn't a valid number!\n";
    }

    return 0;
}
```

***

### Quick Recap

* `std::cin >> variable;` reads input, stops at whitespace.
* `std::getline(std::cin, stringVar);` reads a **full line**, spaces included — use this for names/sentences.
* You can chain reads: `std::cin >> a >> b;`
* `std::cin` can be checked in an `if` to detect bad/invalid input.
* Mixing `cin >>` and `getline` in the same program needs care — `cin >>` leaves a leftover newline that can mess up the next `getline` (a common next-level gotcha to look out for).


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