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Lambda Expressions

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Lambda expressions in C++ are a powerful feature introduced in C++11 that allow you to define anonymous functions directly in your code. They are particularly useful for short snippets of code that are used only once or a few times, such as in algorithms or event handling.

Key Concepts

  1. Syntax: The basic syntax of a lambda expression is:

    [capture](parameters) -> return_type { body }
    • Capture: Specifies which variables from the surrounding scope are accessible inside the lambda.
    • Parameters: The input parameters for the lambda function.
    • Return Type: (Optional) Specifies the return type of the lambda function.
    • Body: The code that defines what the lambda function does.

  2. Capture Clause: The capture clause can be:

    • [ ] : No variables are captured.
    • [=] : All variables in the surrounding scope are captured by value.
    • [&] : All variables in the surrounding scope are captured by reference.
    • [a, &b] : a is captured by value, and b is captured by reference.

  3. Return Type: If the return type is not specified, the compiler will deduce it automatically.

Practical Examples

Example 1: Basic Lambda Expression

#include <iostream>

int main() {
    auto greet = []() {
        std::cout << "Hello, World!" << std::endl;
    };

    greet(); // Output: Hello, World!
    return 0;
}

Explanation:

  • auto greet = []() { ... }; defines a lambda expression that takes no parameters and returns nothing.
  • greet(); calls the lambda function.

Example 2: Lambda with Parameters

#include <iostream>

int main() {
    auto add = [](int a, int b) -> int {
        return a + b;
    };

    std::cout << "Sum: " << add(3, 4) << std::endl; // Output: Sum: 7
    return 0;
}

Explanation:

  • auto add = [](int a, int b) -> int { ... }; defines a lambda that takes two integers and returns their sum.
  • add(3, 4); calls the lambda function with arguments 3 and 4.

Example 3: Capturing Variables

#include <iostream>

int main() {
    int x = 10;
    int y = 20;

    auto add = [x, y]() -> int {
        return x + y;
    };

    std::cout << "Sum: " << add() << std::endl; // Output: Sum: 30
    return 0;
}

Explanation:

  • auto add = [x, y]() -> int { ... }; captures x and y by value.
  • add(); calls the lambda function, which uses the captured values of x and y.

Example 4: Capturing by Reference

#include <iostream>

int main() {
    int x = 10;

    auto increment = [&x]() {
        x++;
    };

    increment();
    std::cout << "x: " << x << std::endl; // Output: x: 11
    return 0;
}

Explanation:

  • auto increment = [&x]() { ... }; captures x by reference.
  • increment(); calls the lambda function, which increments the value of x.

Exercises

Exercise 1: Simple Lambda

Write a lambda function that takes two integers and returns their product.

Solution:

#include <iostream>

int main() {
    auto multiply = [](int a, int b) -> int {
        return a * b;
    };

    std::cout << "Product: " << multiply(5, 6) << std::endl; // Output: Product: 30
    return 0;
}

Exercise 2: Capturing Variables

Write a lambda function that captures an integer by reference and increments it by 5.

Solution:

#include <iostream>

int main() {
    int num = 10;

    auto incrementByFive = [&num]() {
        num += 5;
    };

    incrementByFive();
    std::cout << "num: " << num << std::endl; // Output: num: 15
    return 0;
}

Exercise 3: Using Lambda in STL Algorithm

Use a lambda expression to sort a vector of integers in descending order.

Solution:

#include <iostream>
#include <vector>
#include <algorithm>

int main() {
    std::vector<int> numbers = {1, 4, 2, 8, 5, 7};

    std::sort(numbers.begin(), numbers.end(), [](int a, int b) {
        return a > b;
    });

    for (int num : numbers) {
        std::cout << num << " "; // Output: 8 7 5 4 2 1
    }
    std::cout << std::endl;

    return 0;
}

Common Mistakes and Tips

  • Forgetting to capture variables: Ensure that you capture all necessary variables from the surrounding scope.
  • Incorrect capture mode: Use [=] for capturing by value and [&] for capturing by reference. Mixing them up can lead to unexpected behavior.
  • Overusing lambdas: While lambdas are powerful, overusing them can make the code harder to read. Use them judiciously.

Conclusion

Lambda expressions are a versatile feature in C++ that can simplify your code and make it more expressive. By understanding how to define and use them, you can write more concise and readable code. In the next topic, we will explore multithreading, which allows you to run multiple threads concurrently in your C++ programs.

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