Summary - Lambda
- C++ course
- Intermediate
- Lesson 81/116
- 5 min
Combine capture, STL and std::function to write a filter function.
Summary - Lambda
In this group you learned what lambdas are, the [](){} syntax, capture modes, use with STL and std::function. Now we combine these to write a filter function.
First we will count even numbers using a lambda with std::count_if. Then we will store that lambda in a std::function and reuse it. This shows how the combination of lambda + STL + std::function works in a real scenario.
Practice Count even numbers with a lambda
For v = {1, 2, 3, 4, 5, 6}, use std::count_if with a lambda to count even numbers. The lambda is [](int x) { return x % 2 == 0; }. Print the result with std::cout.
Starter code
#include <iostream>
#include <vector>
#include <algorithm>
int main() {
std::vector<int> v = {1, 2, 3, 4, 5, 6};
// Count even numbers with std::count_if and a lambda, print
return 0;
}Sample solution
#include <iostream>
#include <vector>
#include <algorithm>
int main() {
std::vector<int> v = {1, 2, 3, 4, 5, 6};
int n = std::count_if(v.begin(), v.end(), [](int x) { return x % 2 == 0; });
std::cout << n;
return 0;
}Expected output
3
Practice Store the lambda in std::function
Add #include <functional>. Store the lambda with std::function<bool(int)> isEven = [](int x) { return x % 2 == 0; };. Then count even numbers with std::count_if(v.begin(), v.end(), isEven) and print the result.
Starter code
#include <iostream>
#include <vector>
#include <algorithm>
#include <functional>
int main() {
std::vector<int> v = {1, 2, 3, 4, 5, 6};
// Store the isEven lambda in a function object, count with count_if
return 0;
}Sample solution
#include <iostream>
#include <vector>
#include <algorithm>
#include <functional>
int main() {
std::vector<int> v = {1, 2, 3, 4, 5, 6};
std::function<bool(int)> isEven = [](int x) { return x % 2 == 0; };
int n = std::count_if(v.begin(), v.end(), isEven);
std::cout << n;
return 0;
}Expected output
3
Lambda recap
Lambda expressions are anonymous functions in C++. They use the syntax [capture](params) -> type { body }. The capture clause captures outer variables: [=] by value, [&] by reference.
They form a powerful pair with STL algorithms: std::sort, std::find_if, std::count_if and similar take lambdas for custom conditions. std::function stores a lambda in an object for reuse. In the next group you will learn smart pointers, which will make memory management safe.
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What is used to store a lambda for later calling?
auto f = [](int x) { return x; };- std::sort
- std::find
- std::function Correct answer
- std::count
Explanation: std::function is the general type for storing lambdas.
2 Complete the count_if call
int n = std::____(v.begin(), v.end(), [](int x) { return x % 2 == 0; });int n = std::count_if(v.begin(), v.end(), [](int x) { return x % 2 == 0; });Explanation: std::count_if counts elements matching a condition.