C++ lambdas and the standard algorithms
The <algorithm> header provides functions that work on ranges given by two iterators: sort, find_if, count_if, transform, accumulate, and more. Many take a predicate or comparison function. A lambda expression writes such a function in place: [capture](parameters) { body }. The capture list copies or references variables from the surrounding scope.
C++
#include <algorithm>
#include <iostream>
#include <numeric>
#include <vector>
void show(const char* label, const std::vector<int>& v) {
std::cout << label << ":";
for (int x : v) {
std::cout << " " << x;
}
std::cout << "\n";
}
int main() {
std::vector<int> v = {5, 2, 8, 1, 9, 3};
std::sort(v.begin(), v.end(), [](int a, int b) { return a > b; });
show("descending", v);
auto it = std::find_if(v.begin(), v.end(), [](int x) { return x < 4; });
std::cout << "first below 4: " << *it << "\n";
int threshold = 4;
auto above = std::count_if(v.begin(), v.end(), [threshold](int x) { return x > threshold; });
std::cout << "above " << threshold << ": " << above << "\n";
std::vector<int> squares(v.size());
std::transform(v.begin(), v.end(), squares.begin(), [](int x) { return x * x; });
std::cout << "sum of squares: " << std::accumulate(squares.begin(), squares.end(), 0) << "\n";
v.erase(std::remove_if(v.begin(), v.end(), [](int x) { return x % 2 != 0; }), v.end());
show("evens only", v);
}