The STL and professional C++ · Lesson 18 of 21

Algorithms and lambdas

sort, find_if, count_if and accumulate, with one-line functions.

11 minutes3 games

The <algorithm> header has dozens of ready-made functions that work on any container through iterators: begin() and end(). Instead of writing a loop each time, use them: quicker to write, well tested, and clearer to read.

A lambda is a small unnamed function written right where it's needed: [](int x) { return x > 50; }.

C++

#include <algorithm>
#include <iostream>
#include <numeric>
#include <string>
#include <vector>

struct Student {
    std::string name;
    int xp;
};

int main() {
    std::vector<Student> s = {{"Laila", 320}, {"Karim", 150}, {"Omar", 90}, {"Nour", 410}};

    std::sort(s.begin(), s.end(), [](const Student& a, const Student& b) { return a.xp > b.xp; });
    for (const auto& st : s) std::cout << st.name << " ";
    std::cout << "\n";

    int threshold = 100;
    auto above = std::count_if(s.begin(), s.end(), [threshold](const Student& st) { return st.xp > threshold; });
    std::cout << above << " above " << threshold << "\n";

    auto it = std::find_if(s.begin(), s.end(), [](const Student& st) { return st.name[0] == 'K'; });
    if (it != s.end()) std::cout << "found " << it->name << "\n";

    int total = std::accumulate(s.begin(), s.end(), 0, [](int sum, const Student& st) { return sum + st.xp; });
    std::cout << "total " << total << "\n";
    return 0;
}

Part	Meaning
[]	captures nothing
[threshold]	captures a copy of threshold
[&total]	captures total by reference (can change it)
[=] / [&]	everything it uses, by copy / by reference
(int x)	parameters
{ return … }	body

What's the output?

#include <algorithm>
#include <iostream>
#include <vector>
int main() {
    std::vector<int> v = {5, 2, 8, 2, 9};
    std::sort(v.begin(), v.end());
    auto last = std::unique(v.begin(), v.end());
    v.erase(last, v.end());
    for (int x : v) std::cout << x;
    std::cout << " " << v.size() << "\n";
}

What will this code print?

Order the lines

Order the code that sorts names by length, shortest first.

Click the lines in the right order

Exercise

From {7, 3, 10, 4, 15, 8}: sort the numbers in descending order and print them on one line, then the count of even numbers on a second line.

Expected output:

15 10 8 7 4 3
3

C++

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

int main() {
    std::vector<int> v = {7, 3, 10, 4, 15, 8};
    
    return 0;
}

Learn more from

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#include <algorithm> #include <iostream> #include <numeric> #include <string> #include <vector> struct Student { std::string name; int xp; }; int main() { std::vector<Student> s = {{"Laila", 320}, {"Karim", 150}, {"Omar", 90}, {"Nour", 410}}; std::sort(s.begin(), s.end(), [](const Student& a, const Student& b) { return a.xp > b.xp; }); for (const auto& st : s) std::cout << st.name << " "; std::cout << "\n"; int threshold = 100; auto above = std::count_if(s.begin(), s.end(), [threshold](const Student& st) { return st.xp > threshold; }); std::cout << above << " above " << threshold << "\n"; auto it = std::find_if(s.begin(), s.end(), [](const Student& st) { return st.name[0] == 'K'; }); if (it != s.end()) std::cout << "found " << it->name << "\n"; int total = std::accumulate(s.begin(), s.end(), 0, [](int sum, const Student& st) { return sum + st.xp; }); std::cout << "total " << total << "\n"; return 0; }

Part

Meaning

[]

captures nothing

[threshold]

captures a copy of threshold

[&total]

captures total by reference (can change it)

[=] / [&]

everything it uses, by copy / by reference

(int x)

parameters

{ return … }

body

#include <algorithm> #include <iostream> #include <vector> int main() { std::vector<int> v = {5, 2, 8, 2, 9}; std::sort(v.begin(), v.end()); auto last = std::unique(v.begin(), v.end()); v.erase(last, v.end()); for (int x : v) std::cout << x; std::cout << " " << v.size() << "\n"; }