Ready-Made Algorithms: sort, find and more

C++ comes with a toolbox of ready-made algorithms in <algorithm>. They work on a range given by v.begin() (the start) and v.end() (just past the end).

Sorting and reversing

#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;

int main() {
    vector<int> v = {42, 7, 19, 3, 25};
    sort(v.begin(), v.end());              // smallest to largest
    for (int x : v) cout << x << " ";
    cout << "\n";

    reverse(v.begin(), v.end());           // flip the order
    for (int x : v) cout << x << " ";
    cout << "\n";
    return 0;
}

Output

3 7 19 25 42 
42 25 19 7 3 

Tip: For largest-to-smallest in one step: sort(v.begin(), v.end(), greater<int>());

Smallest, largest, count and sum

#include <iostream>
#include <vector>
#include <algorithm>
#include <numeric>
using namespace std;

int main() {
    vector<int> v = {4, 8, 15, 16, 23, 42, 8};
    cout << "Smallest: " << *min_element(v.begin(), v.end()) << "\n";
    cout << "Largest:  " << *max_element(v.begin(), v.end()) << "\n";
    cout << "How many 8s: " << count(v.begin(), v.end(), 8) << "\n";
    cout << "Sum: " << accumulate(v.begin(), v.end(), 0) << "\n";
    return 0;
}

Output

Smallest: 4
Largest:  42
How many 8s: 2
Sum: 116

min_element and max_element give back the position of the item; the * in front reads the value at that position. (You'll understand why in the Pointers module.)

Is it in there? find

#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;

int main() {
    vector<int> v = {5, 10, 15, 20};
    auto it = find(v.begin(), v.end(), 15);
    if (it != v.end()) {
        cout << "Found 15 at index " << (it - v.begin()) << "\n";
    }
    if (find(v.begin(), v.end(), 99) == v.end()) {
        cout << "99 is not in the list\n";
    }
    return 0;
}

Output

Found 15 at index 2
99 is not in the list

Remember: When find doesn't find anything, it returns v.end(). Always compare with it.