The C++ algorithm Library: sort, find, count

The C++ <algorithm> library provides powerful and commonly used functions for working with vectors and other STL containers. In this article, we explore three of the most frequently used functions: sort, find, and count, explained with practical examples.

1) sort Function

std::sort sorts a range (such as a vector or array) in ascending order by default. Sorting usually runs with an average time complexity of O(n log n).

Basic Usage

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

int main() {
    vector<int> v = {30, 10, 40, 20};

    sort(v.begin(), v.end());

    for (int n : v)
        cout << n << " ";
}

Output: 10 20 30 40

Descending Sort

sort(v.begin(), v.end(), greater<int>());

Custom Comparator

sort(v.begin(), v.end(), [](int a, int b) {
    return a % 10 < b % 10; // sort by last digit
});

2) find Function

std::find searches for a value within a range and returns an iterator to the first match. If the value is not found, it returns the end() iterator.

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

int main() {
    vector<int> v = {10, 20, 30, 40};

    auto it = find(v.begin(), v.end(), 30);

    if (it != v.end())
        cout << "Found: " << *it;
    else
        cout << "Not found!";
}

Searching Strings

vector<string> names = {"Alice", "Emma", "John"};

auto it = find(names.begin(), names.end(), "Emma");

3) count Function

std::count counts how many times a value appears within a range.

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

int main() {
    vector<int> v = {10, 20, 20, 30, 20};

    int c = count(v.begin(), v.end(), 20);

    cout << "Count of 20: " << c;
}

Output: Count of 20: 3

Conditional Counting (count_if)

int c = count_if(v.begin(), v.end(), [](int x) {
    return x % 2 == 0; // count even numbers
});

4) Practical Example — Counting Words

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

int main() {
    vector<string> words = {"cpp", "java", "cpp", "python", "cpp"};

    int cppCount = count(words.begin(), words.end(), "cpp");

    cout << "\"cpp\" appears " << cppCount << " times";
}

5) Using sort, find, and count Together

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

int main() {
    vector<int> v = {50, 20, 10, 20, 40};

    sort(v.begin(), v.end()); // 10 20 20 40 50

    auto it = find(v.begin(), v.end(), 20); // first occurrence of 20

    int c = count(v.begin(), v.end(), 20); // count 20s

    cout << "First 20 found at index: " << (it - v.begin()) << endl;
    cout << "Total count of 20: " << c;
}

6) When to Use Each?

• sort: For sorting any container or range
• find: For locating a specific element quickly
• count: For counting occurrences of a value

The <algorithm> library is one of the most powerful tools in the STL for performing high-performance operations on containers.

7) TL;DR

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