is_sorted() in C++ STL

In C++, is_sorted() is a built-in function used to check whether the element of the given range is sorted or not in ascending order. In this article, we will learn about is_sorted() function in C++.

Let’s take a quick look at a simple example that illustrates is_sorted() method:

#include <bits/stdc++.h>
using namespace std;

int main() {
    vector<int> v = {1, 5, 6, 8, 9};

    // Checking if vector v is sorted or not
    if (is_sorted(v.begin(), v.end()))
        cout << "Sorted";
    else
        cout << "Not Sorted";
    return 0;
}

Given Vector is Sorted

Explanation: By default, is_sorted() function checks whether the range is sorted or not in ascending order. As the vector v is sorted in ascending, the function returned true. Let's look at is_sorted() in detail.

This article covers the syntax, usage, and common examples of is_sorted() method in C++:

Syntax of is_sorted()

The is_sorted() is defined inside <algorithm> header file.

is_sorted(first, last);
is_sorted(first, last, comp);

Parameters

• first: Iterator to the first element of given range.
• last: Iterator to the element just after the last element of given range.
• comp(optional): It is a custom comparison function used to change the sorting order to be checked. By default, it uses the < operator which check if the range is sorted in ascending order.

Return Value

• Returns true, if the range is sorted in the given order.
• Returns false, if the range is not sorted in given order.

Examples of is_sorted()

The following examples demonstrates the use of is_sorted() method in different scenario:

Check if an Array is Sorted in Ascending Order

#include <bits/stdc++.h>
using namespace std;

int main() {
    int arr[] = {4, 7, 8, 9};
    int n = sizeof(arr)/sizeof(arr[0]);

    // Check whether array is sorted in ascending
  	// order
    if (is_sorted(arr, arr + n))
        cout << "Sorted";
    else
        cout << "Not Sorted";
    return 0;
}

Given Array is Sorted

Check if a List is Sorted in Descending Order

#include <bits/stdc++.h>
using namespace std;

// Comparator function
bool comp(int a, int b) {
    return a > b;
}

int main() {
    list<int> l = {9, 7, 6, 3};

    // Check vector Sorted in descending order
    if (is_sorted(l.begin(), l.end(), comp))
        cout << "Sorted";
    else
        cout << "Not Sorted";
    return 0;
}

Given Vector is Sorted

Explanation: As by default, is_sorted() checks for ascending order, we have to provide custom comparison function to check for descending order.

Check if Vector of String is in Lexicographically Ordered

#include <bits/stdc++.h>
using namespace std;

int main() {
    vector<string> v = {"hi", "geeks", "welcome", "to" "geeksforgeeks"};

    // Check vector sorted in user
    // defined order or not
    if (is_sorted(v.begin(), v.end()))
        cout << "Sorted";
    else
        cout << "Not Sorted";
    return 0;
}

Given Vector is Sorted

Check if Set is Sorted in Descending Order

#include <bits/stdc++.h>
using namespace std;

// Comparator function
bool comp(int a, int b) {
    return a > b;
}

int main() {
    set<int> s = {9, 7, 6, 3};

    // Check vector Sorted in descending order
    if (is_sorted(s.begin(), s.end(), comp))
        cout << "Sorted";
    else
        cout << "Not Sorted";
    return 0;
}

Not Sorted

Explanation: The set is already sorted in ascending order. We have used is_sorted() with custom comparator to check if the set is in descending order.