Sorting a vector in C++

Prerequisites : std::sort in C++, vector in C++, initialize a vector in C++.

CPP

// C++ program to sort a vector in non-decreasing
// order.
#include <bits/stdc++.h>
using namespace std;

int main()
{
    vector<int> v{ 1, 5, 8, 9, 6, 7, 3, 4, 2, 0 };

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

    cout << "Sorted \n";
    for (auto x : v)
        cout << x << " ";

    return 0;
}

Output

Sorted 
0 1 2 3 4 5 6 7 8 9 

How to sort in descending order?
sort() takes a third parameter that is used to specify the order in which elements are to be sorted. We can pass “greater()” function to sort in descending order. This function does comparison in a way that puts greater elements before.

CPP

// C++ program to sort a vector in non-increasing
// order.
#include <bits/stdc++.h>
using namespace std;

int main()
{
    vector<int> v{ 1, 5, 8, 9, 6, 7, 3, 4, 2, 0 };

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

    cout << "Sorted \n";
    for (auto x : v)
        cout << x << " ";

    return 0;
}

Output

Sorted 
9 8 7 6 5 4 3 2 1 0 

How to sort in a particular order?
We can also write our own comparator function and pass it as a third parameter.

The comparator function checks, if the statement returned, is true or false and returns a bool value which is passed to the sort function.

For example, lets say Interval i1 = { 6 , 8 } and Interval i2 = { 1, 9 }. When this is passed to the comparator function, it compares i1.start and i2.start. Since, i1.start (=6) < i2.start (=1), the comparator function returns false. This means that Interval i1 should not be placed before Interval i2. Below is the code for this function.

CPP

// A C++ program to sort vector using
// our own comparator
#include <bits/stdc++.h>
using namespace std;

// An interval has start time and end time
struct Interval {
    int start, end;
};

// Compares two intervals according to starting times.
bool compareInterval(Interval i1, Interval i2)
{
    return (i1.start < i2.start);
}

int main()
{
    vector<Interval> v { { 6, 8 }, { 1, 9 }, { 2, 4 }, { 4, 7 } };

    // sort the intervals in increasing order of
    // start time
    sort(v.begin(), v.end(), compareInterval);

    cout << "Intervals sorted by start time : \n";
    for (auto x : v)
        cout << "[" << x.start << ", " << x.end << "] ";

    return 0;
}

Output

Intervals sorted by start time : 
[1, 9] [2, 4] [4, 7] [6, 8] 

How to sort the array in descending order based on some parameter using a comparator function?

A comparator function can be passed in such a manner so that the elements in the array get sorted in descending order.

C++

// A C++ program to sort vector using
// our own comparator
#include <bits/stdc++.h>
using namespace std;

// An interval has start time and end time
struct Interval {
    int start, end;
};

// Compares two intervals according to ending times in descending order.
bool compareInterval(Interval i1, Interval i2)
{
    return (i1.end > i2.end);
}

int main()
{
    vector<Interval> v { { 6, 8 }, { 1, 9 }, { 2, 4 }, { 4, 7 } };

    // sort the intervals in decreasing order of
    // end time
    sort(v.begin(), v.end(), compareInterval);

    cout << "Intervals sorted by ending time in descending order : \n";
    for (auto x : v)
        cout << "[" << x.start << ", " << x.end << "] ";

    return 0;
}

Output

Intervals sorted by ending time in descending order : 
[1, 9] [6, 8] [4, 7] [2, 4]