Sort in Python and C++

Sort in Python

1. In-place method

This method sorts a list in-place, which means it modifies the original list directly without returning a new list.

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listA = [1, 5, 4, 3]
listA.sort()
 

By default, it sorts a list in ascending order ([1, 3, 4, 5]). To sort it in descending order, use reverse parameter:

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listA = [1, 5, 4, 3]
listA.sort(reverse=True)
 

2. Return a new list

This method creates a new sorted list and does not modify the original list.

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listA = [1, 5, 4, 3]
sorted_listA = sorted(listA)
sorted_rv_listA = sorted(listA, reverse=True)
 

3. Key parameter

How to sort a lists of pairs? Use key paramter.

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listOfPairs = [(1, 4), (5, 2), (10, 7), (8, 9)]
# Sort by first item
sorted_listOfPairs = sorted(listOfPairs, key=lambda x: x[0])
# Sort by second item
sorted_listOfPairs = sorted(listOfPairs, key=lambda x: x[1])
 

Sort in C++

1. Header file

sort is in <algorithm> header file.

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#include <algorithm>

using namespace std;
 

2. In-place method is default

By default, it modifies the original vector.

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vector<int> v = {1, 5, 4, 3};
sort(v.begin(), v.end());
 

As the same as Python, it sorts a list in ascending order ({1, 3, 4, 5}).

3. Return a new vector

If you want to return a new sorted vector without modifying the original one, create a new function, pass the original vector directly (not reference) and sort it inside:

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#include <vector>
#include <algorithm>

using namespace std;

vector<int> not_in_place_sort(vector<int>& original) {
    sort(original.begin(), original.end());
    return original;
}

int main() {
    vector<int> v = {1, 5, 3, 4};
    vector<int> v_sorted = not_in_place_sort(v);
    return 0;
}
 

4. Comparator

C++ sort has comparator parameter. comparator is optional. With this comparator, you can sort a vector in descending order, you can sort a vector of pairs, etc.

5. Sort in descending order using comparator

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vector<int> v1 = {1, 3, 5, 4, 2};
sort(v1.begin(), v1.end(), greater<int>);
 

6. Sort a vector of pairs

For comparator, it should be a function that returns boolean value (binary function).

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bool comp(pair<int, int> left, pair<int, int> right) {
	return left.first < right.first;
}

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

Instead of defining a new function, you can use lambda function:

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sort(v.begin(), v.end(),
	[] (const auto& lhs, const auto& rhs) {
    return lhs.first < rhs.first;
    });