Determining Partition Size

After using the partition() function, you can determine the size of each partition by examining the subrange returned by the function.

This subrange represents the second partition, where the predicate returned false. The elements before this subrange belong to the first partition.

Here’s an example to illustrate how you can find the size of each partition:

1#include <algorithm>
2#include <iostream>
3#include <vector>
4
5int main() {
6  std::vector<int> A = {1, -6, 7, 2, 5, 4};
7
8  auto isEven = [](int x) { return x % 2 == 0; };
9
10  auto SecondSR = std::ranges::partition(A, isEven);
11
12  // Size of the second partition
13  std::cout << "Second Partition Size: "
14            << SecondSR.size() << '\n';
15
16  // Create a subrange for the first partition
17  auto FirstSR = std::ranges::subrange(
18    A.begin(), SecondSR.begin());
19
20  // Size of the first partition
21  std::cout << "First Partition Size: "
22            << FirstSR.size() << '\n';
23
24  // Output elements in both partitions
25  std::cout << "First Partition: ";
26  for (int x : FirstSR) std::cout << x << ", ";
27  std::cout << "\nSecond Partition: ";
28  for (int x : SecondSR) std::cout << x << ", ";
29}

1Second Partition Size: 3
2First Partition Size: 3
3First Partition: 4, -6, 2,
4Second Partition: 7, 5, 1,

In this example:

• The isEven predicate determines if an element is even.
• std::ranges::partition() partitions the vector A into even and odd numbers.
• SecondSR represents the subrange of odd numbers.
• FirstSR is constructed using iterators from the beginning of A to the beginning of SecondSR.

By calculating the sizes of FirstSR and SecondSR, you get the sizes of the first and second partitions, respectively. This method allows you to work with partitions efficiently after using partition().