How to Count Elements in a Multi-Dimensional Container

Counting elements in a multi-dimensional container requires iterating through each dimension. Here's how you can do it with std::ranges::count_if() for a 2D vector:

1#include <algorithm>
2#include <iostream>
3#include <vector>
4
5int main() {
6  std::vector<std::vector<int>> Numbers{
7    {1, 2, 3},
8    {4, 4, 5},
9    {6, 7, 8}
10  };
11
12  auto isFour{[](int x) { return x == 4; }};
13  int count = 0;
14
15  for (const auto& row : Numbers) {
16    count += std::ranges::count_if(row, isFour);  
17  }
18
19  std::cout << "Count of fours: " << count;
20}

1Count of fours: 2

In this example:

• We have a 2D vector Numbers containing integers.
• The lambda function isFour() checks if a given number is 4.
• We use a loop to iterate through each row of the 2D vector and apply std::ranges::count_if() to count the number of 4s in each row.
• The counts from each row are accumulated in the count variable.

This approach can be extended to higher-dimensional containers by adding more nested loops. For instance, for a 3D vector, you would need an additional loop to iterate through each 2D sub-vector.

By using std::ranges::count_if() within the appropriate nested loops, you can count elements in multi-dimensional containers efficiently, taking advantage of the power and flexibility of C++20 ranges.