Using Custom Operators with Fold

Using fold_left_first() with custom operators allows you to customize how elements in your collection are combined. This can be particularly useful for operations that are more complex than standard arithmetic.

1. Define the Custom Operator: A custom operator can be a function object, lambda expression, or a function pointer that takes two arguments of the element type and returns the result of combining them.
2. Use the fold_left_first() algorithm: fold_left_first() takes a range and a binary operation but does not require an initial value since it uses the first element of the range as the initial value.

Here's an example using a lambda expression as the custom operator to calculate the sum of absolute values:

1#include <algorithm>
2#include <iostream>
3#include <optional>
4#include <vector>
5
6int main() {
7  std::vector<int> numbers{1, -2, 3, -4, 5};
8
9  auto abs_sum = [](int x, int y) {
10    return std::abs(x) + std::abs(y); };
11
12  std::optional<int> result =
13    std::ranges::fold_left_first(numbers, abs_sum);
14
15  if (result.has_value()) {
16    std::cout << "Result: " << result.value();
17  }
18}

1Result: 15

Explanation

• The lambda abs_sum takes two integers and returns the sum of their absolute values.
• std::ranges::fold_left_first(numbers, abs_sum) uses this lambda to fold the elements of the numbers vector.
• The result is wrapped in a std::optional to handle the case of an empty range.

You can also use a function object (functor) for more complex operations:

1#include <algorithm>
2#include <iostream>
3#include <optional>
4#include <vector>
5
6struct Multiply {
7  int operator()(int x, int y) const {
8    return x * y;
9  }
10};
11
12int main() {
13  std::vector<int> numbers{1, 2, 3, 4, 5};
14
15  std::optional<int> result =
16    std::ranges::fold_left_first(
17      numbers, Multiply{});
18
19  if (result.has_value()) {
20    std::cout << "Result: " << result.value();
21  }
22}

1Result: 120

Explanation

• The Multiply struct defines an operator() that multiplies two integers.
• std::ranges::fold_left_first(numbers, Multiply{}) uses this functor to multiply the elements of the numbers vector.
• The result is wrapped in a std::optional.

fold_left_first() returns a std::optional, which makes it easy to handle cases where the input range is empty:

1#include <algorithm>
2#include <iostream>
3#include <optional>
4#include <vector>
5
6int main() {
7  std::vector<int> empty_numbers{};
8
9  auto sum = [](int x, int y) { return x + y; };
10
11  std::optional<int> result =
12    std::ranges::fold_left_first(
13      empty_numbers, sum);
14
15  if (!result.has_value()) {
16    std::cout << "The input range is empty";
17  }
18}

1The input range is empty

In summary, using fold_left_first() with custom operators involves defining a custom binary operation and passing it to the algorithm.

This allows for flexible and powerful ways to process collections, handling both standard and complex operations with ease.