C++23 Fold Algorithms

# Using Custom Operators with Fold

## How do I use fold_left_first() with custom operators?

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 standardÂ arithmetic.

## Steps to Use Custom Operators

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.

## Example with Lambda

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

#include <algorithm>
#include <iostream>
#include <optional>
#include <vector>

int main() {
std::vector<int> numbers{1, -2, 3, -4, 5};

auto abs_sum = [](int x, int y) {
return std::abs(x) + std::abs(y); };

std::optional<int> result =
std::ranges::fold_left_first(numbers, abs_sum);

if (result.has_value()) {
std::cout << "Result: " << result.value();
}
}
Result: 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.

## Example with Function Object

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

#include <algorithm>
#include <iostream>
#include <optional>
#include <vector>

struct Multiply {
int operator()(int x, int y) const {
return x * y;
}
};

int main() {
std::vector<int> numbers{1, 2, 3, 4, 5};

std::optional<int> result =
std::ranges::fold_left_first(
numbers, Multiply{});

if (result.has_value()) {
std::cout << "Result: " << result.value();
}
}
Result: 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.

## Handling Edge Cases

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

#include <algorithm>
#include <iostream>
#include <optional>
#include <vector>

int main() {
std::vector<int> empty_numbers{};

auto sum = [](int x, int y) { return x + y; };

std::optional<int> result =
std::ranges::fold_left_first(
empty_numbers, sum);

if (!result.has_value()) {
std::cout << "The input range is empty";
}
}
The input range is empty

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

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

This Question is from the Lesson:

### C++23 Fold Algorithms

An introduction to the 6 new folding algorithms added in C++23, providing alternatives to std::reduce and std::accumulate

Answers to questions are automatically generated and may not have been reviewed.

This Question is from the Lesson:

### C++23 Fold Algorithms

An introduction to the 6 new folding algorithms added in C++23, providing alternatives to std::reduce and std::accumulate

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