Deciding between `fold_left()`

and `fold_right()`

depends on how you want to process the elements in your collection and the specific requirements of yourÂ operation.

The `fold_left()`

function processes elements from left to right, starting with the first element and moving towards the last. It's suitable when the order of combining elements from the beginning to the end isÂ important.

`fold_right()`

processes elements from right to left, starting with the last element and moving towards the first. It's useful when you need to combine elements from the end to theÂ beginning.

If your operation is associative and commutative, the result of `fold_left()`

and `fold_right()`

will be the same. For example, addition and multiplication are both associative andÂ commutative.

If your operation is * not* associative or

`fold_left()`

and `fold_right()`

becomesÂ critical.Both `fold_left()`

and `fold_right()`

require an initial value. However, `fold_left_first()`

and `fold_right_last()`

do not require an initial value, as they use the first and last element of the range, respectively, as the initialÂ value.

Let's look at an example where the operation is notÂ commutative:

```
#include <algorithm>
#include <iostream>
#include <vector>
int main() {
std::vector<int> numbers{1, 2, 3, 4, 5};
// Using fold_left() with subtraction
int fold_left_result = std::ranges::fold_left(
numbers, 0, std::minus<>());
std::cout << "Fold left result: "
<< fold_left_result << "\n";
// Using fold_right() with subtraction
int fold_right_result = std::ranges::fold_right(
numbers, 0, std::minus<>());
std::cout << "Fold right result: "
<< fold_right_result << "\n";
}
```

```
Fold left result: -15
Fold right result: 3
```

In the example above, the results of `fold_left()`

and `fold_right()`

differ significantly due to the non-commutative nature of the subtractionÂ operation.

- Use
`fold_left()`

when the operation logically combines elements from the beginning to the end, such as calculating a running total. - Use
`fold_right()`

when the operation logically combines elements from the end to the beginning, such as parsing or evaluating expressions in reverse.

In summary, the choice between `fold_left()`

and `fold_right()`

depends on the specific requirements of your operation and the nature of the elements in yourÂ collection.

Understanding how your operation interacts with the direction of folding will help you make the rightÂ decision.

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`