swift-algorithms | Commonly used sequence and collection algorithms for Swift | Learning library

They're both not in the swift standard library (yet) because they haven't passed the Swift Evolution process. This is explained in the Swift Algorithms announcement:

It’s our ambition for the standard library to include a rich, pragmatic set of generic algorithms. We think the Algorithms package can help realize this goal by serving as a low-friction venue to build out new families of related algorithms—giving us an opportunity to iteratively explore the problem space and learn how different algorithms connect and interact—before graduating them into the standard library.

Generally speaking you should prefer the functional idiom (ie reduce) over the for loop, for a couple of reasons:

1. The functional version conveys intent. In this case we are reducing an array into a dictionary -- not the best example, but usually reduce transforms a vector into a single scalar through some combining function.

2. The functional version is correct by default since its been tested. This may seem trivial in the case of reduce, but its much less so if you look at something like shuffled. How easily can you look at a for loop and tell me if its performing a Fisher Yates shuffle and if that shuffle is implemented correctly? Similarly a pipeline of functions is much easier to read than a bunch of sequential for loops or a single for loop that does 5 different things.

3. The functional version is often (but not in this case) immutable and immutable values are easier to reason about than mutable state, because they never change.

4. The functional methods on Swift.Sequence mostly have analogous methods on Combine.Publisher. That means you can use a single set of functional idioms across all sequences, whether or not they are synchronous or asynchronous/reactive.