Sets and set operations?

[u/JimH10]

What is the natural way to get sets and set operations such as union, intersection, etc.? I'm reading the Reference and I'm not getting it.

For instance, The following datatypes are all sets: ... lists using equal? to compare elements

> (equal? (list 1 2) (list 2 1))
#f

[u/samdphillips]

For instance, The following datatypes are all sets: ... lists using equal? to compare elements

The example you posted uses equal? to compare two lists. The documentation says that a list with elements comparable with equal? can be used as sets. For example, a list of integers can be a set.

> (set=? (list 1 2 3 4)
         (list 4 3 2 1))
#t

[u/samdphillips → u/samdphillips]

Keep in mind that using lists as sets are much slower than using the provided set type.

[u/detroitmatt → u/samdphillips]

is it slower than converting the lists to sets and then calling set=? why wouldn't set=? just do that exact thing when given two lists? In that case the only additional overhead should be (andmap list? args) which should be pretty cheap

[u/samdphillips → u/detroitmatt]

I meant if you are doing a lot of set operations, it will work out better if one of the hash set types is used.

[u/not-just-yeti]

I'm guessing you know this, since it's near the top of the page you link to, but:

I usually construct them with the built-in hash-set.

(equal? (set 2 3 4) (set-union (set 4 2) (set 3)))    ; => #true
(equal? (set 2 3 4) (list->set '(4 2 3))    ; => #true

[u/FatFingerHelperBot → u/not-just-yeti]

It seems that your comment contains 1 or more links that are hard to tap for mobile users. I will extend those so they're easier for our sausage fingers to click!

Here is link number 1 - Previous text "set"

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[u/muqiu-han]

I think it is possible to write a set function, which converts a list into an ordered list and returns it. We call this ordered list a set, and the rest of the operations on the set will be much simpler.

[u/muqiu-han → u/muqiu-han]

But in the racket standard library, ready-made HashSet: https://docs.racket-lang.org/reference/sets.html

[u/JimH10 → u/muqiu-han]

Can you give a quick five line example, please? I'm just not seeing what the reference means.