sorting a list based on the order of its items in another list

[Deyan Yotsov]

Hello,

I have one list of potentially up to 2 million strings: big-list.

Then I have another list, small-list, the elements of which are a subset
of the elements of big-list.

I want to sort the elements in small-list so that they become sorted in
the same way in which they are sorted in big-list.

The most obvious way to do this, for me, is:

(filter #(some #{%} small-list) big-list)

(one by one we take the elements from big-list, we check if they are in
small-list, and if they are, we add them (in big-list order) to the result)

But of course this performs extremely poorly, especially if small-list
is say 1M elements, and big-list is 2M elements.

Suggestions?

Thank you,

Deyan

[Deyan Yotsov]

PS: I just realized that the code, as I wrote it, will not work, as
nesting of # is forbidden.

My actual code is:

(defn is-element-in-list [e l]
  (some #{e} l))

(filter #(is-element-in-list % small-list) big-list)

Sorry about that.

Deyan

[Gary Trakhman]

How about (filter (set small-list) big-list) ?

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[Ray Miller]

Very similar to your solution: (filter (set small-list) big-list)

[Deyan Yotsov]

Thank you very much, Ray Miller and Gary Trakhman!

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