CODEWITHSUNDEEP
clojure
m3nthal
m3nthal

Reputation: 433

How can I remove specific element from a list?

I know this might be a silly question, but I don't get it. I have some data:

(def x (range 1 14))

-> (1 2 3 4 5 6 7 8 9 10 11 12 13)

I want to return a list without "3" for example. Googling "clojure remove item from list" took me to this:

  (remove pred coll)

So I tried to use example with even?:

  (remove even? x) 

  -> (1 3 5 7 9 11 13)

Great! It works with my data! I just need to change pred. And my first guess would be:

  (remove (= 3) x)

  java.lang.ClassCastException: java.lang.Boolean cannot be cast to clojure.lang.IFn

Ok, we don't need to evaluate (= 3), so let's put # before:

  (remove #(= 3) x)

  clojure.lang.ArityException: Wrong number of args (1)` passed to...

I know it is trivial, but still how can I do it?

Upvotes: 11

Views: 7947

Answers (2)

sloth
sloth

Reputation: 101162

You should use:

(remove #(= 3 %) x)

#(= 3) does not take a parameter (but remove tries to pass one to that function).

#(= 3 %) takes one parameter, and calls = with that parameter (%) and 3.

Upvotes: 9

user3594595
user3594595

Reputation:

I like using sets for this kind of thing. So clean..

Remove elements:

(remove #{3 5} [1 2 3 4 5 6 7 8 9])

Keep elements:

(keep #{7 5 3} [1 2 3 4 5 6 7 8 9])

Check if element exists:

(some #{5} [1 2 3 4 5 6 7 8 9])

This works because when a set is used as a function of one argument, it returns the argument if it was inside of the set. In the remove example, the elements 3 and 5 cause the set to return a truthy value: themselves. The same thing happens for both the keep and some examples, except that the some example has the added benefit of returning the first element in your collection that's also in the set.

Upvotes: 17

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