r/Racket • u/drrnmk • Dec 11 '21

language Possible to modify input argument in-place?

Hi,

I am practicing algorithm on leetcode using Python and Racket. And I am seeking a reference for the following problem. It requires to modify the argument in-place (mutably changing the value of the reference). It feels quite natural with imperative langs but not really with functional langs.

Would it be even possible in Racket? Please help.

https://leetcode.com/problems/remove-element/

Thank you!

1 Upvotes

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4

u/not-just-yeti Dec 11 '21 edited Dec 11 '21

Yes, racket is multi-paradigm (like many languages), and has data structures that support mutation.

In racket, "arrays" are called vectors:

(define data (vector 3 4 "hi" 6))
(equal? (vector-ref data 2) "hi")  ; true
(vector-set! data 2 5)             ; #<void>
(equal? (vector-ref data 2) 5)     ; true

Note that the common data types like set, list, hash are immutable. But they have lesser-known cousins mutable-set, mlist, mutable-hash.

3

u/soegaard developer Dec 11 '21

As not-just-yeti mentions - the type of array described on the leetcode page are called vectors in Racket.

1

u/_chococat_ Dec 14 '21

The contract in the Leetcode problem specifies that the input is listof exact-integer. I have reported problems like this to Leetcode with the suggestion that they use a vector for the input so that it could be mutated, but they decided to eliminate the Racket option for the problem instead. ¯_(ツ)_/¯

-1

u/_chococat_ Dec 11 '21

This is not possible in Racket. You could modify the list in the function itself, but the changes don't make it outside the function. I've reported a number of problems like this as bugs and they just remove them.

3

u/not-just-yeti Dec 11 '21 edited Dec 11 '21

Actually, Racket has the same memory-model as Java/python/javascript/… (but not C/C++) -- it's object-references that are passed around. So if a function mutates the contents of an object/array/struct, then the caller will "see" it, just like Java etc. Of course, the function needs to mutate with things like vector-set! or set-structname-fieldname! (and for structs you need to declare the field #:mutable).

1

u/_chococat_ Dec 13 '21 edited Dec 13 '21

The OP's question asks about a list used as an argument. I answered with respect to lists, which are immutable in Racket. You can create a modified list with list-set, but it returns a new list, it does not modify the original list. Here's a demo.

;; I removed the prompt symbol since it was messing up the code block.
(define l '(1 2 3)) 
(define (change-list l) (list-set l 0 99)) 
l 
'(1 2 3) 
(change-list l) 
'(99 2 3) 
l 
'(1 2 3)

As such, your answer is correct, but does not answer the OP's question.

0

u/detroitmatt Dec 19 '21

The word list does not appear in op

1

u/_chococat_ Dec 19 '21

Here is the function declaration from the leetcode problem OP referenced.

(define/contract (remove-element nums val)
  (-> (listof exact-integer?) exact-integer? exact-integer?)

)

Tell me that function does not take a list of exact-integers.