CODEWITHSUNDEEP
recursionschemeracket
Apollo
Apollo

Reputation: 146

Problem while defining variables in racket

I am trying to create a recursive function which picks n items from a list returning the picked values and the list without the values, but when I create my variables I get this error:

new-list: unbound identifier in: new-list

Here is my code:

(define(pick-randomr list n picked) ;;Picked always called as empty list
  (if(= n 0) (values list picked)
     ((let* ([aux list]
             [r (random (length aux))]              
             [value (list-ref aux r)]
             [new-picked (cons value picked)]
             [new-list (values (remove value aux))])
        (values aux r new-list))
      (pick-randomr new-list (- n 1) new-picked))))

EDIT: The line that goes:

 (values aux r new-list)

is just to not have an empty body

Upvotes: 0

Views: 159

Answers (1)

Óscar López
Óscar López

Reputation: 236004

There are a couple of problems with your syntax:

  • You should not use list as a parameter name, it conflicts with a built-in procedure of the same name.
  • Don't surround let* with two brackets, that's a common mistake, brackets are not like curly braces in other languages, you must not use them to define a block of statements, use begin for that - but we don't need it in this particular case.
  • The first error you got stated that you must not define a let* with an empty body. But the expression you added there isn't right, you must write the expressions that use the variables inside the let*, otherwise the new-list variable won't be visible.

This is what you meant to write:

(define (pick-randomr lst n picked)
  (if (= n 0)
      (values lst picked)
      (let* ([aux lst]
             [r (random (length aux))]              
             [value (list-ref aux r)]
             [new-picked (cons value picked)]
             [new-list (values (remove value aux))])
        (pick-randomr new-list (- n 1) new-picked))))

Let's test it:

(pick-randomr '(1 2 3 4 5) 2 '())
=> '(1 2 5)
   '(3 4)

Upvotes: 1

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