CODEWITHSUNDEEP
schemelispracket
Sam
Sam

Reputation: 19

Check to see if an item in Racket is in a list without using member

I'm trying to see if a certain element is in a list, for example (in-list? '+ '(- + / *)) would return #t, while (in-list? '> '(- + / *)) would return #f.

So far I have this for my code:

(define in-list?
  (lambda (s lst)
    (if (eq? s (first lst))
      #t
      (in-list? s (rest lst)))))

When I check using (in-list? '+ '(- + / *)), it returns true, but using the other example I get an error message that says:

first: contract violation
expected: (and/c list? (not/c empty?))
given: '()

I'm confused on why this error is happening and was wondering if someone could help me understand. Thanks!

Upvotes: 1

Views: 875

Answers (1)

ggorlen
ggorlen

Reputation: 57446

You're missing a base case to check if the list is empty. Here's a minimal repro, calling first on an empty list

> (first '())
. . first: contract violation
  expected: (and/c list? (not/c empty?))
  given: '()

You can use empty? to check if the list is empty as the first condition branch and return false instead of calling first on an empty list:

(define in-list?
  (lambda (s lst)
    (cond
      [(empty? lst) #f]
      [(eq? s (first lst)) #t]
      [else (in-list? s (rest lst))])))

Upvotes: 3

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