Checking if element is present in list in racket

Question

How to check if an element is present in a list, both taken as input from the function call, without using the lambda? I was trying member? but could not get it.

(define (find-string (lst lst str ua)
    (cond ((member? ua lst) #t)
    (else #f))

Goffer

It seems that you're trying to specify the type of each parameter in the procedure declaration? it doesn't work like that in Racket... – Shirelyshirey

This may help. – Melisma

Answer 1

The use of member would work it's just that you are adding extra "?" in front of the function none is required

 (member 2 (list 1 2 3 4)) [1]

would return true

another way around is writing ones own recursive function

(define (is-in-list list value)
 (cond
  [(empty? list) false]
  [(= (first list) value) true]
  [else (is-in-list (rest list) value)]))

[1]https://docs.racket-lang.org/reference/pairs.html#%28def._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._member%29%29

Klemperer

I am unsure if you are asking a question or not. Please avoid sentences that require a questionmark at their end. – Oldworld

? was an operator not for question updated by adding quote on it – Klemperer

Answer 2

First, the way with lambda and ormap (for testing):

; ismember? :: String List-of-Strings -> Bool
(define (ismember1? str strs) (ormap [lambda (s) (string=? s str)] strs) )

Second way, with for/or, without lambda:

(define (ismember2? str strs) 
   (for/or ([s (in-list strs)])
      (string=? s str) ) )

Third way, with member, without lambda:

(define (ismember3? str strs) (if [member str strs] #t #f) )

Refer to the official Racket documentation for member.

Notice that the last version is actually the worst in terms of performance.

Jeffers

the OP's order of arguments is reversed though. so their intended function could be better named "contains". – Thorvaldsen