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))

Answer 1

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.

Answer 2

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