Styjun

I need to define the function plPrefixContained – that consumes 5 strings and
returns the first one that contains the string "pl" as a prefix – if one such
exists, and returns #f otherwise.

What I'm trying to do is to use the prefixes function to go over all the strings in the list and check their prefixes, put them in a new list and to output the first string as the result.
(I will handle the #f case later) my code is down below but it keeps giving me the error-

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

any help would be appreciated

(: plPrefixContained : String String String String String -> String)
(define (plPrefixContained x y z w v) 
 (list-ref (prefixes (list x y z w v) '()) 0))


(: prefixes : (Listof String) (Listof String) -> (Listof String))
(define (prefixes lis em)
 (cond
 [(and (eq? (string-ref (first lis) 0) "p") (eq? (string-ref (first lis) 1) "l"))
 (cons (first lis) em)]
 [else
 (prefixes (rest lis) em)]))

this is how I want my output to be like-for example

(test (plPrefixContained "yypl" "opl" "lpTT" "plpl" "lol")
 =>
 "plpl")

There are two problems:

• intensional equality eq?, instead of extensional equality such as equal? or string=?
  


• comparing string / char, instead of comparing char / char or string / string

You are using eq?, which always makes me suspicious. eq? uses "intensional" equality, which is basically pointer equality, meaning that a string which is allocated somewhere in memory won't necessarily be eq? even if it has the same characters. You can see this with (eq? "abc123" (string-append "abc" "123")).

If you're dealing with strings, lists, or any other data which "contains" things, you should avoid eq?. Instead you should use an "extensional" equality predicate such as equal?, or even better, a predicate specific to the types of values you expect, such as string=?. Here's how they behave better than eq?:

> (eq? "abc123" (string-append "abc" "123"))
#f
> (equal? "abc123" (string-append "abc" "123"))
#t
> (string=? "abc123" (string-append "abc" "123"))
#t

Since you're comparing using the strings "p" and "l", I should be able to substitute eq? with string=? in your code:

(: prefixes : (Listof String) (Listof String) -> (Listof String))
(define (prefixes lis em)
 (cond
 [(and (string=? (string-ref (first lis) 0) "p") (string=? (string-ref (first lis) 1) "l"))
 (cons (first lis) em)]
 [else
 (prefixes (rest lis) em)]))

However, this reveals the second problem, which I only spotted after seeing the error message:

string=?: contract violation
 expected: string?
 given: #y
 argument position: 1st
 other arguments...:
 "p"

The string=? isn't working because its first argument, the result of string-ref, is a character (like #y), not a string. To fix this, use char=? instead of string=?, and compare with the characters #p and #l instead of the strings "p" and "l".

(: prefixes : (Listof String) (Listof String) -> (Listof String))
(define (prefixes lis em)
 (cond
 [(and (char=? (string-ref (first lis) 0) #p) (char=? (string-ref (first lis) 1) #l))
 (cons (first lis) em)]
 [else
 (prefixes (rest lis) em)]))