CODEWITHSUNDEEP
racket
Lawrence Woodman
Lawrence Woodman

Reputation: 1444

How to compare a symbol with a symbol taken from a list

If I compare two symbols using equal? I get different results depending on whether one of the symbols is from a list and one is not.

This is demonstrated below:

; The following returns #t  
(equal? (list-ref '('a 'b) 1) (list-ref '('a 'b) 1))

; But this return #f
(equal? 'b (list-ref '('a 'b) 1))

What is the best way to compare two symbols when one is from a list and one is not? If you can help me understand why this is the case then even better.

Upvotes: 1

Views: 2045

Answers (1)

uselpa
uselpa

Reputation: 18917

You weren't comparing symbols:

> (list-ref '('a 'b) 1)
''b
> (symbol? (list-ref '('a 'b) 1))
#f

due to double quoting:

> (list-ref '(a b) 1)
'b
> (symbol? (list-ref '(a b) 1))
#t

So your initial case becomes

> (equal? 'b (list-ref '(a b) 1))
#t

If your list only contains symbols, it's more common to use eq? to compare them (it's supposed to be faster):

> (eq? 'b (list-ref '(a b) 1))
#t

If you double quoted because your list may contain other types, then use

> (list 'a 'b)
'(a b)

instead of

> '(a b)
'(a b)

Upvotes: 6

Related Questions