CODEWITHSUNDEEP
racket
Paradox
Paradox

Reputation: 4556

Is there a function to check whether an element exists in a list?

In racket, I'm aware of the member function which returns the first occurrence of an element if it exists or #f otherwise, but is there a function which just returns #t or #f for whether the element exists in a list?

Upvotes: 1

Views: 1619

Answers (2)

Alexis King
Alexis King

Reputation: 43842

The answer to this question is member. In Racket, everything that is not #f is true. Therefore, member works just fine for testing the presence of an element in a list:

> (if (member 3 '(1 2 3)) 'present 'absent)
'present
> (if (member 3 '(1 2)) 'present 'absent)
'absent

There’s no need to specifically produce #t upon success because everything in Racket that branches only cares about #f and non-#f values.

There are some rare situations in which you really do want a boolean value, such as if you are going to store the result in some data structure and don’t want to retain an unnecessary reference to the list. The usual idiom for doing this in Racket is (and v #t), where v is some value, since this is equivalent to writing (if v #t #f). Therefore, you could just surround your call to member with that:

> (and (member 3 '(1 2 3)) #t)
#t

Since this is so rarely necessary, this is more idiomatic in Racket than writing a separate member? function.

Upvotes: 5

Ben Kovitz
Ben Kovitz

Reputation: 5020

I don't think there's a standard function for this, but see true? and without-truthiness in the predicates library.

Defining true? is pretty easy:

(define (true? x)
    (if x #t #f))

Upvotes: -1

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