Design a function that consumes a [Listof Number] and checks if 0 is in the list.
I want to figure this out using lambda. I've been reading the book, and this is what I've came up with so far. I hope im close.
(define (five? lon)
(..... (lambda (x)
(= x 5))
lon))
how would you finish this (if right)?
Yup! Checkout the ormap function.
procedure
(ormap proc lst ...+) → any
proc : procedure?
lst : list?
From the Racket documentation:
Similar to map in the sense that proc is applied to each element of lst, but
The result is #f if every application of proc produces #f; and
The result is that of the first application of proc producing a value other than #f, in which case proc is not applied to later elements of the lsts; the application of proc to the last elements of the lsts is in tail position with respect to the ormap call.
Assuming that the number that's being searched is 5 (as inferred from the code and not the problem description) - there's a procedure that already does what's being asked, it's called member:
(member 5 '(1 3 5 7))
=> '(5 7)
Which, according to the documentation:
Locates the first element of lst that is equal? to v. If such an element exists, the tail of lst starting with that element is returned. Otherwise, the result is #f.
But if we were to implement a similar procedure from scratch, this is how the solution would look, fill-in the blanks:
(define five?
(lambda (lon)
(if <???> ; is the list empty?
<???> ; then the number is not in the list
(or <???> ; otherwise the current element is 5 or
(five? <???>))))) ; the number is in the rest of list
Don't forget to test your code:
(five? '())
=> #f
(five? '(1 2 3 6 7 9))
=> #f
(five? '(1 2 3 5 7 9))
=> #t