CODEWITHSUNDEEP
schemeracket
user2113651
user2113651

Reputation: 153

Extracting only numbers within a list

Is there a way to extract only numbers within a list? I'm using the beginner language package so I cannot use filter which is a bummer.

(list a 1 2 b d 3 5) => 1 2 3 5 etc I want to use this as a part of my helper function but I cannot figure it out!

thanks!

Upvotes: 2

Views: 1122

Answers (1)

Óscar López
Óscar López

Reputation: 236004

Ideally this problem should be solved using the filter higher-order procedure, like this:

(filter number? '(a 1 2 b d 3 5))
=> '(1 2 3 5)

... But because this looks like a homework, I'll give you some hints on how to solve the problem by hand, just fill-in the blanks:

(define (only-numbers lst)
  (cond (<???>                        ; is the list empty?
         <???>)                       ; return the em´pty list
        (<???>                        ; is the 1st element in the list a number?
         (cons <???>                  ; then cons the first element
               (only-numbers <???>))) ; and advance the recursion
        (else                         ; otherwise
         (only-numbers <???>))))      ; simply advance the recursion

Notice that this solution follows a well-known template, a recipe of sorts for recursively processing a list and in turn creating a new list as output. Don't forget to test your procedure:

(only-numbers '(a 1 2 b d 3 5))
=> '(1 2 3 5)

(only-numbers '(1 2 3 4 5))
=> '(1 2 3 5)

(only-numbers '(a b c d e))
=> '()

Upvotes: 4

Related Questions