CODEWITHSUNDEEP
schemeracket
user1322801
user1322801

Reputation: 849

Merging lists in recursive procedure in Scheme

I am trying to perform the following action - i have a function that recieves a list and a number, for each occurrance of this number it should duplicate it in the output list, here is what i have so far:

(define (extendList lst num)
  (define data (car lst))
  (define nextData (cdr lst))
  (cond ((and (number? data)(= data num)) ( list num num (extendList nextData num)))
      (list? data) (list (extendList data num) (extendList nextData num))
      (else ( list data (extendList (cdr lst) num) ))))

The thing is that it just doesnt call to this function again in recursion - all it does is adding the next element to the output, for example here is my input and my expected output:

(extendList '(6 x y 6) 6) ; output should be: (6 6 x y 6 6)
(extendList '(6 x (y 6) 6) 6) ;output should be: (6 6 x (y 6 6) 6 6)

but for both of the inputs it just returns the following:

'(6 6 x)

What am i doing wrong here? thanks in advance!

Upvotes: 0

Views: 154

Answers (1)

Sorawee Porncharoenwase
Sorawee Porncharoenwase

Reputation: 6502

There are several problems:

  1. You use cond incorrectly. There's supposed to be a parenthesis around (list? data) (list (extendList data num) (extendList nextData num))

    I suggest to use bracket around each branch (bracket and parenthesis are interchangeable), so that it's easier to read:

    (define (extendList lst num)
  (define data (car lst))
  (define nextData (cdr lst))
  (cond
    [(and (number? data)(= data num)) (list num num (extendList nextData num))]
    [(list? data) (list (extendList data num) (extendList nextData num))]
    [else (list data (extendList (cdr lst) num))]))

  2. You write a recursive function without a base case! What will happen if I call (extendList '() 6)?

  3. (list num num (extendList nextData num)) will create a list with three elements. I don't think this is what you actually want. You might want to use list* or cons instead. The same problem happens with other branches as well.

Upvotes: 2

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