Delete element from List in Scheme

Question

I have list in this form

( (1 3) (2 2) (3 1) (4 5) (5 1)))

and I want to delete an item let's say (3 1)

So the result will be

( (1 3) (2 2) (4 5) (5 1)))

I have written something like this and I do not know why it is not running correctly.

(define (deleteItem list item)
  (cond
    ((equal? item (car  list)) (cdr list))
    (cons (car  list)(deleteItem(cdr list) item))))

Answer 1

There's a built-in function for this, it's called remove:

(define lst
  '((1 3) (2 2) (3 1) (4 5) (5 1)))

(remove '(3 1) lst)
=> '((1 3) (2 2) (4 5) (5 1))

… But I guess you need to implement it from scratch. Some suggestions for your code:

• You should not use list as a parameter name, that'll clash with a built-in function. Let's call it lst instead
• You're missing the base case necessary form most list procedures: what happens if the list is empty?
• You're also missing the else part in the last condition

With all the above fixes in place, the procedure will work:

(define (deleteItem lst item)
  (cond ((null? lst)
         '())
        ((equal? item (car lst))
         (cdr lst))
        (else
         (cons (car lst) 
               (deleteItem (cdr lst) item)))))

(deleteItem lst '(3 1))
=> '((1 3) (2 2) (4 5) (5 1))