Scheme (Dr racket) filter issues

Question

I'm new to scheme and am trying to write a quick sort in it and I can't seem to get the filter function figured out. I believe filter takes two arguments, a procedure that can be applied to each element in a list and then a list onto which the procedure is applied.

#lang racket

;Quick sort
(define(quickSort l)
    (cond
        [(null? l)('(()))]
        [(append(quickSort(filter(lambda(x)(< x car l))(cdr l))))(list(car l))(quickSort(filter(lambda(x)(>= x car l)(cdr l) cdr l)))]
        )
  )

(quickSort '(5 9 4 6 8 7 1 9))

This gives me a contract violation that says the (< x car l) should be ?real, and what it is getting is the car procedure.

I tried extracting the filter function and running it by itself, like so:

(filter(lambda(x)(< x car '(3 5 1 7 8))(cdr '(3 5 1 7 8))))

but this is telling me there is only one argument. I'm not sure why. It appears to me that there are two.

So, any ideas? I initially thought that it might be a problem with the arguments to the filter function, but now I'm thinking that maybe lambda can't accept the (car l) parameter? Would it not evaluate before being passed?

Dan D. Dan D. · Accepted Answer · 2014-11-17T00:26:32

Indent your code and then you'll see that there are syntactical errors. I have corrected them in the following:

;Quick sort
(define (quickSort l)
  (cond [(null? l) '()]
        [else (append (quickSort (filter (lambda (x) (< x (car l))) (cdr l))) 
                      (list (car l))
                      (quickSort (filter (lambda (x) (>= x (car l))) (cdr l))))]))

Scheme (Dr racket) filter issues

1 Answers