## Some Idea

Discussion of Scheme and Racket
dsjoka
Posts: 14
Joined: Thu Feb 10, 2011 1:30 pm

### Some Idea

Hey,

I need some help with a program. I don't expect anyone to write it for me. I just need help with the ideas or maybe some pseudo code to help me.

Problem:

Write a Scheme function that returns the number of zeros in a given simple list of numbers.

Thanks.

Warren Wilkinson
Posts: 117
Joined: Tue Aug 10, 2010 11:24 pm
Location: Calgary, Alberta
Contact:

### Re: Some Idea

In common lisp:

Code: Select all

``````(count 0 list)
``````
If this is a homework assignment (and you're expected to implement count yourself) think of the problem recursively.

The number of zeros in a list is: the number of zeros in the rest of the list plus 1 (if this item is a zero) or 0 (if this item is not zero).
Also, the number of zeros in an empty list is zero.
Need an online wiki database? My Lisp startup http://www.formlis.com combines a wiki with forms and reports.

dsjoka
Posts: 14
Joined: Thu Feb 10, 2011 1:30 pm

### Re: Some Idea

sorry I'm new to this

is this the right idea? And can you tell me if I placed the () right?

(DEFINE (countzeros list)
(IF (<>(car(list)0)
0
(+1(countzeros(cdr(list)))
))

Thanks.

dsjoka
Posts: 14
Joined: Thu Feb 10, 2011 1:30 pm

### Re: Some Idea

now that I think of this, wouldn't it be wrong.

Because if the car of the list is not equal to zero and we return 0, it will reset the count????

Warren Wilkinson
Posts: 117
Joined: Tue Aug 10, 2010 11:24 pm
Location: Calgary, Alberta
Contact:

### Re: Some Idea

Yep, your right -- you've identified a problem with your implementation. Your IF statement says "If the first element of the list is NOT zero -- then the count is zero". What you probably meant to say is "If the first element of the list is NOT zero, then the count is the count-of-zeros in the rest of the list."

I see another problem involving an empty list --- trace the program mentally with an empty list, I think you'll find your program never finishes.
Need an online wiki database? My Lisp startup http://www.formlis.com combines a wiki with forms and reports.

dsjoka
Posts: 14
Joined: Thu Feb 10, 2011 1:30 pm

### Re: Some Idea

I have no clue whats wrong.

I get the error:
procedure application: expected procedure, given: 0 (no arguments)
code is:

Code: Select all

``````(define (countzeros list)
(cond
((null? car list) 0)
((= (car list) 0 )) (+ 1 (countzeros(cdr list)))
(else (countzeros(cdr list)))))``````
I tried (countzeros (0))
Which should return 1

Is my logic even correct? If so am I missing (), or did I put a + sign or = sign in the wrong place?

This is suppose to output the number of zeros is a number list.

gugamilare
Posts: 406
Joined: Sat Mar 07, 2009 6:17 pm
Location: Brazil
Contact:

### Re: Some Idea

You've forgotten one pair of parenthesis:

Code: Select all

``````(define (countzeros list)
(cond
((null? (car list)) 0)
((= (car list) 0 )) (+ 1 (countzeros(cdr list)))
(else (countzeros(cdr list)))))``````
Also, you should call (countzeros '(0)) or (countzeros (list 0)), not (countzeros (0)).

dsjoka
Posts: 14
Joined: Thu Feb 10, 2011 1:30 pm

### Re: Some Idea

ok so when I do that it only returns true and not how many zeros there is?
> (countzeros '(0))
#t
> (countzeros (list 0))
#t
> (countzeros '(0 0))
#t
>

gugamilare
Posts: 406
Joined: Sat Mar 07, 2009 6:17 pm
Location: Brazil
Contact:

### Re: Some Idea

Sorry, there is one parenthesis misplaced. You should also change the first condition - you don't want to test if the first element of the list is null, but rather if the list is null.

Code: Select all

``````(define (countzeros list)
(cond
((null? list) 0)
((= (car list) 0 ) (+ 1 (countzeros (cdr list))))
(else (countzeros (cdr list)))))``````