[plt-scheme] sicp exercise 2.23

Try your for-each as in:
(for-each (lambda (x) (newline) (display x) nil) (list 1 2 3))
But you are close. Do you need 'if' in for-each-helper?
Jos
----- Original Message ----- 
From: "Benjamin L. Russell" <dekudekuplex at yahoo.com>
To: "Jason Wang" <randomtalk at gmail.com>; "John Clements" 
<clements at brinckerhoff.org>
Cc: "PLT Scheme List" <plt-scheme at list.cs.brown.edu>
Sent: Wednesday, April 30, 2008 5:35 AM
Subject: Re: [plt-scheme] sicp exercise 2.23


> No, don't use "begin"!  One of the main points of this exercise is to 
> learn to use side-effects.
>
> Here is my solution and the results of a sample run:
>
> --
> (define nil '())
> ;Value: nil
>
> (define (for-each proc items)
>  (if (null? items)
>      nil
>      (for-each proc (for-each-helper proc items))))
> ;Value: for-each
>
> (define (for-each-helper proc items)
>  (if (null? (proc (car items)))
>      nil
>      (cdr items)))
> ;Value: for-each-helper
>
> (for-each (lambda (x) (newline) (display x))
>   (list 57 321 88))
>
> 57
> 321
> 88
> ;Value: ()
> --
>
> It's actually quite simple; just use a side-effect in the if-clause.  No 
> need for even a "cond".  Nobody said a test-clause couldn't be used for a 
> side effect!
>
> Benjamin L. Russell
>
> --- On Wed, 4/30/08, John Clements <clements at brinckerhoff.org> wrote:
>
>> From: John Clements <clements at brinckerhoff.org>
>> Subject: Re: [plt-scheme] sicp exercise 2.23
>> To: "Jason Wang" <randomtalk at gmail.com>
>> Cc: "PLT Scheme List" <plt-scheme at list.cs.brown.edu>
>> Date: Wednesday, April 30, 2008, 3:35 AM
>> On Apr 28, 2008, at 11:04 PM, Jason Wang wrote:
>>
>> >
>> > However, my problem is that SICP hasn't introduced
>> begin yet...
>>
>> I don't have a copy of SICP handy (I guess I can just
>> tear up my
>> Schemer Membership Card right now...), but
>> "begin" seems to me to be
>> the natural choice to express what you want; it's true
>> that there are
>> other constructs that behave in a begin-like way, and in
>> fact you can
>> write begin as a function (for a fixed number of arguments)
>> quite
>> easily, but I would suggest just using 'begin',
>> especially as you've
>> been through HtDP already.
>>
>> John
>> Clements_________________________________________________
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