From leavens@larch.cs.iastate.edu Sun Feb  6 21:07:14 2005
Date: Sun, 6 Feb 2005 21:07:14 -0600 (CST)
From: Gary T. Leavens <leavens@larch.cs.iastate.edu>
To: Adam Nelson <nelsonad@iastate.edu>
Cc: Com S 342 TAs -- Brian Patterson <patterbj@cs.iastate.edu>, Daniel Patanroi <danpatan@cs.iastate.edu>
Subject: Re: Homework 3 Question 4

Hi Adam,

On Sun, 6 Feb 2005, Adam Nelson wrote:

> How do you set up the define statement for an unknown number of arguments as
> in question 4?
>
> IE in the statement:
> (define append* (lambda (???)
> what would be put in the place of the question marks?

You have to use the form

   (define append*
     (lambda args
        ; code here that uses args as a list
        ...))

See also the R^5RS which says about procedure formals in the section on
procedures:

"<Formals> should have one of the following forms:

* (<variable1> ...): The procedure takes a fixed number of arguments;
   when the procedure is called, the arguments will be stored in the
   bindings of the corresponding variables.

* <variable>: The procedure takes any number of arguments; when the
   procedure is called, the sequence of actual arguments is converted
   into a newly allocated list, and the list is stored in the binding of
   the <variable>."

This last syntax for formals is the one you want for this problem.

         Gary T. Leavens
         Department of Computer Science, Iowa State University
         229 Atanasoff Hall, Ames, Iowa 50011-1041 USA
         http://www.cs.iastate.edu/~leavens  phone: +1-515-294-1580

---------------------------

From leavens@larch.cs.iastate.edu Mon Feb  7 14:26:09 2005
Date: Mon, 7 Feb 2005 14:26:09 -0600 (CST)
From: Gary T. Leavens <leavens@larch.cs.iastate.edu>
To: Adam Nelson <nelsonad@iastate.edu>, Brian Patterson <patterbj@cs.iastate.edu>
Cc: Daniel Patanroi <danpatan@cs.iastate.edu>
Subject: Re: Homework 3 Question 5

Hi Brian, Adam,

Sorry I didn't see your query earlier, Adam.  Brian's answer is
right, but I have one technical quibble with it.  The first argument
to append-map is a procedure of type (-> (S) (list-of T)), which means
that it takes an argument of type S and returns a list of values of
type T.  It can't return an arbitrary type, but has to return a list
type.  otherwise, Brian's answer, including examples is exactly right.
Thanks, Brian.

On Mon, 7 Feb 2005, Brian Patterson wrote:

> Append-map takes two arguments: A function from type S to type T and a list 
> of Ss.  So, for example, you need to give append-map a function and 
> append-map should run that function on each element of the list.  So, an 
> example input if S is integers and T is booleans is a function that checks if 
> each element of the list is equal to 3, returning a list of just n if not and 
> a null list otherwise applied to a list of numbers.  So the two inputs are
>
> (lambda (n) (if (= n 3) '() (list n)))
>
> and
>
> '(3 4 2 3 7)
>
> so
>
> (append-map (lambda (n) (if (= n 3) '() (list n))) '(3 4 2 3 7))
>
> returns
>
> (4 2 7)
>
> as the null lists get appended into the list of non-3 items.  So there are 
> only two arguments and your function should start (define append-map (lambda 
> (f lst) .... ) with f being a function you use later on elements of lst.  The 
> main point of this problem (as opposed to problems 3 and 4) is using a 
> passed-in function to do stuff.
>
> Brian
>
> On Feb 7, 2005, at 12:18 PM, Adam Nelson wrote:
>
>> I don't understand what append-map is looking for.  I understand that the 
>> list-
>> of-S is the initial list, but I don't understand what the other two 
>> arguments
>> are for.

         Gary T. Leavens
         Department of Computer Science, Iowa State University
         229 Atanasoff Hall, Ames, Iowa 50011-1041 USA
         http://www.cs.iastate.edu/~leavens  phone: +1-515-294-1580

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