CIS 425 Frequently Asked Questions

Scheme
Q:

How do I insert debugging print statements in a Scheme program?
A: The display function takes a single parameter and prints its value at the time it is executed. Unlike any other function we have studied in Scheme, this function has a side effect. In fact, the only reason to execute display is to have that side-effect: The value returned by the function is useless. To start printing on a new line, use newline. You can put these together in a simple function that prints all its arguments on one line: 
(define print-all 
  (lambda (ls)
    (map display ls)
    (newline)))
The resulting function will take a list of arguments and print them on a single line: 
> (define a '(1 2 3))
> (define b '(4 5))
> (print-all (list "hello " a b))
hello (1 2 3)(4 5)
Note that you only get a space between two values if you explicitly insert it.

There's another feature you need to know if you're going to print values in any of the three clauses of an if. If you want to print debugging information in one of those clauses, you have to both call display to print the value and then actually do the required computation. How do you do two things in the same expression? Use the begin function. This evaluates all of its arguments in left-to-right sequence but only returns the value of the last one:

> (define ls '(1 2 3)) ; assume a and b defined as above
> (if (begin (print-all (list ls)) (null? ls))
    (begin (print-all (list a)) a)
    (begin (print-all (list b)) b))
(1 2 3)
(4 5)
(4 5)
Note that the last (4 5) is the printed result of the if, whereas the first two lines are side-effects resulting from calls to print-all while the clauses of the if were executing.
Q:

What's the Scheme equivalent of % (the C/Java modulo operator)?
A: modulo. As in 
> (modulo 5 2)
1
Q:

I'm getting tired of having to re-type my functions at the Chez prompt, and would really like to save my work in the file mywork.scm, which I can then load. How do I do this?
A: 
> (load "mywork.scm")
Q:

Okay: how about something that will let me give names to interesting values (such as functions and lists) so that I don't have to keep typing them?
A: 
> (define mylist '(1 2 3 4 5 6 7 8 9))
> mylist
(1 2 3 4 5 6 7 8 9)
Q:

Cool. How do I exit Chez when I'm done?
A: 
> (exit)
Q:

About car: in class lectures, we've learned that car returns the first element in a list, and cdr returns the "rest of the list". So the expression (car '((1 2) 3)) ought to return 1.
         But in tutorial, we've been told that (car '((1 2) 3)) actually returns the list (1 2) (which is what Chez Scheme really does anyway). Could you explain the apparent conflict?
A: The key is to remember that lists are elements, too. car always gives you the contents of the address register in a cons cell. If this cell is the head of a list beginning with a number, you'll get a number; if the list begins with a sublist, the value returned by car will be the sublist itself (really, what you're getting is the list's address in memory).

A couple of random finer points that have come to us as mail messages:

  1. When you want to return a primitive value as a result (e.g. the number 5 or the value of the variable x), the expression to use is the number or variable itself. In particular, it's a common rookie mistake to write "(5)" or "(x)" instead of "5" or "x"

  2. The basic syntactic form of an if expression is

    (if E1 E2 E3)
    In particular, don't use the keyword else here as you do with cond.

To CIS 425 Homepage ...

Maintainer of this page: The GTF