Homework 09:
Due by 11:59pm on Friday, 4/19
Instructions
Download hw09.zip.
Our course uses a custom version of Scheme (which you will build for Project 4) included in the starter ZIP archive. To start the interpreter, type
python3 scheme
. To run a Scheme program interactively, typepython3 scheme -i <file.scm>
. To exit the Scheme interpreter, type(exit)
.
Submission: When you are done, submit with python3 ok
--submit
. You may submit more than once before the deadline; only the
final submission will be scored. Check that you have successfully submitted
your code on okpy.org. See Lab 0 for more instructions on
submitting assignments.
Using Ok: If you have any questions about using Ok, please refer to this guide.
Readings: You might find the following references useful:
Grading: Homework is graded based on effort, not correctness. However, there is no partial credit; you must show substantial effort on every problem to receive any points.
Q1: List Comprehensions
Recall that list comprehensions in Python allow us to create lists out of iterables:
[<map-expression> for <name> in <iterable> if <conditional-expression>]
Use a macro to implement list comprehensions in Scheme that can create lists
out of lists. Specifically, we want a list-of
macro that can be called as
follows:
(list-of <map-expression> for <name> in <list> if <conditional-expression>)
Calling list-of
will return a new list constructed by doing the following for
each element in <list>
:
- Bind
<name>
to the element. - If
<conditional-expression>
evaluates to a truth-y value, evaluate<map-expression>
and add it to the result list.
Here are some examples:
scm> (list-of (* x x) for x in '(3 4 5) if (odd? x))
(9 25)
scm> (list-of 'hi for x in '(1 2 3 4 5 6) if (= (modulo x 3) 0))
(hi hi)
scm> (list-of (car e) for e in '((10) 11 (12) 13 (14 15)) if (list? e))
(10 12 14)
Hint: You may use the built-in
map
andfilter
procedures. Check out the Scheme Built-ins reference for more information.You may find it helpful to refer to the
for
loop macro introduced in lecture. The filter expression should be transformed using alambda
in a similar way to the map expression in the example.
(define-macro (list-of map-expr for var in lst if filter-expr)
'YOUR-CODE-HERE
)
Use Ok to test your code:
python3 ok -q list-comp
Optional (not graded): Recall also that the if <conditional>
portion of
the Python list comprehension was optional. Modify your macro so that the
Scheme list comprehension does not require a conditional expression.
Refer to the macro form in the Scheme Specification for an explanation of how to do optional macro parameters.