Homework 6: Scheme, Scheme Lists, Tail Calls

Due by 11:59pm on Thursday, August 4

Instructions

Download hw06.zip. Inside the archive, you will find a file called hw06.scm, along with a copy of the ok autograder.

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 correctness. Each incorrect problem will decrease the total score by one point. There is a homework recovery policy as stated in the syllabus. This homework is out of 2 points.

Required Questions


Getting Started Videos

These videos may provide some helpful direction for tackling the coding problems on this assignment.

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YouTube link

Code Writing Questions

Q1: Thane of Cadr

Define the procedures cadr and caddr, which return the second and third elements of a list, respectively. If you would like a quick refresher on scheme syntax consider looking at Lab 08 Scheme Refresher.

(define (cddr s)
  (cdr (cdr s)))

(define (cadr s)
  'YOUR-CODE-HERE
)

(define (caddr s)
  'YOUR-CODE-HERE
)

Use Ok to unlock and test your code:

python3 ok -q cadr-caddr -u
python3 ok -q cadr-caddr

Q2: Interleave

Implement the function interleave, which takes a two lists lst1 and lst2 as arguments. interleave should return a new list that interleaves the elements of the two lists. (In other words, the resulting list should contain elements alternating between lst1 and lst2.)

If one of the input lists to interleave is shorter than the other, then interleave should alternate elements from both lists until one list has no more elements, and then the remaining elements from the longer list should be added to the end of the new list.

(define (interleave lst1 lst2)
  'YOUR-CODE-HERE
)

Use Ok to unlock and test your code:

python3 ok -q interleave -u
python3 ok -q interleave

Q3: My Filter

Write a procedure my-filter, which takes a predicate pred and a list lst, and returns a new list containing only elements of the list that satisfy the predicate. The output should contain the elements in the same order that they appeared in the original list.

Note: Make sure that you are not just calling the built-in filter function in Scheme - we are asking you to re-implement this!

(define (my-filter pred lst)
  'YOUR-CODE-HERE
)

Use Ok to unlock and test your code:

python3 ok -q filter -u
python3 ok -q filter

Q4: Concatenate

Write a tail-recursive function concatenate, which takes in s, a list of lists, and concatenates all the elements together into one list.

Hint: Recall the built-in append function

(define (concatenate s)
'YOUR-CODE-HERE
)

Use Ok to unlock and test your code:

python3 ok -q concatenate -u
python3 ok -q concatenate