# Homework 6: Scheme, Scheme Lists, Tail Calls hw06.zip

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.

To see these videos, you should be logged into your berkeley.edu email.

## Code Writing Questions

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)))

'YOUR-CODE-HERE
)

'YOUR-CODE-HERE
)``````

Use Ok to unlock and test your code:

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

### 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``````