Assignment 3 (Scheme) Solution



Scheme Functions

Your solutions to the assignment problems are to be your own work. Refer to the course academic integrity statement in the syllabus.

This assignment provides some practice in Scheme programming. You will write Scheme functions (and test functions) for the given problems. You will use the Racket environment, which can be downloaded from After installing Dr. Racket, be sure to select R5RS as the language on the language menu. You can edit your code in the upper panel of the Racket IDE and save it from there. Or edit in any text editor and load it into Racket. (Make sure to save your file with .scm extension.)

Turning in your assignment

Turn in a plain text file named HW3.scm containing legal scheme code — you should be able to take the contents of your file and run it through racket without errors. Create your own test cases for each function and provide test functions comparing the output of your testcases to the correct output.

To submit your assignment, turn in your file by uploading on the Assignment3 (Scheme) DROPBOX on Blackboard (under AssignmentSubmisions menu). Be sure to include your name as a comment at the top of the file. You may turn in your assignment up to 5 times. Only the last one submitted will be graded. Please let the instructor know if you need to resubmit it a 6th time.

The work you turn in is to be your own personal work. You may not copy another student’s code or work together on writing code. You may not copy code from the web, or anything else that lets you avoid solving the problems for yourself.


The assignment will be marked for correctness and effective programming. You will lose points : (1) if your functions give errors for certain test cases, (2) if they don’t produce the correct output, (3) if you don’t provide test functions, and (4) if you don’t explain your code with appropriate comments.

General rules

Unless directed otherwise, you should implement your functions using recursive definitions that you build up from the basic built-in functions such as cons, car, cdr, list, etc. Don’t use set! and don’t define anything except functions.


  1. (deepSum L) 15%

The function deepSum takes a single argument, L, a list, and returns the sum of elements (atoms) contained in L and the sublists in L. You can assume that list L contains either integers or sublists containing integers.


(deepSum(1 (2 3 4) (5) 6 7 (8 9 10) 11)) returns 66.

(deepSum( () )) returns 0.

(Note: deepSum should check whether each element in L is a list or not. Remember that the predicate function (pair? L) returns true if L is a pair value (such as a list). (pair? ‘()) returns #f (i.e., empty list is not a pair value.) )

  1. (numbersToSum sum L) – 15%

The function numbersToSum that takes an int (called sum)(which you can assume is positive), and an int list L (which you can assume contains positive numbers) and returns a list. The returned list should include the first n elements from the input list such that the first n elements of the list add to less than sum, but the first (n + 1) elements of the list add to sum or more. Assume the entire list sums to more than the passed in sum value.


(numbersToSum 100 ‘(10 20 30 40)) returns (10 20 30)

(numbersToSum 30 ‘(5 4 6 10 4 2 1 5)) returns (5 4 6 10 4)

  1. (isSorted L) – 15%

The function isSorted takes a list (which contains positive numbers) and returns true if the elements

in the list are in ascending order. isSorted will return false otherwise. (Hint: use length function

to check the length of a list)


(isSorted ‘(1 4 5 6 10)) returns #t

(isSorted ‘(1 3 6 5 10)) returns #f

(isSorted ‘(1)) returns #t

  1. (mergeUnique2 L1 L2) – 10%

The function mergeUnique2 takes two lists of integers, L1 and L2, each already in ascending order, and returns a merged list that is also in ascending order and that doesn’t include any duplicates. The resulting list should be the union of the elements of the two lists. Duplicates should be eliminated during the merge. You may assume that input lists, L1 and L2, don’t contain any duplicates. Examples:

(mergeUnique2 ‘(4 6 7) ‘(3 5 7)) returns (3 4 5 6 7 )

(mergeUnique2 ‘(1 5 7) ‘(2 5 7)) returns (1 2 5 7)

(mergeUnique2 ‘() ‘(3 5 7)) returns (3 5 7)

(fold L)

Just use the definition given in class. You’ll need it for the function below.

  1. (mergeUniqueN Ln) – 10%

  1. Using mergeUnique2 function defined above and the fold function, define mergeUniqueN which takes a list of lists, each already in ascending order, and returns a new list containing all of the elements in ascending order. (You may assume that the sublists in Ln don’t contain any duplicate values.)

(Provide an answer using fold; without using recursion).


(mergeUniqueN ‘()) returns ()

(mergeUniqueN ‘((2 4 6) (1 4 5 6))) returns (1 2 4 5 6)

(mergeUniqueN ‘((2 4 6 10) (1 3 6) (8 9))) returns (1 2 3 4 6 8 9 10)

Note that here fold returns a list, not just a simple value; but the solution is, in fact, a very simple use of fold once you choose the right value for the base case corresponding to the empty list as input.

  1. In a comment, discuss the question of how many cons operations your function uses to produce its result, in terms of the sizes of the input lists. Explain your answer. For this problem, I suggest looking first at how many cons operations are used by mergeUnique2 for lists of length len1 and len2. If mergeUniqueN is used for a list with a single sublisy, what is the answer? For 2 sublists? For n sublists?

6. (matrixMap f M) – 10%

A matrix M can be represented in Scheme as a list of lists, for example M='((1 2) (3 4)) Without using recursion, write a function matrixMap, which takes a function f and a matrix M as arguments and returns a matrix consisting of f applied to the elements of M.


(matrixMap (lambda (x) (* x x)) ‘((1 2) (3 4)) ) returns ((1 4) (9 16)) (matrixMap (lambda (x) (+ 1 x)) ‘((0 1 2) (3 4 5)) ) returns ((1 2 3) (4 5 6))

  1. (avgOdd L) – 10%

Without using recursion, write a function avgOdd which takes a list L and returns the average of the odd values in L.


(avgOdd ‘(1 2 3 4 5)) returns 3

(avgOdd ‘(1 3 5)) returns 3

(avgOdd ‘(1 2 4 6)) returns 1

Use (odd? n) and (even? n) predicate functions to check if a given number n is odd or even.

  1. (unzip L) – 15%

In class we defined a function named zip that takes two lists as its arguments and produces a list of pairs as its output. For this problem, define the inverse of zip without using recursion, namely unzip, that takes a list of two-element lists as input and produces a list of two lists as output. (Hint: Call the map function twice to get the first and second values from pairs) What should (unzip ‘()) be?


(unzip ‘((1 2) (3 4) (5 6))) returns ((1 3 5) (2 4 6))

(unzip ‘((1 “a”) (5 “b”) (8 “c”))) returns ((1 5 8) (“a” “b” “c”))

Scheme resources:

R5RS (the standard)

How to Design Programs (with links to the DrScheme web page) Structure and Interpretation of Computer Programs Programming Languages: Application and Interpretation