CptS355 – Assignment 1 recursive function list_diff solution


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1. list_diff – 15%
a) [10pts] Define a recursive function list_diff which takes two lists as input and returns a list of
the non-common elements from the two input lists. The output list should include the elements from
the first list that don’t appear in the second list and the elements from the second list that don’t appear
in the first list. You should not eliminate the duplicates in the output. The order of the elements in the
output can be arbitrary.
The type of the list_diff function should be compatible with the following:
list_diff :: Eq a => [a] -> [a] -> [a]
> list_diff [1,6,1,3,4,3,5] [3,8,5,4,7,4,8]
> list_diff “We care about our world.”
“We care most about the humans in our world!”
> list_diff [“alpha”,”gamma”,”beta”,”pi”,”theta”]
2. replace – 15%
The function replace takes a list, a value v1, a replacement value v2, and a number n as input, and it
replaces the first n occurrences of v1 in the list with v2. Note that, if n is greater than or equal to the
number of occurrences of v1 in the list, then it will replace all occurrences of v1 with v2.
The type of the replaces function should be compatible with one of the following (depending on the
comparison you apply on the n value, the type of your function will be different):
replace :: (Num t, Eq t, Eq a) => [a] -> a -> a -> t -> [a]
replace :: (Ord t, Num t, Eq a) => [a] -> a -> a -> t -> [a]
> replace “CptS 355 is offered at 5:55pm” ‘5’ ‘2’ 3
“CptS 322 is offered at 2:55pm”
> replace [(1,10),(2,20),(3,30),(1,10),(4,40)] (1,10) (0,0) 2
> replace [1,1,2,1,2,3,1,2,3,4,1,2,3,4,5] 4 40 3
> replace [1,1,2,1,2,3,1,2,3,4,1,2,3,4,5] 6 60 1
3. max_date – 10%
Assume we represent a date value as a 3-tuple which includes the month, day, and year. For example,
(1, 21, 2022) is the tuple representing the date Jan 21, 2022.
Define a recursive function max_date which takes a list of date tuples as input and returns the tuple
representing the most recent date (i.e., max date). If there are more than one tuple having the max
date, it should return one of them.
Hint: Write a helper function that returns the max of the two given date tuples and use It in your
The type of the max_date function should be compatible with the following:
max_date :: (Ord a1, Ord a2, Ord a3) => [(a2, a3, a1)] -> (a2, a3, a1)
> max_date [(12,1,2021), (11,30, 2021), (2,1,2022), (1,5,2021), (12,15,2021),
(2,1,2022), (12,1,2021), (1,5,2022)]
> max_date [(11,26,2021), (1,26,2022), (1,27,2021), (1,26,2022), (1,27,2021),
(11, 26, 2021)]
> max_date [(11, 30, 2021)]
4. num_paths – 10%
Consider a robot in a (m X n) grid who is only capable of moving right or down in the grid (can’t move
left, up, or diagonal). The robot starts at the top left corner, (1,1), and is supposed to reach to the
bottom right corner: (m,n). Write a function numPaths that takes the grid length and width
(i.e.,m,n) as argument and returns the number of different paths the robot can take from the start to
the end. Give an answer using recursion.

For example, the 2×2 , there are 2 paths the robot can take from (1,1) to (2,2). For the 3×3 grid, the
robot has 6 different paths.
> num_paths 4 3
> num_paths 5 8
> num_paths 9 10
> num_paths 1 1000
5. find_courses and max_count – 25%
Assume that we store the list of CptS courses and the programming languages that are used in those
classes as a list of tuples. The first element of each tuple is the course major + number and the second
element is the list of the programming languages that are used in that course. See below for an
progLanguages =
[ (“CptS121” , [“C”]),
(“CptS122” , [“C++”]),
(“CptS223” , [“C++”]),
(“CptS233” , [“Java”]),
(“CptS321” , [“C#”]),
(“CptS322” , [“Python”, “JavaScript”]),
(“CptS355” , [“Haskell”, “Python”, “PostScript”, “Java”]),
(“CptS360” , [“C”]),
(“CptS370” , [“Java”]),
(“CptS315” , [“Python”]),
(“CptS411” , [“C”, “C++”]),
(“CptS451” , [“Python”, “C#”, “SQL”]),
(“CptS475” , [“Python”, “R”])
(a) Write a Haskell function find_courses that takes the list of courses (similar to progLanguages)
and a programming language name (for example “Java” ) as input and returns the list of the courses
which use that programming language.
The type of the find_courses function should be compatible with one of the following:
find_courses :: Eq t1 => [(a, [t1])] -> t1 -> [a]
find_courses :: (Foldable t1, Eq t2) => [(a, t1 t2)] -> t2 -> [a]
Note: Foldable is the typeclass which admits a folding operation, for example a list. A fold aggregates the
elements of a structure using a combining function.
> find_courses progLanguages “Python”
> find_courses progLanguages “C++”
> find_courses progLanguages “Go”
[ ]

(b) Write a Haskell function max_count that takes the list of courses as input (similar to
progLanguages) and returns the course with max number of programming languages. It returns a
two-tuple including the course major + number and the number of programming languages it uses.
In the above list, the course associated with the most number of languages is “CptS355”.
The type of the max_count function should be compatible with one of the following:
max_count :: [(a1, [a2])] -> (a1, Int)
max_count :: Foldable t => [(a1, t a2)] -> (a1, Int)
> max_count progLanguages
5. split_at_duplicate – 20%
Write a function split_at_duplicate that takes a list as input and splits the input list at the
consecutive duplicate elements. The goal is to produce a result in which the elements of the original list
have been collected into ordered sub-lists each containing the elements between the repeated
duplicate elements in the input list. The function will return a nested list including the sublists obtained
by the splits. The duplicate values should be included in the sublists.
The type of split_at_duplicate should be compatible with the following:
split_at_duplicate :: Eq a => [a] -> [[a]]
> split_at_duplicate [1,2,3,1,1,4,5,5,5,6,7,6,6,8,9,9]
> split_at_duplicate [10,10,10,10,10,10,10,10]
> split_at_duplicate [1,2,3,4,5,3,4,5,6,7,8]
> split_at_duplicate ([]::[Int])
(5%) Testing your functions
Install HUnit
We will be using the HUnit unit testing package in CptS355. See
http://hackage.haskell.org/package/HUnit for additional documentation.
Windows (using cabal installer) :
Run the following commands on the terminal.
cabal update
cabal v1-install HUnit
Mac (using stack installer)
Run the following commands on the terminal.
stack install HUnit
Check the attached HUnit_HowtoInstall.pdf document for other options to install HUnit.
Running Tests
The HW1SampleTests.zip file includes 6 .hs files where each one includes the HUnit tests for a
different HW problem. The tests compare the actual output to the expected (correct) output and raise
an exception if they don’t match. The test files import the HW1 module (HW1.hs file) which will include
your implementations of the HW problems.
You will write your solutions to HW1.hs file. To test your solution for each HW problem run the
following commands on the command line window (i.e., terminal):
$ ghci
$ :l P1_HW1tests.hs
P1_HW1tests> run
Repeat the above for other HW problems by changing the test file name, i.e. , P2_HW1tests.hs,
P3_HW1tests.hs, etc.
You are expected to add at least 2 more test cases for each problem. Write your tests for all problems
in HW1_tests.hs file – the template of this file is provided to you in the HW1 assignment page. Make
sure that your test inputs cover all boundary cases. Also, your test inputs should not be same or very
similar to the given sample tests.
Note : For problem 5(b), it is sufficient to provide one additional test case. For all other problems, please
give two tests.
In HUnit, you can define a new test case using the TestCase function and the list TestList
includes the list of all test that will be run in the test suite. So, make sure to add your new test cases to
the TestList list. All tests in TestList will be run through the “runTestTT tests” command.
The instructor will further explain this during the lecture.
To run your own test file run the following command on the GHCI prompt:
$ :l HW1tests.hs
HW1tests> run
If you don’t add new test cases or if your tests are very similar to the given tests, you will be deduced 5%
in this homework.
Haskell resources:
• Learning Haskell, by Gabriele Keller and Manuel M T Chakravarty (http://learn.hfm.io/)
• Real World Haskell, by Bryan O’Sullivan, Don Stewart, and John Goerzen
• Haskell Wiki: https://wiki.haskell.org/Haskell
• HUnit: http://hackage.haskell.org/package/HUnit