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Homework 2 Solved
This homework is meant to prepare you for upcoming exams. You will be expected to be familiar with the concepts covered in this homework and be able to write grammatically-correct Haskell code. Each question has a time estimate; you’ll know you're ready for the exam when you can solve them roughly within their time constraints.
You must turn in a PDF file with your answers via Gradescope - you may include both typed and hand-written solutions, so long as they are legible and make sense to our TAs. Make sure to clearly label each answer with the problem number you’re solving so our TAs can more easily evaluate your work.
For all questions where you are writing Haskell code, you should give your functions a type definition.
1.
a) (2 min.) Use the map function to write a Haskell function named scale_nums that takes in a list of Integers and an Integer named factor. It should return a new list where every number in the input list has been multiplied by factor.
Example:
scale_nums [1, 4, 9, 10] 3 should return [3, 12, 27, 30].
You may not define any nested functions. Your solution should be a single, one-line map expression that includes a lambda.
b) (2 min.) Use the filter and all functions to write a Haskell function named only_odds that takes in a list of Integer lists, and returns all lists in the input list that only contain odd numbers (in the same order as they appear in the input list). Note that the empty list vacuously satisfies this requirement.
Example:
only_odds [[1, 2, 3], [3, 5], [], [8, 10], [11]] should return [[3, 5], [], [11]].
You may not define any nested functions. Your solution should be a single, one-line filter expression that includes a lambda.
c) (2 min.) In Homework 1, you wrote a largest function that returns the larger of two words, or the first if they are the same length:
largest :: String -> String -> String largest first second =
if length first >= length second then first else second
Use one of foldl or foldr and the largest function to write a Haskell function named largest_in_list that takes in a list of Strings and returns the longest String in the list. If the list is empty, return the empty string. If there are multiple strings with the same maximum length, return the one that appears first in the list.
Do not use the map, filter or maximum functions in your answer.
Your answer should be a single, one-line fold expression.
Example:
largest_in_list ["how", "now", "brown", "cow"] should return
"brown".
largest_in_list ["cat", "mat", "bat"] should return "cat".
2.
a) (5 min.) Write a Haskell function named count_if that takes in a predicate function of type (a -> Bool) and a list of type [a]. It should return an Int representing the number of elements in the list that satisfy the predicate. Your solution must use recursion. Do not use the map, filter, foldl, or foldr functions in your solution.
Examples:
count_if (\x -> mod x 2 == 0) [2, 4, 6, 8, 9] should return 4. count_if (\x -> length x > 2) [“a”, “ab”, “abc”] should return 1.
b) (3 min.) Now, reimplement the same function above (call it
count_if_with_filter), but use the filter function in your solution.
c) (3 min.) Now, reimplement the same function above (call it
count_if_with_fold) but use either foldl or foldr in your solution.
3.
a) (3 min.) Explain the difference between currying and partial application.
b) (4 min.) Suppose we have a Haskell function with type a -> b -> c. Consider the other two function types:
i. (a -> b) -> c
ii. a -> (b -> c)
Is a -> b -> c equivalent to i, ii, both, or neither? Why?
c) (2 min.) Consider the following Haskell function:
foo :: Integer -> Integer -> Integer -> (Integer -> a) -> [a]
foo x y z t = map t [x,x+z..y]
Rewrite the implementation of foo as a chain of lambda expressions that each take in one variable to demonstrate the form of a curried function.
4. Consider the following Haskell function:
f a b
=
let c =
\a -> a
-- (1)
d =
\c -> b
--
(2)
in \e f
-> c d e
--
(3)
a) (1 min.) What variables (if any) are captured in the lambda labeled (1)?
b) (1 min.) What variables (if any) are captured in the lambda labeled (2)?
c) (1 min.) What variables (if any) are captured in the lambda labeled (3)?
d) (4 min.) Suppose we invoke f in the following way:
f 4 5 6 7
What are the names of the variables that the passed in values (4, 5, 6, and 7) are bound to? Which of them (if any) are actually referenced in the implementation of f? Explain.
5. (5 min.) C allows you to point to functions, as in the following code snippet:
int add(int a, int b) {
return a + b;
}
int main() {
• Declare a pointer named `fptr` that points to a function
• that takes in two int arguments and returns another int int (*fptr)(int, int);
• Assign the address of the `add` function to `fptr`
fptr = &add;
• Invoke the function using `fptr`. This returns 8. (*fptr)(3, 5);
}
Function pointers are first-class citizens like any other pointer in C: they can be passed as arguments, used as return values, and assigned to variables. As a reminder, however, C does not support nested functions.
Compare function pointers in C with closures in Haskell. Are Haskell closures also first-class citizens? What (if any) capabilities do function pointers have that closures do not (and vice versa)?
6. Haskell allows you to define your own custom data types. In this question, you’ll look at code examples and use them to write your own (that is distinct from the ones shown).
Consider the following code example:
data Triforce = Power | Courage | Wisdom
wielder :: Triforce -> String
wielder Power = "Ganon"
wielder Courage = "Link"
wielder Wisdom = "Zelda"
princess = wielder Wisdom
a) (2 min.) Define a new Haskell type InstagramUser that has two value constructors (without parameters) - Influencer and Normie.
b) (2 min.) Write a function named lit_collab that takes in two InstagramUsers and returns True if they are both Influencers and False otherwise.
Consider the following code example:
data Character = Hylian Int | Goron | Rito Double | Gerudo |
Zora
describe :: Character -> String
describe (Hylian age) = "A Hylian of age " ++ show age describe Goron = "A Goron miner"
describe (Rito wingspan) = "A Rito with a wingspan of " ++ show wingspan ++ "m"
describe Gerudo = "A mighty Gerudo warrior"
describe Zora = "A Zora fisher"
c) (2 min.) Modify your InstagramUser type so that the Influencer value constructor takes in a list of Strings representing their sponsorships.
d) (3 min.) Write a function is_sponsor that takes in an InstagramUser and a String representing a sponsor, then returns True if the user is sponsored by sponsor (this function always returns False for Normies).
Consider the following code example:
data Quest = Subquest Quest | FinalBoss
count_subquests :: Quest -> Integer
count_subquests FinalBoss = 0
count_subquests (Subquest quest) = 1 + count_subquests quest
e) (2 min.) Modify your InstagramUser type so that the Influencer value constructor also takes in a list of other InstagramUsers representing their followers (after their sponsors).
f) (3 min.) Write a function count_influencers that takes in an InstagramUser and returns an Integer representing the number of Influencers that are following that user (this function always returns 0 for Normies).
g) (2 min.) Use GHCi to determine the type of Influencer using the command
:t Influencer. What can you infer about the type of custom value constructors?
7. Consider the following Haskell data type:
data LinkedList = EmptyList | ListNode Integer LinkedList
deriving Show
a) (3 min.) Write a function named ll_contains that takes in a LinkedList and an Integer and returns a Bool indicating whether or not the list contains that value.
Examples:
ll_contains (ListNode 3 (ListNode 6 EmptyList)) 3 should return True.
ll_contains (ListNode 3 (ListNode 6 EmptyList)) 4 should return False.
b) (3 min.) We want to write a function named ll_insert that inserts a value at a given zero-based index into an existing LinkedList. Provide a type definition for this function, explaining what each parameter is and justifying the return type you chose.
c) (5 min.) Implement the ll_insert function. If the insertion index is 0 or negative, insert the value at the beginning. If it exceeds the length of the list, insert the value at the end. Otherwise, the value should have the passed-in insertion index after the function is invoked.
8. In this question, we’ll examine how the choice of programming language/paradigm can affect the difficulty of a given task.
a) (5 min.) Using C++, write a function named longestRun that takes in a vector of booleans and returns the length of the longest consecutive sequence of true values in that vector.
Examples:
Given {true, true, false, true, true, true, false}, longestRun(vec) should return 3.
Given {true, false, true, true}, longestRun(vec) should return 2.
b) (10 min.) Using Haskell, write a function named longest_run that takes in a list of Bools and returns the length of the longest consecutive sequence of True values in that list.
Examples:
longest_run [True, True, False, True, True, True, False] should return 3.
longest_run [True, False, True, True] should return 2.
c) (10 min.) Consider the following C++ class:
#import <vector>
using namespace std;
class Tree {
public:
unsigned value;
vector<Tree *> children;
Tree(unsigned value, vector<Tree *> children) { this->value = value;
this->children = children;
}
};
Using C++, write a function named maxTreeValue that takes in a Tree pointer root and returns the largest value within the tree. If root is nullptr, return 0.
This function may not contain any recursive calls.
d) (5 min.) Consider the following Haskell data type: data Tree = Empty | Node Integer [Tree]
Using Haskell, write a function named max_tree_value that takes in a Tree and returns the largest Integer in the Tree. Assume that all values in the tree are non-negative. If the root is Empty, return 0.
Example:
max_tree_value (Node 3 [(Node 2 [Node 7 []]), (Node 5 [Node
• []])]) should return 7.
9. (10 min.) Write a Haskell function named fibonacci that takes in an Int argument n. It should return the first n numbers of the Fibonacci sequence (for this problem, we’ll say that the first two numbers of the sequence are 1, 1).
Examples:
fibonacci 10 should return [1,1,2,3,5,8,13,21,34,55]. fibonacci -1 should return [].
Hint: You may find it easier to build the list in reverse in a right-to-left manner, then use the reverse function.
10. (20 min.) Super Giuseppe is a hero trying to save Princess Watermelon from the clutches of the evil villain Oogway. He has a long journey ahead of him, which is comprised of many events:
data Event = Travel Integer | Fight Integer | Heal Integer
Super Giuseppe begins his adventure with 100 hit points, and may never exceed that amount. When he encounters:
A Travel event, the Integer represents the distance he needs to travel. During this time, he heals for ¼ of the distance traveled (floor division).
A Fight event, the Integer represents the amount of hit points he loses from the fight.
A Heal event, the Integer represents the amount of hit points he heals after consuming his favorite power-up, the bittermelon.
If Super Giuseppe has 40 or fewer life points after an Event, he enters defensive mode. While in this mode, he takes half the damage from fights (floor division), but he no longer heals while traveling. Nothing changes with Heal events. Once he heals above the 40 point threshold following an Event, he returns to his normal mode (as described above).
Write a Haskell function named super_giuseppe that takes in a list of Events that comprise Super Giuseppe’s journey, and returns the number of hit points that he has at the end of it. If his hit points hit 0 or below at any point, then he has unfortunately Game Over-ed and the function should return -1.
Examples:
super_giuseppe [Heal 20, Fight 20, Travel 40, Fight 60, Travel 80, Heal 30, Fight 40, Fight 20] should return 10. super_giuseppe [Heal 40, Fight 70, Travel 100, Fight 60, Heal 40] should return -1.
