Name: ID :
CSE 130, Winter 2012: Midterm Examination
Feb 14th, 2012
• Do not start the exam until you are told to.
• This is a open-book, open-notes exam, but with no computational devices allowed (such as calcula- tors/cellphones/laptops).
• Do not look at anyone else’s exam. Do not talk to anyone but an exam proctor during the exam.
• Write your answers in the space provided.
• Wherever it gives a line limit for your answer, write no more than the specified number of lines. The rest will be ignored.
• Work out your solution in blank space or scratch pa- per, and only put your answer in the answer blank given.
• The points for each problem are a rough indicator of the difficulty of the problem.
1. 45 Points
2. 15 Points
3. 25 Points
4. 0 Points
TOTAL 85 Points
• Good luck!
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1. [ 45 points ] We are going to implement merge sort, in small steps.
a. [ 15 points ] First, you will write a function split : ’a list -> ’a list * ’a list. This function
splits a given list in two parts along the middle. Here are examples of it running:
# split [23;1;8;3];;
– : int list * int list = ([23; 1], [8; 3])
# split [23;1;8;3;6];;
– : int list * int list = ([23; 1], [8; 3; 6])
# split [23;1;8;3;6;20];;
– : int list * int list = ([23; 1; 8], [3; 6; 20])
# split [“a”;”b”;”c”];;
– : string list * string list = ([“a”], [“b”; “c”])
# split [“a”];;
– : string list * string list = ([], [“a”])
To do this, you will use fold_left, whose type is (’a -> ’b -> ’a) -> ’a -> ’b list -> ’a. You can also assume a function length: ’a list -> int which returns the length of a list. Now, fill in the implementation of split below:
let rec split l =
(* additional let declarations if you need any *)
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let base = _______________________________________ in
let fold_fn (i,l1,l2) elmt =
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let (_, l1, l2) = List.fold_left fold_fn base l in
(l1,l2)
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b. [ 15 points ] You will now write a merge function of type ’a list -> ’a list -> ’a list. This function takes two lists that are already sorted using the ordering <=, and merges them into a sorted list. Here are examples of merge: # merge [2;4;6;8] [1;3;5];; – : int list = [1; 2; 3; 4; 5; 6; 8] # merge [2;10;20] [1;2;3;4;5;8;10;12];; – : int list = [1; 2; 2; 3; 4; 5; 8; 10; 10; 12; 20]Fill in the implementation of merge below: let rec merge l1 l2 =match (l1, l2) with| ([], l) -> __________________________________________________________________
| (l, []) -> __________________________________________________________________
| _____________________________________________________________________________
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c. [ 15 points ] We are now ready to write merge_sort, whose type is ’a list -> ’a list. You should use the split and merge functions above to implement merge_sort. Recall that merge sort works by splitting the input list in two, recursively sorting the two sub-lists, and then merging the two results of the recursive calls into a sorted list. Here are examples of running merge_sort:
# merge_sort [2;10;3;2;1];;
– : int list = [1; 2; 2; 3; 10]
# merge_sort [-10;0;10;-20;100;-100];;
– : int list = [-100; -20; -10; 0; 10; 100]
Fill in the implementation of merge_sort below: let rec merge_sort l =
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2.
[ 15 points ] Assume you are given the following two functions: explode : string -> char list
implode : char list -> string
Given a string s, (explode s) returns the list of characters in the string, and given a list of characters l,
(implode l) returns a string that contains all the characters in the list. For example: # explode “abc”;;
– : char list = [’a’; ’b’; ’c’]
# implode [’a’; ’b’; ’c’];;
– : char list = “abc”
Also, assume that you have the traditional map, with type (’a -> ’b) -> ’a list -> ’b list. Using map, explode and implode, write a function replace: string -> string that replaces the hyphen character, ’-’, with space, ’ ’:
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3. [25points]
a. [ 13 points ] In this problem you are going to write a function:
app: (’a->’b)list->’a->’blist
Given a list of functions l, (app l x) returns a list where each element of the list is the application of
the corresponding function from l to x. For example:
# let incr x = x+1;;
val incr : int -> int =
# let decr x = x-1;;
val decr : int -> int =
# app [incr;decr] 10;;
– : int list = [11; 9]
Implement app below using map:
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b. [ 12 points ] Now, consider the following code:
# let [f1;f2] = app [(=);(<)] 2;; val f1 : int -> bool =
val f2 : int -> bool =
For each expression below, write down what it evaluates to:
(f1 1)
(f1 2)
(f1 3)
(f2 1)
(f2 2)
(f2 3)
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4. [ 0 points ] Circle the correct answer. When Sorin was a kid, he did which of the following sports: 1. Snow Rugby
2. Luging
3. Curling
4. Figure Skating 5. Speed Skating
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