COMPSCI 220 Midterm 2: Fall 2019
Name:
Spire ID:
DO NOT OPEN EXAM BOOKLET UNTIL INSTRUCTED TO DO SO.
Time for Exam 60 minutes
Instructions Do not begin your exam until instructed. Please read all rules carefully before beginning
your exam. You will have one hour to complete all exam problems to the best of your ability.
Points This exam has 6 questions worth 100 points total.
Question Difficulty The expected difficulty of each question is marked with ?s. Harder questions have
more ?s. This should help you plan your time.
Rules
1. All electronics (including but not limited to cellphones, tablets, computers, smart watches, and calcu-
lators) must be turned off and placed out of sight in your backpack.
2. In order to receive credit for your midterm you must write your name and Spire ID on the first page,
and your name on all other pages.
3. All answers must fit within the boxes allocated for that question. Any work outside of the box will not
be taken into account during grading.
4. There will be no talking or leaving the exam room room during testing.
5. We have provided exam notes, which you may write on. You may not use any other material or notes.
6. All work must be your own and compliant with the Universities Academic Honesty Policy. Any
exhibition of Academic Dishonesty will be reported to the Academic Honesty Board.
Version A
1
COMPSCI 220 Midterm 2: Fall 2019 Name:
Question 1 What do the following programs display? Note that none of these programs produce errors.
Part a (5 points) Difficulty: ?
function F1(x) {
if (x === 5) {
return;
}
console.log(x);
F1(x + 1);
}
F1(1);
Part b (5 points) Difficulty: ?
function F2() {
console.log(’A’);
function inner() {
console.log(’B’);
}
console.log(’C’);
}
F2();
Part c (5 points) Difficulty: ?
function F3(x) {
return {
get: function() { return x; },
set: function(y) { x = y; }
};
}
let A = F3(200);
let B = F3(10);
B.set(300);
console.log(A.get());
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COMPSCI 220 Midterm 2: Fall 2019 Name:
Part d (5 points) Difficulty: ??
function F4(x) {
return {
x: x,
get: function() { return x; },
set: function(y) { x = y; }
};
}
let o = F4(3);
o.set(100);
console.log(o.x);
console.log(o.get());
Part e (5 points) Difficulty: ??
function F5(arr) {
return {
get: function(index) { return arr[index]; },
set: function(newArr) { arr = newArr; }
}
}
let a = [11, 22, 33];
let o = F5(a);
a[0] = 44;
console.log(o.get(0));
o.set([100, 200, 300]);
console.log(a[1]);
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COMPSCI 220 Midterm 2: Fall 2019 Name:
Question 2 Consider the following function.
function F7(f, g, h) {
f(function(x) {
if (x > 10) {
h(function() { console.log(’A’); });
}
else {
g(function() { console.log(’B’); })
}
})
}
Part a (5 points) Difficulty: ??
Give values of f, g, and h such that F7(f, g, h) displays the following output. Your answer
must not use console.log or raise errors.
A
B
Part b (5 points) Difficulty: ??
Give values of f, g, and h such that F7(f, g, h) displays the following output. Your answer
must not use console.log or raise errors.
B
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COMPSCI 220 Midterm 2: Fall 2019 Name:
Question 3 (5 points) Difficulty: ?
Consider the following function.
function F9(f, g) {
f(function(x) { console.log(x); });
}
Give values for f and g such that F9(f, g) displays the output:
220
Your answer must not use console.log or raise errors.
Question 4 (5 points) Difficulty: ??
Consider the following function.
function F8(f, r, n) {
if (n === 0) {
return r;
}
else {
return F8(f, f(n, r), n - 1);
}
}
Give a value for f, such that F8(f, 1, n) returns the value 1 × 2 × · · · × n. (The function must not
display anything. i.e., do not use console.log.)
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COMPSCI 220 Midterm 2: Fall 2019 Name:
Question 5 This question concerns streams, using the implementation in the exam notes. In your answers,
you may assume that the stream is unbounded (i.e., you can ignore sempty).
Part a (5 points) Difficulty: ?
Describe the following stream, or just list the first five values that it produces.
nats.filter(n => n % 3 === 0).filter(n => n % 6 === 0)
Note that nats is the stream of natural numbers (definition is in the exam notes).
Part b (5 points) Difficulty: ?
Write a stream of the multiples of four, i.e. 0, 4, 8, 12, · · ·. Your code can call the functions and
definitions in the exam notes. You do not have to to reproduce them in your solution.
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COMPSCI 220 Midterm 2: Fall 2019 Name:
Part c (5 points) Difficulty: ? ? ?
Write a function called alt with the following type:
alt(stream1: Stream, stream2: Stream): Stream
The result of alt should be a new stream that contains all the elements of stream1 and stream2
interleaved, starting with the first element of stream1.
This is an example of alt’s behavior:
let positives = snode(1, memo0(() => positives.map(x => x + 1)));
let negatives = snode(-1, memo0(() => negatives.map(x => x - 1)));
alt(positives, negatives) // produces 1, -1, 2, -2, 3, -3, ...
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COMPSCI 220 Midterm 2: Fall 2019 Name:
Part d (10 points) Difficulty: ??
Write a function called index with the following type:
index(stream: Stream): Stream<{ index: number, value: A}>
This function returns a stream of objects that contains the same values as the input stream. How-
ever, each value is accompanied with a number that specifies its index in the stream, starting with
zero.
For example, if stream produces the following values:
100
200
300
...
then index(stream) should produce the following values:
{ index: 0, value: 100 }
{ index: 1, value: 200 }
{ index: 2, value: 300 }
...
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COMPSCI 220 Midterm 2: Fall 2019 Name:
Part e (10 points) Difficulty: ??
Write a function called split with the following type:
split(stream: Stream): { evens: Stream, odds: Stream }
This function returns an object with two streams, where evens has alternating elements of stream,
including the first element, and odds has alternating elements of stream, excluding the first element.
Hint: Use index as a helper function.
Part f (5 points) Difficulty: ? ? ?
Describe the mystery stream, or just list the first five values that it produces.
let X = snode(0, memo0(() => X.map(n => n + 2)));
let Y = snode(1, memo0(() => Y.map(n => n + 2)));
let mystery = alt(X, Y);
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COMPSCI 220 Midterm 2: Fall 2019 Name:
Question 6 This question concerns error handling with results, using the implementation in the exam notes.
Part a (5 points) Difficulty: ?
What does the following program display?
function E0(n) {
if (n === 0) { return new Success(10); }
else { return new Failure(0); }
}
E0(10).then(function(v) { console.log(v); });
console.log("Done");
Part b (5 points) Difficulty: ?
Consider the following function:
function E2(r) {
r.then(function(n) {
console.log("A");
});
}
Give a value for r such that E2(r) displays the following output:
A
Your answer must not use console.log or raise errors.
Part c (5 points) Difficulty: ??
What does the following program display?
function E1(n) {
if (n === 0) {
return new Success(0);
}
else {
return E1(n - 1).then(r => { console.log(n); return new Success(r + n) });
}
}
E1(3);
End of exam.
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