Scala - Coursera Course Discussion Thread

[Joe San]

Well I did try to implement the map function. What I did was to check if the element belongs to the set and if yes, I apply the function to the set. I pass the result of the function to my already implemented singleton set which returns a Set. In the else block, I return false. Now, when I tried to test it using the following:

assert(contains(map(set1To4, (x: Int) => x + x)), 6) // This should result in true as 3 applied to x+x would be 6, but it returns false.

Later I tried to print the f(x) and s(x) and to my surprise, the s(x) prints 6

if(s(x)) // println(s(x) prints 6. From where did it get this 6?
  apply f(x) and set it to a val
  call singletonSet with the val from the above statement
else
  false

I do not get why it prints 6 and evaluates to false.

[chris webster]

Well, it's difficult to discuss the assignment without giving away the answer, and I don't understand what your code is trying to do, but I think you are making things way too complicated. You don't need your singleton set at all for this exercise, and the actual implementation of map()can be done in half a line of code that uses some of the functions we've already implemented. Of course, figuring out that half line of code was pretty hard (at least for me), so I suggest you take a look at David Hsu's suggestion on the thread I started for this question in the Assignments / Week 2 forum on the course website. David's formulation of the problem helped me figure out how to implement the requirement, so hopefully it will help you too.

[Joe San]

David's suggestion hints that we use the exists method. But when using exists method, I would end up iterating.

Wouldn't I be needing a variable that I declare on my map function? A map function returns a Set. A Set here is something that takes an Int and returns a Boolean. So defining that on the map function body would look like:

map(s: Set, f: Int => Int): Set = {
  (x: Int) => apply the function on the Set and return a boolean
}

Where I'm exactly stuck is when I call this function using my test case:

assert(map(s1, (x: Int) => x + x), 2)

The value of x is 2. But s1 is 1. But the map function should evaluate to 2. I think I'm missing some basic understanding of anonymous functions.

[Joe San]

I mimiced my forall implementation and I was able to arrive at my answer for the map function. I will now have to reduce it to one line by making use of the exists method. Thanks for the pointer to your post. It helped me to finish it.

[Matthew Brown]

Joe Harry wrote:Where I'm exactly stuck is when I call this function using my test case:

assert(map(s1, (x: Int) => x + x), 2)

The value of x is 2. But s1 is 1. But the map function should evaluate to 2. I think I'm missing some basic understanding of anonymous functions.

Surely the map should evaluate to a set that contains 2, not 2 itself?

My tip for getting a concise expression would be to step away from the code, and try and express what the result of the map function means. Complete the sentence "map(s1, f) contains x if...". Bonus points if you use the word "exists" somewhere.

[Joe San]

Matthew Brown wrote:

My tip for getting a concise expression would be to step away from the code, and try and express what the result of the map function means. Complete the sentence "map(s1, f) contains x if...". Bonus points if you use the word "exists" somewhere.

if x exists in s and maps to the result of applying the function on x (f(x)).

[Matthew Brown]

Joe Harry wrote:if x exists in s and maps to the result of applying the function on x (f(x)).

No, x doesn't have to exist in s. Using your doubling example, 6 doesn't exist in {1, 2, 3}, but does exist in {2, 4, 6}.

[Joe San]

Matthew Brown wrote:

Joe Harry wrote:if x exists in s and maps to the result of applying the function on x (f(x)).

No, x doesn't have to exist in s. Using your doubling example, 6 doesn't exist in {1, 2, 3}, but does exist in {2, 4, 6}.

then, it would be as follows:

Set s has an element which when applied the function f(?) would get x where ? is an element in the Set s

[Matthew Brown]

Pretty much, yes. And "s has an element <insert condition here>" is exactly what the exists method is for.

[Joe San]

Matthew Brown wrote:Pretty much, yes. And "s has an element <insert condition here>" is exactly what the exists method is for.

f(?) == x where ? is the element in s. I did infact complete the map function but I did not use the exists method. I'm having some mis-understanding with using the anonymous functions. I will have to think about calling the exists method instead of iterating once again in my map.

[Wen Tong Lin]

Hello everyone,

Its good to see this thread. I was wondering if anybody can give me a hint on how to define the filter function? The description says "Returns the subset of `s` for which `p` holds.". I'm confuse why p is not define as a Set, but instead it's a function with the same signature and return type.

My understanding was:

s = {1,2,3,4,5}
p = {2,3,4}

then it returns {2,3,4} ?

[Joe San]

Wen Tong Lin wrote:Hello everyone,

Its good to see this thread. I was wondering if anybody can give me a hint on how to define the filter function? The description says "Returns the subset of `s` for which `p` holds.". I'm confuse why p is not define as a Set, but instead it's a function with the same signature and return type.

My understanding was:

s = {1,2,3,4,5}
p = {2,3,4}

then it returns {2,3,4} ?

Though technically a Predicate P is also a Set (that takes an Int and gives a Boolean), here it would be better to think about it as a Predicate or a Condition rather than a Set.

Your filter implementation should be able to satisfy whatever you give as the Predicate:

For example:

val filteredSet = filter(veryBigSet, x => (x < 5)) // veryBigSet consists of numbers anything less than 5.

So if you call the filter method on a Set of numbers say 1 to 10, it should filter out everything that does not pass the Predicate. One hint for this would be to think about it as the inverse of your intersect method.

[Wen Tong Lin]

Thank you Joe. I think I got it now. Let me give it a try.

[Wen Tong Lin]

Here's the filter method:

...deleted...

test case:

test("filter will use predicate to every set") {
new TestSets {
val w = singletonSet(1)
val x = singletonSet(2)
val y = singletonSet(3)
val z = singletonSet(-4)
val a = union(w,x)
val b = union(a,y)
val c = union(b,z)
//c = {1,2,3,-4}
val f = filter(c, x => (x < 0))
// f = {1,2,3}
assert(contains(f,1), "f contains 1")
assert(contains(f,2), "f contains 2")
assert(contains(f,3), "f contains 3")
assert(!contains(f,-4), "f does not contain -4")
}

I am still digesting how this functional style of programming works, considering, we are using a function to hold the data. I am used to storing data in an array. In here the data is stored as closure.

[Matthew Brown]

Hi Wen. I hope you don't mind, but I've deleted your answer to the question. It's against the Coursera honor code to make any solutions for assignments available to anyone else. I know this is a very small example, but I still think we should keep to discussing assignments in general terms.

[Wen Tong Lin]

Hi Mathew. I apologize for posting the solution, I didn't know about that. Thanks for removing it.

[Wen Tong Lin]

Hi guys,

I'm stuck with implementing map method. My solution looks very imperative. Any hints? This is what I did.

loop from -bound to +bound assign value to i
if s(i) is true return a function that does f(i)

[chris webster]

Wen Tong Lin wrote:Hi guys,

I'm stuck with implementing map method. My solution looks very imperative. Any hints? This is what I did.

loop from -bound to +bound assign value to i
if s(i) is true return a function that does f(i)

A lot of us had problems with this - me included. I suggest you look at this thread on the course forum, especially David Hsu's advice.

You can implement the map() function in a single short line of code, re-using some of the functions you created earlier in the assignment. For example, your "forall()" function may already check the valid range of integers, and "exists()" is supposed to re-use forall() as well, so maybe you don't need to re-implement the iterative stuff here?

[Wen Tong Lin]

Thanks Chris. I already submitted my exercise without the answer. Let me check that thread, for the sake of learning.

[chris webster]

Wen Tong Lin wrote:Thanks Chris. I already submitted my exercise without the answer. Let me check that thread, for the sake of learning.

You can re-submit your assignment with the completed function if you want.

[Matthew Brown]

Discussion continues on week 3 at https://coderanch.com/t/594086/Scala/Scala-Coursera-Week...