Implementing exercise binary-search #615

samuelhnrq · 2017-10-29T13:36:26Z

On the works!

NobbZ · 2017-10-29T18:11:02Z

What kind of Container will you use?

If it is a non continious memory store (eg. a List), the binary search is nonsense (binary search gets even worse than a linear search due to subslicing).

samuelhnrq · 2017-10-29T18:22:49Z

Does it even matter bro? Aren't you overthinking it? it's not the point to make the better possible way to search an element in an array, rather then figuring out how to implement the algorithm, even if I knew what the heck were you talking about and indeed it were a lot slower, I don't think it would matter anyway, but that's just IMHO, what do you even mean? It's going to be slow?

NobbZ · 2017-10-29T20:27:00Z

Yes, it does matter.

The README sells the binary search as superior while it isn't for lists, so either implement the exercise in a way that it is not using a List but some kind of Array, or don't do it at all!

At least that's my opinion, but I am not a maintainer here, so at the end @exercism/haskell descides.

Average-user · 2017-10-29T20:30:49Z

@samosaara This is supposed to be an example, therefore, the implementation matters.

NobbZ · 2017-10-29T20:32:59Z

PS: As far as I can remember the best possible implementations of binary search in a linked List ar O(n log n), but most are near to O(n²).

In worst case (element is found at the very last possible step) you'll have copied parts of the list log n times, you have to iterate each copied slice 1 to 1.5 times on naiv implementations (first half time for the element in the middle, another half time when you need left subslice, or a full time when needing the right subslice).

In a proper Array (as in a C array, meaning a continious indexed memory area), subslicing is basically a NOOP (when done correctly by the runtime) as well as indexed access does not involve itration of the full memory, but only calculating the exact point and accessing it for reading.

Average-user · 2017-10-29T20:34:27Z

@NobbZ Sequences can do the work here right?

NobbZ · 2017-10-29T20:38:36Z

Yes, they can perform better than Lists when using their splitAt, drop, and take functions.

samuelhnrq · 2017-10-29T20:44:20Z

Oh so the whole discussion was to have the best ever, swag, 1337 h@x0r, code golf, example solution, well, ok whatever I'll use sequences. Guess I missed the point, thought you guys were complaining about the exercise itself.

Average-user · 2017-10-29T20:50:07Z

@samosaara It isn't about having the best ever, thats impossible. The idea is to show when is useful to actually use a binary search algorithm, and apparently with lists it isn't.

NobbZ · 2017-10-29T20:50:16Z

I'm fine with the exercise, as long as the binary search is used with a correct datatype to not suggest bad practices and sell them as a good one.

petertseng · 2017-10-29T22:15:15Z

I thought https://hackage.haskell.org/package/array-0.5.2.0/docs/Data-Array.html would have been sufficient. Wouldn't O(1) access be more suited to showing binary search than O(log(min(i, n-i))) that I see in https://hackage.haskell.org/package/containers-0.5.10.2/docs/Data-Sequence.html#v:index ? Or is even that sufficient? I guess since there would be at most O(log n) bisections one would call the runtime O(log^2 n), so you are saying that is good enough, right? Even if it is good enough, would it still be better to use an Array? If I miss something, please educate me, thank you.

Average-user · 2017-10-29T22:19:31Z

@petertseng I just said sequences because it is what I've used. Probably arrays work too, but I can't find where it says that the access is constant.

NobbZ · 2017-10-29T22:44:28Z

If Arrays are O(1) in random reading and also can be easily sliced, then of course they are perfect for this algorithm, but I can't find a slicing accessor in the docs…

petertseng · 2017-10-29T23:05:47Z

I can't find where it says that the access is constant.
If Arrays are O(1) in random reading

Okay, y'all are right, https://hackage.haskell.org/package/array-0.5.2.0/docs/Data-Array.html only specifies that "a programmer may reasonably expect rapid access to the components" without specifically explaining how rapid is the rapid referred to.

and also can be easily sliced

Shrug. Can just keep track of the left/right index of the subarray you're searching, and thereby avoid a need to slice, just keep the same array throughout.

samuelhnrq · 2017-10-29T23:12:12Z

Okay, so, I guess I'm not that proficient in Haskell to implement with the kinda of quality you guys are requesting, for the example, so I guess I would need help (accepting PR on my fork XD) or at least time to implement this 😅

Average-user · 2017-10-29T23:17:09Z

@samosaara I think that you have time. And the only "challenge" here, it is to get used to Data.Array.

For example you can try to implement the algorithm in Lists, as your previous intentions were. And then translate it, keeping in mind the differences between the two containers.

samuelhnrq · 2017-11-01T23:44:17Z

Oh wait but just to be sure, I should do change the signature of the exercise itself or just convert back and forth in the example implementation?

Average-user · 2017-11-02T00:17:17Z

@samosaara Probably the first, so that the users that are going to implement this, will be forced to use a container in which this is efficient.

petertseng · 2017-11-05T04:07:03Z

exercises/binary-search/stack.yaml

@@ -0,0 +1 @@
+resolver: lts-8.21


before I forget: I moved to lts-9.11 now so this should change too

samuelhnrq · 2017-11-05T11:28:22Z

Well, there is a problem, I cant implement one of the tests because there is no way to represent an empty Data.Array, Do I ignore it? What should I do?

petertseng · 2017-11-05T11:42:19Z

I was able to find http://mail.haskell.org/pipermail/haskell-cafe/2004-October/007098.html by asking my preferred search engine about empty arrays, but I can't say whether this information is still up to date since it is ten years old. Is it in fact out of date?

samuelhnrq · 2017-11-05T12:18:12Z

exercises/binary-search/test/Tests.hs

               , value       = 13
               , expected    = Nothing
               }
        , Case { description = "nothing is included in an empty array"
-               , input       = []
+               , input       = listArray (1, 0) [] -- There is no way to represent an empty array


Ignore the dumb comment, there is. it is working

samuelhnrq · 2017-11-05T15:47:29Z

Actually I have doubts if I got that right, I didn't find a way to split the array itself, and the way I'm doing it there is not much of a point, because I'm just making it into lists so I can split them... Which was the whole point of this discussion, that doing that was slow. Did I miss a method or that's the way to do it?

NobbZ · 2017-11-05T20:36:11Z

In general (so regardless of the language) these algorithms avoid to copy the array. The best way to do this is to pass it around as is, and provide 2 additional parameters which determine upper and lowerbound of the active subslice.

find xs e = go (lowerIdx xs) (upperIdx xs)
  where
    go l u | xs[(l + u) / 2] == e = True
           | xs[(l + u) / 2] <  e = go (1 + (l + u) / 2) u
           | otherwise            = go l (-1 + (l + u) / 2)

This is only a rough draft and psedo code using C-like array accessors…

But it should show the overall concept

petertseng

Nice job on the array without copying.

Here's a question that I couldn't find answered in the code currently.

I saw https://github.com/exercism/problem-specifications/blob/4599c2ecbd5ff03d098b4a6eaaea0ab7dbc518db/exercises/binary-search/canonical-data.json#L73-L81 case, where you would want to have Nothing of searching for 7 in array1. I think that case should be added to the tests as well.

petertseng · 2017-11-26T12:09:50Z

exercises/binary-search/test/Tests.hs

+
+
+data Case a = Case { description :: String
+                 , input       :: Array Int a


I would indent this line two spaces further so that the comma lines up with the open bracket on the previous row.

petertseng · 2017-11-26T12:09:56Z

exercises/binary-search/test/Tests.hs

+
+data Case a = Case { description :: String
+                 , input       :: Array Int a
+                 , value       :: a


I would indent this line two spaces further so that the comma lines up with the open bracket on the previous row.

same

petertseng · 2017-11-26T12:10:00Z

exercises/binary-search/test/Tests.hs

+data Case a = Case { description :: String
+                 , input       :: Array Int a
+                 , value       :: a
+                 , expected    :: Maybe Int


I would indent this line two spaces further so that the comma lines up with the open bracket on the previous row.

same

petertseng · 2017-11-26T12:10:10Z

exercises/binary-search/test/Tests.hs

+                 , input       :: Array Int a
+                 , value       :: a
+                 , expected    :: Maybe Int
+                 }


I would indent this line two spaces further so that the comma lines up with the open bracket on the previous row.

same except now I'm talking about the closing bracket

petertseng · 2017-11-26T12:12:44Z

exercises/binary-search/test/Tests.hs

+               , expected    = Nothing
+               }
+        ]
+        where array1 = listArray (0, 6) [1, 3, 4, 6, 8, 9, 11]


I'm considering whether this should go above the cases so that I can see it before I see the cases it's going to be used in. I'm not sure, so let me know what you think about it.

petertseng · 2017-11-26T12:13:18Z

exercises/binary-search/README.md

+
+## Source
+
+Wikipedia [https://en.wikipedia.org/wiki/Isogram](https://en.wikipedia.org/wiki/Isogram)


why isogram?

sshine · 2019-03-08T10:53:39Z

Hello @samosaara! I went and improved on this solution in #807, since it was so close to being finished.

A few complexities have been added to the track since you started this PR, so I've addressed those.

Thanks for doing all the groundwork.

Implementing exercise binary-search

078f092

Everything but example

c26de9e

Adds to config.json

f5bbc8a

petertseng reviewed Nov 5, 2017

View reviewed changes

Array collection, new resolver, example implementation

0b6c2d6

samuelhnrq commented Nov 5, 2017

View reviewed changes

samuelhnrq added 2 commits November 5, 2017 10:18

Removes dumb comment

cadc26a

Removes unecessary parentheses

f1ba38c

New way without copying

6356637

petertseng suggested changes Nov 26, 2017

View reviewed changes

sshine mentioned this pull request Mar 8, 2019

binary-search: Add exercise (closes #615) #807

Merged

sshine closed this in 8787e59 Mar 12, 2019

Provide feedback

Saved searches

Use saved searches to filter your results more quickly

Implementing exercise binary-search #615

Implementing exercise binary-search #615

samuelhnrq commented Oct 29, 2017

NobbZ commented Oct 29, 2017

samuelhnrq commented Oct 29, 2017

NobbZ commented Oct 29, 2017

Average-user commented Oct 29, 2017 •

edited

Loading

NobbZ commented Oct 29, 2017 •

edited

Loading

Average-user commented Oct 29, 2017

NobbZ commented Oct 29, 2017

samuelhnrq commented Oct 29, 2017 •

edited

Loading

Average-user commented Oct 29, 2017

NobbZ commented Oct 29, 2017

petertseng commented Oct 29, 2017 •

edited

Loading

Average-user commented Oct 29, 2017

NobbZ commented Oct 29, 2017

petertseng commented Oct 29, 2017

samuelhnrq commented Oct 29, 2017

Average-user commented Oct 29, 2017 •

edited

Loading

samuelhnrq commented Nov 1, 2017

Average-user commented Nov 2, 2017

petertseng Nov 5, 2017

samuelhnrq commented Nov 5, 2017

petertseng commented Nov 5, 2017

samuelhnrq Nov 5, 2017

samuelhnrq commented Nov 5, 2017 •

edited

Loading

NobbZ commented Nov 5, 2017 •

edited

Loading

petertseng left a comment •

edited

Loading

petertseng Nov 26, 2017

petertseng Nov 26, 2017

petertseng Nov 26, 2017

petertseng Nov 26, 2017

petertseng Nov 26, 2017

petertseng Nov 26, 2017

sshine commented Mar 8, 2019



		data Case a = Case { description :: String
		, input :: Array Int a


		## Source

		Wikipedia [https://en.wikipedia.org/wiki/Isogram](https://en.wikipedia.org/wiki/Isogram)

Implementing exercise binary-search #615

Implementing exercise binary-search #615

Conversation

samuelhnrq commented Oct 29, 2017

NobbZ commented Oct 29, 2017

samuelhnrq commented Oct 29, 2017

NobbZ commented Oct 29, 2017

Average-user commented Oct 29, 2017 • edited Loading

NobbZ commented Oct 29, 2017 • edited Loading

Average-user commented Oct 29, 2017

NobbZ commented Oct 29, 2017

samuelhnrq commented Oct 29, 2017 • edited Loading

Average-user commented Oct 29, 2017

NobbZ commented Oct 29, 2017

petertseng commented Oct 29, 2017 • edited Loading

Average-user commented Oct 29, 2017

NobbZ commented Oct 29, 2017

petertseng commented Oct 29, 2017

samuelhnrq commented Oct 29, 2017

Average-user commented Oct 29, 2017 • edited Loading

samuelhnrq commented Nov 1, 2017

Average-user commented Nov 2, 2017

Choose a reason for hiding this comment

samuelhnrq commented Nov 5, 2017

petertseng commented Nov 5, 2017

Choose a reason for hiding this comment

samuelhnrq commented Nov 5, 2017 • edited Loading

NobbZ commented Nov 5, 2017 • edited Loading

petertseng left a comment • edited Loading

Choose a reason for hiding this comment

Choose a reason for hiding this comment

Choose a reason for hiding this comment

Choose a reason for hiding this comment

Choose a reason for hiding this comment

Choose a reason for hiding this comment

Choose a reason for hiding this comment

sshine commented Mar 8, 2019

Average-user commented Oct 29, 2017 •

edited

Loading

NobbZ commented Oct 29, 2017 •

edited

Loading

samuelhnrq commented Oct 29, 2017 •

edited

Loading

petertseng commented Oct 29, 2017 •

edited

Loading

Average-user commented Oct 29, 2017 •

edited

Loading

samuelhnrq commented Nov 5, 2017 •

edited

Loading

NobbZ commented Nov 5, 2017 •

edited

Loading

petertseng left a comment •

edited

Loading