What is the complexity for Seq.append? Is it O(1) time and space?
I might add that (a) I should certainly hope so, and (b) I failed to google my way to complexity bounds for Seq module members. If someone know of such, I'd love to have a link to it.
EDIT. I did check the source code before asking. The implementation is here, which will make you look, eventually, at this function, the implementation of which is not immediately accessible to me, and the comments before which puzzles me.
Yes, it is O(1) because it basically just create a new object that encapsulates the two original sequences.
See source code here
Related
What is the rationale behind allowing rebinding of variables in Elixir, when Erlang doesn't allow that?
Most functional languages don't allow rebinding of variables in the same scope. So Elixir allowing this does definitely give it an non-functional, imperative feel. Erlang's problem is rather lack of scope, or to be more precise that there is only one scope in a whole function clause. We did have serious discussions whether to introduce scope but in the end we decided against it as it was backwards incompatible with the existing system. And developers hate backwards inconsistent changes.
The Erlang way has one serious benefit: when you get it wrong you generally get an error so you can SEE the mistake. This compared just getting strange behaviour when a variable doesn't have the value you expect it to have which is MUCH harder to detect and correct.
Personally I think that the problem of new variable names, for example using the number scheme, is greatly overblown. Compared to the time it takes me to work out WHAT I am going to do changing variable names is trivial. And after a while you just see it without reflecting about it. Honestly.
EDIT:
Also when chaining data through a sequence of functions the actual meaning of the data changes so reusing the same variable name can be very misleading. It can end up just meaning a generic "data I am passing from one function to another".
Here's the rationale straight from the horse's mouth:
http://blog.plataformatec.com.br/2016/01/comparing-elixir-and-erlang-variables/
Because it's simpler.
Take a look at this question posted to the Erlang mailing list in 2009. Specifically this part:
I like pattern matching in the majority of cases, but I find I write
enough code where I need to incrementally update a data structure, and
maintaining that code is a pain when I have code like:
X = foo(),
X1 = bar(X),
X2 = xyzzy(X1),
blah(X2).
and later want to change it to:
X = foo(),
X1 = whee(X),
X2 = bar(X1),
X3 = xyzzy(X2),
blah(X3).
Editor's note--this is the reply to that question.
This goes through IRC a lot. This is a result of poor variable naming
practice, and there is no need to introduce rebinding to "fix" it;
just stop using single letters and counters as variable names.
If for example that was written
FooStateX = foo(),
PostBarX = bar(FooStateX),
OnceXyzziedX = xyzzy(PostBarX),
blah(OnceXyzziedX).
The code demonstrated there isn't all that uncommon in Erlang (note the remark "this goes through IRC a lot"). Elixir's ability to simply rebind names saves us from having to generate new dummy names for things all the time. That's all. It's wise to bear in mind that the original creators of Erlang weren't trying to build a functional language. They were simply pragmatic developers who had a problem to solve. Elixir's approach is simple pragmatism.
I need to know the time complexity of the
sortedArrayUsingComparator function of the NSArray class. A source would be great since I'm likely to mention it in my bachelor's thesis.
I'm sorting an array of locations by distance to the current location.
The only answer I could find was someone saying it was at least T(n)=O(n) but likely T(n)=O(n log n)
How would I know for sure?
By actual trial of NSArray sorting the times are in line with O(n*log(n)).
See blog post
Note that in a comment there is a sort method (PS9110) which is O(n) but is proprietary and patented. The method is quite interesting.
Well to sort an array you have to at least look at every element which is for sure O(n). There are several mathematical papers which show you that there can't be a better sorting algorithm then O(n*log(n)) like Mergesort for example. Since the comparator implements a Mergesort I think the complexity should be O(n*log(n)) for best,average and worst case.
You can find some information about Mergesort here:
Mergesort
And some article concerning the best sorting algorithm time complexity:
Sorting algorithms
I couldn't find the exact implementation of the given method but here is a great article how you can dig deeper in the implementation of Arrays in Objective-C and to have a look at the methods implementation:
Exposing NSMutableArray
I.e, if I have a record
-record(one, {frag, left}).
Is record_info(fields, one) going to always return [frag,
left]?
Is tl(tuple_to_list(#one{frag = "Frag", left = "Left"}))
always gonna be ["Frag", "Left"]?
Is this an implementation detail?
Thanks a lot!
The short answer is: yes, as of this writing it will work. The better answer is: it may not work that way in the future, and the nature of the question concerns me.
It's safe to use record_info/2, although relying on the order may be risky and frankly I can't think of a situation where doing so makes sense which implies that you are solving a problem the wrong way. Can you share more details about what exactly you are trying to accomplish so we can help you choose a better method? It could be that simple pattern matching is all you need.
As for the example with tuple_to_list/1, I'll quote from "Erlang Programming" by Cesarini and Thompson:
"... whatever you do, never, ever use the tuple representations of records in your programs. If you do, the authors of this book will disown you and deny any involvement in helping you learn Erlang."
There are several good reasons why, including:
Your code will become brittle - if you later change the number of fields or their order, your code will break.
There is no guarantee that the internal representation of records will continue to work this way in future versions of erlang.
Yes, order is always the same because records represented by tuples for which order is an essential property. Look also on my other answer about records with examples: Syntax Error while accessing a field in a record
Yes, in both cases Erlang will retain the 'original' order. And yes it's implementation as it's not specifically addressed in the function spec or documentation, though it's a pretty safe bet it will stay like that.
Seems like it's inconsistent in the lists module. For example, split has the number as the first argument and the list as the second, but sublists has the list as the first argument and the len as the second argument.
OK, a little history as I remember it and some principles behind my style.
As Christian has said the libraries evolved and tended to get the argument order and feel from the impulses we were getting just then. So for example the reason why element/setelement have the argument order they do is because it matches the arg/3 predicate in Prolog; logical then but not now. Often we would have the thing being worked on first, but unfortunately not always. This is often a good choice as it allows "optional" arguments to be conveniently added to the end; for example string:substr/2/3. Functions with the thing as the last argument were often influenced by functional languages with currying, for example Haskell, where it is very easy to use currying and partial evaluation to build specific functions which can then be applied to the thing. This is very noticeable in the higher order functions in lists.
The only influence we didn't have was from the OO world. :-)
Usually we at least managed to be consistent within a module, but not always. See lists again. We did try to have some consistency, so the argument order in the higher order functions in dict/sets match those of the corresponding functions in lists.
The problem was also aggravated by the fact that we, especially me, had a rather cavalier attitude to libraries. I just did not see them as a selling point for the language, so I wasn't that worried about it. "If you want a library which does something then you just write it" was my motto. This meant that my libraries were structured, just not always with the same structure. :-) That was how many of the initial libraries came about.
This, of course, creates unnecessary confusion and breaks the law of least astonishment, but we have not been able to do anything about it. Any suggestions of revising the modules have always been met with a resounding "no".
My own personal style is a usually structured, though I don't know if it conforms to any written guidelines or standards.
I generally have the thing or things I am working on as the first arguments, or at least very close to the beginning; the order depends on what feels best. If there is a global state which is chained through the whole module, which there usually is, it is placed as the last argument and given a very descriptive name like St0, St1, ... (I belong to the church of short variable names). Arguments which are chained through functions (both input and output) I try to keep the same argument order as return order. This makes it much easier to see the structure of the code. Apart from that I try to group together arguments which belong together. Also, where possible, I try to preserve the same argument order throughout a whole module.
None of this is very revolutionary, but I find if you keep a consistent style then it is one less thing to worry about and it makes your code feel better and generally be more readable. Also I will actually rewrite code if the argument order feels wrong.
A small example which may help:
fubar({f,A0,B0}, Arg2, Ch0, Arg4, St0) ->
{A1,Ch1,St1} = foo(A0, Arg2, Ch0, St0),
{B1,Ch2,St2} = bar(B0, Arg4, Ch1, St1),
Res = baz(A1, B1),
{Res,Ch2,St2}.
Here Ch is a local chained through variable while St is a more global state. Check out the code on github for LFE, especially the compiler, if you want a longer example.
This became much longer than it should have been, sorry.
P.S. I used the word thing instead of object to avoid confusion about what I was talking.
No, there is no consistently-used idiom in the sense that you mean.
However, there are some useful relevant hints that apply especially when you're going to be making deeply recursive calls. For instance, keeping whichever arguments will remain unchanged during tail calls in the same order/position in the argument list allows the virtual machine to make some very nice optimizations.
is there any condition in which merge sort can be done without extra memory
my prof said it has and he will give bonus point on that.
You want to google in place merge sort.
Here is one of the result :
http://thomas.baudel.name/Visualisation/VisuTri/inplacestablesort.html
Use linked list. This will avoid the O(n) extra space needed during Mergeing of 2 lists. However, you cannot do anything about the space taken by recursion calls i.e O(lg(n)).
Yes, the answer to this is to use in-place merge sort
Given that this is a homework question, I can only point you to The Art of Computer Programming. A good programmer should be able to use the standard references in our field to research a question such as this.