[Erlang Programming Book] Exercise 3-5 reversing

[vinu]

I tried this problem but it seems that I am missing something
fundamental, I rad the text again and could not find anything
consider this

reverse([]) ->
[];
reverse([H|T]) ->
[reverse(T)|H].


it takes a list and reverse it but for following
1> reverse([1,2,3])
prints
[[[[]|3]|2]|1]

I wonder why | did-not get processed.

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[Ale]

2010/4/20 vinu <vinu...@gmail.com>:

> I tried this problem but it seems that I am missing something
> fundamental, I rad the text again and could not find  anything
> consider this
>
> reverse([]) ->
>  [];
> reverse([H|T]) ->
>  [reverse(T)|H].
>
>
> it takes a list and reverse it but for following
> 1> reverse([1,2,3])
> prints
> [[[[]|3]|2]|1]
>
> I wonder why | did-not get processed.

Check you recursive case: [reverse(T)|H]. What does that mean?

Construct a list, with last element H and insert the result of
reverse(T) at the beginning. The tail of the list is [H], while the
head is reverse(T).


One way to do it is like this:

reverse([]) -> [];
reverse([H|T]) -> reverse(T) ++ [H].

The other would be using accumulators like many have shown you before.

>
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> http://www.erlangprogramming.org/
>
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Ale.

[vinu]

I did it your way of course and eventually found out my first solution
did not work because erlang doesnot usually flatten the list
automatically.

On Apr 20, 10:10 pm, Ale <peralta.alejan...@gmail.com> wrote:
> 2010/4/20 vinu <vinu76...@gmail.com>:

[Rakesh Kumar]

reverse([]) -> [];
reverse([H|T]) -> reverse(T) ++ [H].

[Hynek Vychodil]

Your solution has O(n^2) complexity which is not good solution. It is why they stated there should be solved with auxiliary function:

reverse(L) -> reverse(L, []).

reverse([], L) -> L;

reverse([H|T], L) -> reverse(T, [H|L]).

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[Francesco Cesarini]

There is a side note in the book which explains why you should avoid List ++ [Element], especially in recursive calls. With the current implementation of the VM, to append Element to the end of the list, you need to traverse every element in List. Doing this for every iteration in your recursive call with have huge impact on performance.

F