Why The Difference In Prepend vs. Append?

[Onorio Catenacci]

Hi all,

Just curious about something I spotted recently:

iex(1)> L1 = [1,2,3]

[1, 2, 3]

iex(2)> L2 = [4|L1]

[4, 1, 2, 3]                #Prepend behaves as expected

iex(3)> L3 = [L1|4]      

[[1, 2, 3] | 4]             #Append behaves differently.

Is this due to the nature of a linked list (that is O(N) complexity for an append vs. O(1) for a prepend)?  Not that I have a need to append to a list but I am curious about the difference in the behavior.

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Onorio

[Onorio Catenacci]

By the way in my original code I was using lowercase "L"; I changed the case of the variables after I copy/pasted them into my message to make it easier to distinguish between 1 and L here.  Of course if you try that code at an iex prompt it won't work. Obviously not: an uppercase L is an atom (like a module name).  Sorry for my carelessness. As I say, I was just trying to avoid confusion between lowercase L and 1. 

If you use lowercase var names the sample code behaves as I mentioned. Just wanted to save anyone telling me that, of course, L<n> won't work for binding a list to a variable.

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Onorio

[Peter Hamilton]

That is not append, it is prepend in both cases.

[a|b] is shorthand for prepend. Erlang (and many lisps) do not b to be a list. The result of prepending an element to an element is called an improper list. Most list manipulating functions assume a proper list and will break with an improper list, but the language itself does t prevent you from creating them anyway.

In the second case, we are prepending an element (a list) to another element (a single value), creating an improper list.

[Arie van Wingerden]

Hi Onorio,

when you do [ 4 | lst ] variable lst is a list and 4 is added to the front of that list.

When you do [ lst | 4 ] you create a so called "improper list", because 4 is NOT a list and thus you cannot add something to it's front!

General rule (in Erlang and Elixir): avoid improper lists. So prepend always to an existing LIST.

In Lisp and derivatives sometimes improper lists have a good use as pairs. This is not the case for Erlang & Elixir.

Regards,

   Arie

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[Lars Storjord]

Just a small side note: Prepending to a list like that is often called 'cons'ing, after the cons function in Lisp. That is handy to know in case you read more about it in other contexts online.

[Onorio Catenacci]

Thanks Peter, Arie and Lars for your responses.

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Onorio