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String concatenation

February 25th, 2009
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As we all know, Erlang treats String as the array of integers, hence operations on list are applicable on String. 

String concatenation can be done in different ways in Erlang

1> X="I love ".
"I love "
2> Y="Erlang".
"Erlang"
3> string:concat(X, Y).
"I love Erlang"
4> string:join([X, Y], "").
"I love Erlang"
5> X ++ Y.
"I love Erlang"
6> lists:append(X, Y).
"I love Erlang"
7> lists:concat([X, Y]).
"I love Erlang"

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  1. Fabian Alenius
    May 8th, 2009 at 07:03 | #1
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    You could also do X ++ Y, I’m not sure if it’s just syntactic sugar for lists:concat(…) or not but I suspect it might be.

  2. Fabian Alenius
    May 8th, 2009 at 07:09 | #2
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    Ops, I’m blind. Seems you already have an example of ++.

  3. trung
    May 10th, 2009 at 07:15 | #3
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    Thanks, Fabian.
    I’ve looked at the source code for lists.erl, your’re right, lists:concat/1 uses ++ for concatenation:

    concat(List) ->
    flatmap(fun thing_to_list/1, List).

    thing_to_list(X) when is_integer(X) -> integer_to_list(X);
    thing_to_list(X) when is_float(X) -> float_to_list(X);
    thing_to_list(X) when is_atom(X) -> atom_to_list(X);
    thing_to_list(X) when is_list(X) -> X. %Assumed to be a string

    flatmap(F, [Hd|Tail]) ->
    F(Hd) ++ flatmap(F, Tail);

  4. Denny Abraham
    July 15th, 2009 at 18:57 | #4
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    Hi Trung,

    According to Programming Erlang, the source for the lists module is only an illustrative implementation. Use of ++ is discouraged for all but trivially small lists.

    -Denny

  5. trung
    July 16th, 2009 at 06:48 | #5
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    Thanks Denny for the comment.
    Hmm, I though initially that ++ is quite native, why they used it in lists:concat/1 implementation then?

  6. Ben Xu
    August 21st, 2012 at 23:00 | #6
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    @Denny Abraham
    I was also confused about “++” vs. “lists:concat”, then I write a simple test, it seems “lists:concat” was obviously faster. I think Denny is right, the BIFs may not be implemented in Erlang indeed.
    The “string:concat/2″ seems not very useful compared to “lists:concat/1″, since it only takes two arguments.
    Hope the author will make a comparison to these functions ;)

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