String join with

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Revision as of 04:30, 25 July 2007 (edit)
Khigia (Talk | contribs)
(New page: == Problem == You need to combine several items of a list into a string using a separator between each item representation. In python, this is done by: <code caption="Python example"> pyt...)
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Goryachev (Talk | contribs)
(added alternative solution code)
 
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erl> test:string_join(",", [1, 2, 3], fun(X) -> io_lib:format("~B", [X]) end). erl> test:string_join(",", [1, 2, 3], fun(X) -> io_lib:format("~B", [X]) end).
"1,2,3" "1,2,3"
 +</code>
 +
 +== Solution (alternative) ==
 +There is another code which is simplier to read and works faster for me (goryachev):
 +<code caption="Erlang code">
 +my_string_join(Items, Sep) ->
 + lists:flatten(lists:reverse(my_string_join1(Items, Sep, []))).
 +
 +my_string_join1([Head | []], _Sep, Acc) ->
 + [Head | Acc];
 +my_string_join1([Head | Tail], Sep, Acc) ->
 + my_string_join1(Tail, Sep, [Sep, Head | Acc]).
 +</code>
 +
 +Usage:
 +<code caption="Erlang code">
 +erl> string_join:my_string_join(["123", "456", "789"], $,).
 +"123,456,789"
 +</code>
 +
 +
 +And here is a benchmark:
 +<code caption="Erlang code">
 +erl> Items = ["bar foo" || _ <- lists:seq(1, 100000)], ok.
 +ok
 +erl> {Time, _} = timer:tc(string_join, string_join, [",", Items]), Time.
 +163565
 +erl> {MyTime, _} = timer:tc(string_join, my_string_join, [Items, $,]), MyTime.
 +87379
</code> </code>
[[Category:CookBook]][[Category:StringRecipes]] [[Category:CookBook]][[Category:StringRecipes]]

Current revision

[edit] Problem

You need to combine several items of a list into a string using a separator between each item representation.

In python, this is done by:

Python example

python> ','.join(['a', 'b', 'c'])
'a,b,c'
python> ','.join(map(str, [1,2,3]))
'1,2,3'

[edit] Solution

This solution interleave the input list with as many copy of the separator, then use the builtin concat function to create the final string:

Erlang code

string_join(Join, L) ->
    string_join(Join, L, fun(E) -> E end).

string_join(_Join, L=[], _Conv) ->
    L;
string_join(Join, [H|Q], Conv) ->
    lists:flatten(lists:concat(
        [Conv(H)|lists:map(fun(E) -> [Join, Conv(E)] end, Q)]
    )).

Usage is as follow:

Erlang example

erl> test:string_join(",", ["1", "2", "3"]).
"1,2,3"
erl> test:string_join(",", [1, 2, 3], fun(X) -> io_lib:format("~B", [X]) end).
"1,2,3"

[edit] Solution (alternative)

There is another code which is simplier to read and works faster for me (goryachev):

Erlang code

my_string_join(Items, Sep) ->
    lists:flatten(lists:reverse(my_string_join1(Items, Sep, []))).

my_string_join1([Head | []], _Sep, Acc) ->
    [Head | Acc];
my_string_join1([Head | Tail], Sep, Acc) ->
    my_string_join1(Tail, Sep, [Sep, Head | Acc]).

Usage:

Erlang code

erl> string_join:my_string_join(["123", "456", "789"], $,).                 
"123,456,789"


And here is a benchmark:

Erlang code

erl> Items = ["bar foo" || _ <- lists:seq(1, 100000)], ok.
ok
erl> {Time, _} = timer:tc(string_join, string_join, [",", Items]), Time.
163565
erl> {MyTime, _} = timer:tc(string_join, my_string_join, [Items, $,]), MyTime.
87379
Retrieved from "http://www.trapexit.org/String_join_with"
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