[erlang-questions] simple question: how to convert integers to string with fixed digits

Håkan Stenholm <>
Sat Dec 1 19:46:42 CET 2007


Matej Kosik wrote:
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> Håkan Stenholm wrote:
>> Matej Kosik wrote:
>>
>> You can experiment with the io:format(...) (or io_lib:format(...))
>> string, this works in you case:
>>
>> 6> io:format("~2..0w", [2]).
>> 02
>>
>> 10> io:format("~2.16.0B", [255]).
>> FF
>>
>
> These procedures work fine. Reading the manual I was not able to recognize such possibilities. They
> print the term on the terminal in the way I need it.
>
> These parameters also kind be used with
>
> 	io_lib:fwrite/2
>
> even though its results, for example in these cases:
>
> 	io_lib:fwrite("~2.16.0B", [12]).
>
> is weird
>
> 	[["0","C"]]
>
> I would expect:
>
> 	["0C"]
>
> Nonetheless the weird returned object seems to behave as the expected string.
io_lib.erl has tendency to create strings similar to io_lists (io_list() 
= [char() | binary() | io_list()]), there are efficiency reason  for 
doing it like this, e.g. appending two strings like ["foo" | "bar"] 
during pretty printing formating is O(1) rather than O(N) which "foo" ++ 
"bar" - which could in some circumstances result in O(N^2) issues.
 
You might need to use lists:flatten/1 (O(N log N)) if you need a proper 
string() - [char()], rather than a nested list of chars and lists.

One reason why io_lib doesn't apply lists:flatten/1 on it's output is 
another efficiency consideration - when sending byte streams over 
networks or to disk, it is quite simple to traverse the nested (tree) 
structure in O(N log N) time* thereby minimizing the amount of 
operations needed to process the strings**, this is useful when the data 
is mainly intended to be passed on to other processes/servers/users.


*  functions/modules that accept io_list() will do this as well
   as a number of other io related functions
** you save a O(N) traversal over the flattend string
>
> Thank you
> - --
> Matej Kosik
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