Erlang Central

Commas in Numbers

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Problem

You need to format a number for printing with commas (or other locale-dependant marks) in the appropriate places. This is important for creating human-legible print copies of reports.

Solution

Turn the number into a list and then use pattern-matching to insert commas where needed.


-module(number).
-export([partition_integer/2, partition_float/4]).

partition_integer(N,P) ->
    partition_integer(lists:reverse(integer_to_list(N)),P,[]).

partition_integer([A,B,C,D|T],P,Acc) ->
    partition_integer([D|T],P,[P,C,B,A|Acc]);
partition_integer(L,_,Acc) ->
    lists:reverse(L) ++ Acc.

Floating-point numbers generally require additional separator, and also an explicit fractional-part length:


partition_float(N,P,FP,L) ->
    F = tl(tl(hd(io_lib:format("~.*f",[L,N-trunc(N)])))),
    lists:flatten([partition_integer(trunc(N),P),FP,F]).

Here's an example of the above in action:


1> number:partition_integer(1918928282, $,).
"1,918,928,282"
2> number:partition_integer(1982928828, $'). 
"1'982'928'828"
3> number:partition_float(1982928828.12345, $', $., 6).
"1'982'928'828.123450"