Commas in Numbers

From Erlang Community

[edit] 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.

[edit] Solution

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

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