How to separate numbers in three-digit groups in C

[Dave Eisen]

In article <12192 at hub.ucsb.edu> angst at cs.ucsb.edu (Hopelessly in love w/Donna Reed) writes:
>This function will work.  It assumes the number is contained in a string
>(which you can do by using sprintf()) --
>

No it won't. It bombs on -123 for example.