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From: scs@adam.pika.mit.edu (Steve Summit)
Newsgroups: comp.lang.c
Subject: Re: Help needed to round-off a number
Message-ID: <11832@bloom-beacon.MIT.EDU>
Date: 4 Jun 89 18:39:04 GMT
References: <2666@oregon.uoregon.edu> <267@ibd.BRL.MIL>
Sender: daemon@bloom-beacon.MIT.EDU
Reply-To: scs@adam.pika.mit.edu (Steve Summit)
Lines: 18

In article <267@ibd.BRL.MIL> heilpern@brl.arpa (Mark A. Heilpern (IBD) <heilpern>) writes:
>There is a relatively painless way to [truncate or round-off
>a number a certain decimal place]:
>2) multiply your original by 10^n
>2a) add (5 / 10^n) for round-up error
>3) truncate the fraction off of your answer
>4) divide this result by 10^n

If you have normalized the intermediate result by 10^n (in step 2)
then you round by adding one half, not 5/10^n.

Note that the method lets you round to a multiple of anything,
not necessarily limited to powers of ten.  (You may want to
subtract a half and/or use ceil() when rounding negative numbers,
though.)

                                            Steve Summit
                                            scs@adam.pika.mit.edu