The stupid question is the question not asked PerlMonks

### Re: Re: Using (s)printf()

by Hofmator (Curate)
 on Aug 31, 2001 at 16:20 UTC ( #109363=note: print w/replies, xml ) Need Help??

in reply to Re: Using (s)printf()

• The first one (9.3333 -> 9.34) is a typo in the original.
• The second and third one are easily explained by the following
```printf("%.15f", 9.55);   # prints 9.550000000000001
printf("%.15f", 43.55);   # prints 43.549999999999997
This means that due to the limitations of the internal representation, the literal 9.55 is in fact 9.550000000000001 which rounds to 9.56. The same goes for 43.55 but there the representation is slightly smaller than 43.55 so a rounding down occurs.

-- Hofmator

Replies are listed 'Best First'.
Re: Re: Re: Using (s)printf()
by Anonymous Monk on Sep 01, 2001 at 07:29 UTC
Actually, this isn't quite the whole story. Most everyone follows the IEEE convention of "round towards nearest or even."
Examples (rounding all of these to the one's place):

2.51 becomes 3 (this is the 'nearest' rule, which always comes first)
2.49 becomes 2 (again, 'nearest')

However, what happens if you have 2.50 ? Which way do you round it... 'tis no nearer to 2 than to 3. The IEEE standard says if there is a tie, round to the even number.

2.50 becomes 2
3.50 becomes 4

You have to pick up or down... this method is consistent and thus tends to make your errors (statistically) smaller.

Actually, this isn't quite the whole story. Most everyone follows the IEEE convention of "round towards nearest or even."
I'm aware of that - but it wasn't applicable in this case. Btw, (s)printf doesn't seem to get it right:
```printf "%.15f", 0.5;  # prints 0.500000000000000
printf "%.0f",  0.5;  # prints 1 and should print 0
and 0.5 can be represented exactly ...

-- Hofmator

Re: Re: Re: Using (s)printf()
by Zteven (Initiate) on Sep 02, 2001 at 15:08 UTC
Thank you for your explanation. How do I circumvent this problem? I have a little perl-program that pulls these numbers from a file and it has to round them correctly. Isn't there some way to store a number exactly?

What if you want to chop a number but not round it? printf just *wants* to round, no option to tell it not to :(

I needed something that would chop the extra digits (price calculataions) without rounding.

quick fox was to compare the sprintf result with the original and if it was larger remove 0.01 - yuk!...

I ended up using split and substr... - not pretty either

There must be a better way!

Create A New User
Node Status?
node history
Node Type: note [id://109363]
help
Chatterbox?
 [Corion]: Yay. Traditional finance situation averted. Bonds can be quoted in amounts (1_000_000 EUR) or per unit (1 unit). And a traditional error is to trade 2_000_000 piece when you meant to trade 2_000_000 EUR. [Corion]: (one of my scripts simply catches high amounts and I phone people making that trade, ideally before the payment is due) [Corion]: The sad thing is that my script sits at the end of the pipeline and can only look at the payments due today or tomorrow basically, while there are many more systems further up in the pipeline [GotToBTru]: better late than never, I guess [Corion]: GotToBTru: Sure - there is a long and sad story of many frantic cleanups that led us to implement this notification ;)

How do I use this? | Other CB clients
Other Users?
Others lurking in the Monastery: (10)
As of 2017-03-29 11:32 GMT
Sections?
Information?
Find Nodes?
Leftovers?
Voting Booth?
Should Pluto Get Its Planethood Back?

Results (347 votes). Check out past polls.