Re^3: sprintf rounding convention

in reply to Re^2: sprintf rounding convention
in thread sprintf rounding convention

Now I'm confused:

$ perl -v

This is perl, v5.8.8 built for x86_64-linux-gnu-thread-multi

$ perl -e 'printf "%.2f\n%.2f\n", 0.335, 1.335;'
0.34
1.33
[download]

and


C:\>perl -v

This is perl, v5.8.9 built for MSWin32-x86-multi-thread
(with 9 registered patches, see perl -V for more detail)

C:\>perl -e "printf qq{%.2f\n%.2f\n}, 0.335, 1.335;"
0.34
1.34
[download]

Am I missing something (as usual)?

Update:

Following ikegami's suggestion, I tried increasing the precision to 40 digits, and got:

$ perl -v

This is perl, v5.8.8 built for x86_64-linux-gnu-thread-multi

$ perl -e 'printf "%.40f\n%.40f\n", 0.335, 1.335;'
0.3350000000000000199840144432528177276254
1.3349999999999999644728632119949907064438

C:\>perl -v

This is perl, v5.8.9 built for MSWin32-x86-multi-thread
(with 9 registered patches, see perl -V for more detail)

C:\>perl -e "printf qq{%.40f\n%.40f\n}, 0.335, 1.335;"
0.3350000000000000200000000000000000000000
1.3350000000000000000000000000000000000000
[download]

It looks like this variation is just an artifact of 64-bit vs. 32-bit, emphasizing how you should never rely on precise results from rounding.

Comment on Re^3: sprintf rounding convention Select or Download Code

Replies are listed 'Best First'.
Re^4: sprintf rounding convention by Joost (Canon) on Oct 30, 2009 at 23:14 UTC
I think the main problem is that neither of those two numbers can be accurately represented by binary (IEEE) floating points. For what it's worth, I get the same results as your linux test on my intel macbook, v5.8.9 built for darwin-2level and v5.8.8 built for darwin-thread-multi-2level. "What should it profit a man, if he should win a flame war, yet lose his cool?"	[reply]
Re^4: sprintf rounding convention by ikegami (Patriarch) on Oct 31, 2009 at 01:48 UTC
I bet `perl -e'printf "%.40e\n", 0.335'` gives 0.334999... on the system that rounded to 1.33. Let me know if not.	[reply] [d/l]

In Section Seekers of Perl Wisdom