Re^3: sprintf rounding convention

[kennethk]

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

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

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

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.

[Joost]

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.

[ikegami]

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.