At 2:55 PM -0500 1/11/10, Rick Dwyer wrote:
I have been asked to further modify the value to the nearest half cent.

So if the 3rd decimal spot ends in 1 or 2, it gets rounded down to 0
If it ends in 3, 4, 5, 6 it gets rounded to 5. And if it 7, 8 or 9 it gets rounded up to full cents.

Can this be done fairly easily? Not knowing PHP well, I am not aware of the logic to configure this accordingly.



The above described rounding algorithm introduces more bias than simply using PHP's round() function, which always rounds down. IMO, modifying rounding is not worth the effort.

The "best" rounding algorithm is to look at the last digit and do this:

0 -- no rounding needed.
1-4 round down.
6-9 round up.

In the case of 5, then look to the number that precedes it -- if it is even, then round up and if it is odd, then round down -- or vise versa, it doesn't make any difference as long as you are consistent.

Here are some examples:

122.4  <-- round down (122)
122.6 <-- round up (123)
122.5 <-- round up (123)

123.4  <-- round down (123)
123.6 <-- round up (124)
123.5 <-- round down (123)

There are people who claim that there's no difference, or are at odds with this method, but they simply have not investigated the problem sufficiently to see the bias that rounding up/down causes. However, that difference is very insignificant and can only be seen after tens of thousands iterations. PHP's rounding function is quite sufficient.




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