Rounding error

graham

I have a page which calculates website page hit frequencies for the whole site and gives a running total at the bottom.

The total at the bottom does not always equal 100% as it should be ( I get 99.99% etc). I use number_format() set to two decimal places throughout the calculations. Any suggestions to get 100% accuracy please?

best, Graham

mrmufin

From the PHP manual using the round() function:

This is normal and reflects how computers store floating point numbers. 11.5 is not actually 11.5. It is 11.49999999999999999 or 11.50000000000000000001. Floating point numbers are approximated using some level of precision. Think of 1/3. How would you write that? 0.3333333333... when do you stop? How many 3's do you need to accurately write 1/3? To be completely accurate you need an infinite amount of 3's and that just isn't feasible, so at some point you just have to say that you have enough precision.

graham

I understand this. However, the page in question is for an accountant's website (ironic) and as it doesn't quite add up the start-asses in their company are all over mine like a rash. I was hoping to nail the sucker.

best, Graham

diego25

Well, after calculating the percentages (and before outputting them), use an if statement to see if their sum is 100. If it's not, add the difference to the highest percentage.

Something like:

$percent1 = 50;
$percent2 = 49.9;

if ($percent1 + $percent2 != 100) {
$percent1 += 100 - ($percent1 + $percent2);
}

Of course, my code is really simplified in that I didn't find the largest percentage, but I'm sure you can do that...

Diego