PHP rounding error

Question

I'm using PHP 5.2.13 on my linux server. I'm getting weird error when rounding numbers. This is my test case:

<?php
echo "        " . round(1.505, 2) . "
";
echo "       " . round(11.505, 2) . "
";
echo "      " . round(111.505, 2) . "
";
echo "     " . round(1111.505, 2) . "
";
echo "    " . round(11111.505, 2) . "
";
echo "   " . round(111111.505, 2) . "
";
echo "  " . round(1111111.505, 2) . "
";
echo " " . round(11111111.505, 2) . "
";
echo "" . round(111111111.505, 2) . "
";

This is results:

        1.51
       11.51
      111.51
     1111.51
    11111.51
   111111.51
  1111111.5
 11111111.51
111111111.51

Anyone knows what causes this? I can't update PHP, since it's shared server.

Answer 1

This is because the number 1111111.505 can't be represented exactly in floating point notation. The closest it can get is 1111111.5049999999. So what ends up happening is that it converts the number in your code to 1111111.50499999999 and then it does the rounding. Which results in 1111111.5. Floating point numbers have problems in that they can't represent a lot of even seemingly simple decimal numbers with complete accuracy. For instance. the number 0.1 cannot be accurately represented using binary floating numbers. Using Python, if you type in 0.1, it returns 0.10000000000001. Plus or minus a few zeros. This is the reason that some languages such as .Net provide a "Decimal" data type which is able to represent all decimal values within a certain range and number of decimal places. The downside of the decimal data type is that it is slower, and each number takes more space to store.