The mystery of 9.95

JdS · #1 23-Oct-2006, 11:03

I have this script that will accept a price off a web form. Since it's a price, I prefer to save the information to the database as an integer, in other words, in cents.

For some strange, unknown reason, when I test the script and submit a price like 9.95, the script will end up with 994 cents!

Here's an example script you can run yourself:

PHP Code:


<?php
if( isset($_POST['price'])  &&  $_POST['price'] )
{
    $price['dollars'] = floatval( trim($_POST['price']) );
    $price['cents'] = intval( $price['dollars']*100 );
    printf( "<p>\$%s is %d cents.</p>", $price['dollars'], $price['cents'] );
    $form_price_value = &$price['dollars'];
}
else
{
    $form_price_value = null;
}
echo     "<form method=\"post\" action=\"$_SERVER[PHP_SELF]\">"
        .     "<p>Price: <input type=\"text\" name=\"price\" value=\"$form_price_value\" /></p>"
        .     "<p><input type=\"submit\" name=\"submit\" value=\"How much is that in cents?\" /></p>"
        . "</form>"
?>

If you submit 9.95 or 8.95, the results will be 994 and 894 respectively. However anything else will be fine, e.g. 6.95, 7.95, 3.95 etc.

I thought it was a Windows issue... but I find the same result(s) testing the example script above, on this web server (Linux).

It can't be the browsers either. Same results on both, FF and IE.

Anybody with a suggestion?

TurboPT · #2 23-Oct-2006, 11:47

What about just a straight:

PHP Code:


$someVar = $_POST['price'] * 100;

It was also interesting that your example for 19.99 returned 19.98, but the above snippet gives 1999. (and 995 for the original amount)

LuciWiz · #3 23-Oct-2006, 11:57

The problem has to do with floating point precision; see the manual page for PHP discussing this very point. I am sure there is an infinite number of numbers exhibiting this problem, and floating point precision is a problem that plagues any computer programming language with finite representation of floating point numbers - just about everyone there is, considering that the limitation is hardware-related; software developers are perfect. I think there are several implementations in the ML family of PL's that allow for a different handling, and I seem to remember Ruby had something like this, but I imagine you don't care about this

Can you consider acquiring the input data into a string and then performing a simple parsing to determine the number in the "integer" form?

Best regards,
Lucian

JdS · #4 23-Oct-2006, 12:06

TurboPT,

Thank you for the suggestion but I didn't notice a change. Just in case I misunderstood you, here's what I edited in the example script.

PHP Code:


<?php
if( isset($_POST['price'])  &&  $_POST['price'] )
{
    $price['dollars'] = floatval( trim($_POST['price']) );
    //$price['cents'] = intval( $price['dollars']*100.0 );
    $price['cents'] = intval( $_POST['price']*100 );
    printf( "<p>\$%s is %d cents.</p>", $price['dollars'], $price['cents'] );
    $form_price_value = &$price['dollars'];
}
else
{
    $form_price_value = null;
}
echo     "<form method=\"post\" action=\"$_SERVER[PHP_SELF]\">"
        .     "<p>Price: <input type=\"text\" name=\"price\" value=\"$form_price_value\" /></p>"
        .     "<p><input type=\"submit\" name=\"submit\" value=\"How much is that in cents?\" /></p>"
        . "</form>"
?>

Lucian,

I read that page actually, but I was just annoyed because I couldn't understand why it was happening randomly. Why would 9.95 turn into 994 but 6.95 works out fine into 695!

Can I just add 0.000001 to the value? You know, as a quick fix?

LuciWiz · #5 23-Oct-2006, 12:19

Quote:

Originally Posted by JdS

Lucian,

I read that page actually, but I was just annoyed because I couldn't understand why it was happening randomly. Why would 9.95 turn into 994 but 6.95 works out fine into 695!

Can I just add 0.000001 to the value? You know, as a quick fix?

No, that wouldn't really be a fix, since you would still perform operations on floating point numbers, and that is prone to rounding errors. I would recommend reading What Every Computer Scientist Should Know About Floating-Point Arithmetic, but I understand you are in a real hurry to solve the problem. I am also sorry for throwing links at you, but I feel like I am really not qualified to explain those things myself, as I find them to be hardly straight-forward. I can tell you that some time ago Intel actually had to recall an entire series of processors which displayed gross rounding errors for floating point numbers, but otherwise they are accepted as a limitation of the current hardware, which only works with base 2 numbers.

Long story short, wouldn't it be easier to just accept the data as a string, e.g:

"12.345"

and parse the string as:

"12"
"345"

than make

"12345"

out of it and convert that to an integer?

You know I don't program in Php

JdS · #6 23-Oct-2006, 12:20

Brilliant! Thank you Lucian. Simple, and does the job!

TurboPT · #7 23-Oct-2006, 14:02

JdS,
I was thinking more along the lines of: [without using intval()]

PHP Code:


$price['cents'] = $_POST['price']*100;

That's all, but glad you got it working!

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