Question

I’m trying to get float (18,16) with padding zeros to the right, but this isn’t working because it always ends with a 1 instead of a zero. My guess it’s because I’ve specified it wrong but I’m not sure how to do it correct.

Sample:
12.12234 must become 12.122340000000000 and 1.01 must become 1.01000000000000.

To achieve this I’m using sprintf('%18.016f', $fMyFloat); but this always ends with a 1 instead of a 0. Obviously I’m overseeing something, I just don’t know what. Any help is greatly appreciated.

Answer 1

This test code:

$data = array(
    12.12234,
    1.01
);
foreach($data as $fMyFloat){
    echo sprintf('%18.016f', $fMyFloat) . PHP_EOL;
}

… prints this in my 64-bit computer:

12.1223399999999994
1.0100000000000000

You are facing the well-known problem of floating point precision. Converting integers from base 10 to base 2 is pretty straightforward (and exact) but base 10 floating point numbers cannot always be represented exactly in base 2. This issue isn’t specific to PHP but there’s a warning in the PHP manual:

Floating point numbers have limited precision. Although it depends on
the system, PHP typically uses the IEEE 754 double precision format,
which will give a maximum relative error due to rounding in the order
of 1.11e-16. Non elementary arithmetic operations may give larger
errors, and, of course, error propagation must be considered when
several operations are compounded.

Additionally, rational numbers that are exactly representable as
floating point numbers in base 10, like 0.1 or 0.7, do not have an
exact representation as floating point numbers in base 2, which is
used internally, no matter the size of the mantissa. Hence, they
cannot be converted into their internal binary counterparts without a
small loss of precision

Your only chance to achieve such precision is to manipulate numbers as strings. PHP bundles two arbitrary precision libraries that can help you when you need to do match with strings: GNU Multiple Precision and BC Math. Here’s a little hack with bcmath:

$data = array(
    '12.12234',
    '1.01'
);
bcscale(16);
foreach($data as $fMyFloat){
    echo bcdiv($fMyFloat, 1) . PHP_EOL;
}

It this case, though, you can probably use simple string functions:

$data = array(
    '12.12234',
    '1.01'
);
foreach($data as $fMyFloat){
    list($a, $b) = explode('.', $fMyFloat);
    echo $a . '.' . str_pad($b, 16, 0) . PHP_EOL;
}