Strange behaviour from dechex

Anon

Am I going crazy?

I have a function wherein I have the following statements:

echo "resid: $resid ";
$resid = dechex($resid);
echo "resid: $resid ";

And the output returned is:
resid: 7784
resid: 1e67

Both my calculator and I agree that 7784 = 0x1e68 (and not 0x1e67).

So, as reality check I wrote a little php script and as follows:
<?
echo dechex(7784);
echo "\n";
?>

The output was:

1e68

Am I missing something obvious? Am I going insane? Is PHP going insane? Can anyone help me?

Thank you,
Erica

Anon

I ran this test script:

<?
$resid = 7784;
echo "resid: $resid ";
$resid = dechex($resid);
echo "resid: $resid ";
?>

and the output I got was

resid: 7784 resid: 1e68

which is right...it's not 1e67 as you've described.

Diego

Anon

That was my point. I cannot duplicate the error outside of that one script.
My concern is that there is something wrong with the $resid variable, but I tried casting it to an integer with (int) and that didn't change the output at all.

Now the script portion is:
echo "resid: $resid ";
$resid = dechex((int)$resid);
echo "resid: resid ";

With output:
resid: 7784
resid: 1e67

Anon

Where does $resid come from? A database? form input? Can you show us the code where you initialize $resid?

Diego

Anon

This is one way I was able to reproduce your output:

<?
$resid = 7783.99999999999;
echo "resid: $resid ";
$resid = dechex($resid);
echo "resid: $resid ";
?>

Therefore, you'll have to round() $resid before you dechex() it.

This is not a PHP bug or anything...it's a limitation of floating point numbers.

Diego

Anon

Thanks! I got there too. It turns out that $resid is behaving like a

Unfortunately, for various reasons, I can't paste the whole script, but you will get a similar error if you do something like this:

$x = floor(3.00);
$resid = pow(2, $x);
echo dechex($resid);

Even though $x is a whole number and the result of 2^$x is a whole number, and the documentation states of pow() "If possible, this function will return an integer," I still get an incorrect value.
I understand that pow() is probably returning a float because $x is a float, but php is supposedly a dynamically typed language and should be able to do the float -> integer conversion with no problem when passing $resid into dechex. Even when you try (int)$resid, it gives you 7 and not 8, which doesn't make a whole lot of sense.

Hopefully, this illustrates my point with fewer words:
$x = 3.01;
echo "x: $x\n";
$x = floor($x);
echo "x: $x\n";
$resid = pow(2, $x);
echo "resid: $resid\n";
echo "(int)resid: ";
echo (int)$resid;
echo "\n";
echo "Result: ";
echo dechex((int)$resid);
echo "\n";