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Date: Tue, 7 Apr 2009 15:34:40 -0400
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Subject: Re: [PHP-DEV] [PATCH] double to long conversion change
From: danielc@analysisandsolutions.com (Daniel Convissor)

Hi Matt, and everyone:

On Mon, Apr 06, 2009 at 01:00:47PM -0500, Matt Wilmas wrote:

> unless I'm missing something,  
> you're talking about converting long/int to double/float.  That's the  
> opposite of this thread subject, which is how to convert a double to a 
> long when it's out of the range of a long. :-)

It's a two way street.  If the floats don't have enough precision, things 
get jumbled when converting the floats back to integers.


> But, for what you're testing, that's the behavior I'd expect -- once 
> you've reached the precision of a double, you'll only get the closest  
> representation possible (and of course a 64-bit long is more precise than 
> a double since there's no floating point to represent). Also, I assume 
> what can be represented by a double is the same across platforms, if it's 
> IEEE 754.

Yes.  But I was expecting that since long on 64-bit machines holds 64 
bits in PHP (et al), that PHP would use C's long double type for floats 
on 64-bit platforms rather than plain old doubles.  It seems like the 
user-friendly, PHP way to handle the situation, particularly as 64-bit 
computers are commonplace these days.


> Just curious though, you're saying that all whole numbers (from long) 
> below 2^53 are representable? (Powers of 2 should always be OK.)  When 
> writing a big literal number on a 32-bit system, I'm seeing much lower 
> than that (around 2^40)

I'm talking about 64-bit machines.


> Like I said, I figure a double type should 
> behave the same everywhere.  Unless the shell/bash uses a *long* double 
> type (twice as big as a regular double)? *shrug*

Exactly.

The test scripts in question are now available for download from
http://www.analysisandsolutions.com/php/intfloat/

Thanks,

--Dan

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