First question: I just noticed that writefln("%a", 1.2) writes 0x1.3333333333333p+0, while writeln(format("%a", 1.2)) (that is, with std.string.format) writes 0x9.9999999999998p-3 ... wouldn't it be nice to be consistent here? (The former is what printf in gcc gives.) Or am I missing a difference in functionality?

Second question: Just to make sure, this *is* an exact representation of the underlying floating-point number? (I.e., if that'w what I'm after, using %a *is* the way to go?)

-- 
Magnus Lie Hetland
http://hetland.org

On Wed, 02 Mar 2011 13:35:11 +0100, Magnus Lie Hetland wrote:

> First question: I just noticed that writefln("%a", 1.2) writes
> 0x1.3333333333333p+0, while writeln(format("%a", 1.2)) (that is, with
> std.string.format) writes 0x9.9999999999998p-3 ... wouldn't it be nice
> to be consistent here? (The former is what printf in gcc gives.) Or am I
> missing a difference in functionality?

Hm, that's weird.  I'm pretty sure writefln() is doing the right thing here, since that's what printf() does.  I've had a look at the code for format(), and it looks to me like it is using some old formatting code that is being phased out.  I've created a bug report for this, and will look into fixing it shortly:

  http://d.puremagic.com/issues/show_bug.cgi?id=5687


> Second question: Just to make sure, this *is* an exact representation of the underlying floating-point number? (I.e., if that'w what I'm after, using %a *is* the way to go?)

Yes, that's right.

-Lars

"Lars T. Kyllingstad" <public@kyllingen.NOSPAMnet> wrote in message news:iknfsh$13ga$1@digitalmars.com...
> On Wed, 02 Mar 2011 13:35:11 +0100, Magnus Lie Hetland wrote:
>
>> First question: I just noticed that writefln("%a", 1.2) writes
>> 0x1.3333333333333p+0, while writeln(format("%a", 1.2)) (that is, with
>> std.string.format) writes 0x9.9999999999998p-3 ... wouldn't it be nice
>> to be consistent here? (The former is what printf in gcc gives.) Or am I
>> missing a difference in functionality?
>
> Hm, that's weird.  I'm pretty sure writefln() is doing the right thing
> here, since that's what printf() does.  I've had a look at the code for
> format(), and it looks to me like it is using some old formatting code
> that is being phased out.
>

Yea, definitely seems to be the case. Unless it's been fixed recently, format doesn't even support the posix-style positional argument specifiers that were added to writef ages ago (I know I filed a ticket on this, but I don't recall the number offhand).

>> Second question: Just to make sure, this *is* an exact representation of the underlying floating-point number? (I.e., if that'w what I'm after, using %a *is* the way to go?)
>
> Yes, that's right.
>

I'm no floating-point expert, but I would think that the only way to get an exact representation would be to output the raw data in hex (or binary, or octal, etc):

writef("0x%X", cast(ulong)1.2);

On 2011-03-04 03:16:50 +0100, Nick Sabalausky said:

> I'm no floating-point expert, but I would think that the only way to get an exact representation would be to output the raw data in hex (or binary, or octal, etc):
> 
> writef("0x%X", cast(ulong)1.2);

That's also an option, certainly. Then I could just write it as a decimal integer, I guess.

But the point of hex-literals for floats is precicely that they can represent floats exactly (even though it may not *look* like it, as in this case, because lots of digits are required to approximate the decimal value 1.2). So using those shold be safe -- and perhaps more easily read on other platforms, for example (byte order), or into floats with other precisions or the like.

Thanks for the suggestion, though :)

-- 
Magnus Lie Hetland
http://hetland.org