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digitalmars.D.learn - Two questions about "%a"

reply Magnus Lie Hetland <magnus hetland.org> writes:
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
Mar 02 2011
parent reply "Lars T. Kyllingstad" <public kyllingen.NOSPAMnet> writes:
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
Mar 02 2011
parent reply "Nick Sabalausky" <a a.a> writes:
"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);
Mar 03 2011
parent Magnus Lie Hetland <magnus hetland.org> writes:
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
Mar 04 2011