Array#pack and String#unpack query

ruby-talk
1

Hi,

In order to learn a bit of ruby I'm trying to make a module for reading
and writing audio files in WAV format. First and foremost I'd like to
read and write files with 32-bit float bitdepth.

I'm using Array#pack and String#unpack but run into trouble with
precision.

The following of code..

arr = [ 1.0, 0.75, 0.5, 0.1, 0.025, 0.01 ]
p arr.pack("e*").unpack("e*")
p arr.pack("E*").unpack("E*")

...will output this:

[1.0, 0.75, 0.5, 0.100000001490116, 0.025000000372529,
0.00999999977648258]
[1.0, 0.75, 0.5, 0.1, 0.025, 0.01]

How come?

/Anton

···

--
Posted via http://www.ruby-forum.com/.

2

I forgot a thing: I expected the output to be the same for "e*" and
"E*":

[1.0, 0.75, 0.5, 0.1, 0.025, 0.01]
[1.0, 0.75, 0.5, 0.1, 0.025, 0.01]

Am I wrong?

/Anton

Anton Hörnquist wrote:

···

Hi,

In order to learn a bit of ruby I'm trying to make a module for reading
and writing audio files in WAV format. First and foremost I'd like to
read and write files with 32-bit float bitdepth.

I'm using Array#pack and String#unpack but run into trouble with
precision.

The following of code..

arr = [ 1.0, 0.75, 0.5, 0.1, 0.025, 0.01 ]
p arr.pack("e*").unpack("e*")
p arr.pack("E*").unpack("E*")

...will output this:

[1.0, 0.75, 0.5, 0.100000001490116, 0.025000000372529,
0.00999999977648258]
[1.0, 0.75, 0.5, 0.1, 0.025, 0.01]

How come?

/Anton

--
Posted via http://www.ruby-forum.com/.

3

Read http://en.wikipedia.org/wiki/IEEE-754 about how floating point numbers
are stored. 32-bit float doesn't have enough precision to store 0.1's
bitpattern. That is why you should never compare floating point numbers
directly.

Jan

···

On Thursday 11 September 2008 19:59:39 Anton Hörnquist wrote:

> [1.0, 0.75, 0.5, 0.100000001490116, 0.025000000372529,
> 0.00999999977648258]
> [1.0, 0.75, 0.5, 0.1, 0.025, 0.01]
>
> How come?