From: Michael Neumann <neumann@...>
Date: 2001-10-23T22:27:49+09:00
Subject: [ruby-talk:23048] Re: Howto read binary data?

Martin Kahlert wrote:
> Hi!
> I want to read a file containing double values in binary representation,
> i.e. they have been written from C using
> 
> double variable;
> write(fd, &variable, sizeof(variable));

For this purpose, I wrote some time ago a class BinaryReader:

  class BinaryReader

    DEF = [ 
      [ :short,  2, 's' ],
      [ :ushort, 2, 'S' ],

      [ :int,    4, 'i' ],
      [ :uint,   4, 'I' ],

      [ :float,  4, 'f' ],
      [ :double, 8, 'd' ]
    ]

    DEF.each do |meth, size, format|
      eval %{
        def read_#{ meth }(n = 1)
          _read(n, #{size}, '#{ format }')
        end
      }
    end

    def initialize( handle )
      @handle = handle
    end

    private

    def _read(n, size, format)
      bytes = n * size

      str = @handle.read(bytes)
      raise "failure during read" if str.nil? or str.size != bytes 

      val = str.unpack(format * n) 

      if n == 1
        val.first
      else
        val
      end
    end

  end


You can use this as follows:


  file = File.new(...)
  file.binary       # only neccessary on Windows 
  reader = BinaryReader.new( file )

  aFloat = reader.read_float
  int1, int2 = reader.read_int(2)   # read two integers
  # ... read_double, read_ushort etc..


Regards,

  Michael

> How can i read this file from ruby and stuff the data into a ruby double
> variable? (The file has been built on the same machine, so endianess should
> not be a problem).

See Array#pack and String#unpack.

Regards,

  Michael


-- 
Michael Neumann
merlin.zwo InfoDesign GmbH
http://www.merlin-zwo.de