reading/writing binary problem

[Wolfgang]

Hi,

I have some binary files which I need to delete some data at the
beginning of the file (to extract the raw data out of the file). I got a
matlab file from a colleque but I would prefer if I could do it with ruby.

This matlab code does the trick:
function m=write_rad(filename)
file=fopen(filename);
f= fread(file,'uint16');

[r,k]=size(f);
if k==1;
t=r-1048576;
m=f(t+1:r,:);
end
end % function

How can I do this in ruby? I'm completely lost.

Thanks
Wolfgang

[Robert Klemme]

An easy solution is to copy the file:

Untested:

def cut_prefix(file, length)
raise ArgumentError, "negative length" if length < 0

bak = file + ".bak"
File.rename file, bak

File.open(bak,"rb") do |io_r|
io_r.seek length

File.open(file, "wb") do |io_w|
while ( buffer = io_r.read(4096) )
io_w.write(buffer)
end
end
end
end

Another solution would be to do it internally with continuously seeking,
reading, seeking, writing etc.

Kind regards

robert

[ara.t....@noaa.gov]

this sould be easy if i understood that method and the nature of your data ;-)

here's what i understand this method to do :

# open the file

file=fopen(filename);

# read all available unit16 quantities into an [m, n] array

f= fread(file,'uint16');

# check the size of this array, r is the number of elements read

[r,k]=size(f);

# k will be one iff data was read into r dimension

if k==1;

# here i assume 1048576 is the 'row_size' of 1mb. we set t to be the offset
# of the last row

t=r-1048576;

# because matlab is evil and uses one based indexing, we inc t by one and
# extract the last row/block of data. but here it seems like the ith index
# should be t+1:r-1048576 no? otherwise it seems like this would be out of
# bounds?

m=f(t+1:r,:);

in any case this looks alot like you are trying to extract the last 1mb of
uint16 quantities from a data file - is that right? let me know and i'll show
you how to do this.

regards.

-a
--
share your knowledge. it's a way to achieve immortality.
- h.h. the 14th dali lama

[Wolfgang]

Hello,

yes I want to extract a matrix of 1024x1024 of binary data (2bytes for
each value). But ahead of this data is 1048576 bit (13312 bytes) of
crap. I don't know matlab at all and I don't like to start this routine
when I want to import that data into my application, which can not skip
this header. If it is easier to do by a linux command (its running in
cygwin) any help is also welcome.

Wolfgang

PS: I will also test the example of Robert

ara.t....@noaa.gov schrieb:

[ara.t....@noaa.gov]

On Thu, 30 Mar 2006, Wolfgang wrote:

> Hello,
>
> yes I want to extract a matrix of 1024x1024 of binary data (2bytes for each
> value). But ahead of this data is 1048576 bit (13312 bytes) of crap. I don't
> know matlab at all and I don't like to start this routine when I want to
> import that data into my application, which can not skip this header. If it
> is easier to do by a linux command (its running in cygwin) any help is also
> welcome.
>
> Wolfgang
>
> PS: I will also test the example of Robert

ok. so, just to clarify, you want to read a 2mb matrix out of a file after
skiping 13312 bytes of crap. this will do that

def extract_data path
open(path){|f| f.seek(13312); f.read(1024*1024*2)}
end

here data returned is a buffer. if you want a matrix object i __highly__
suggest installing the narray module and using something like

require 'narray'
def extract_matrix path
na = NArray.sint 1024, 1024
open(path){|f| f.seek(13312); na[] = f.read(1024*1024*2)}
na
end

this will return a numerical matrix with nice operators. you can write it to
file with

open('m.dat','w'){|f| f.write na}

and this will write the matix in binary.

[Daniel Harple]

On Mar 29, 2006, at 6:31 PM, ara.t....@noaa.gov wrote:

> open(path){|f| f.seek(13312); f.read(1024*1024*2)}

If you are using Windows don't forget to open it in binary mode:

open(..., 'rb') ...

-- Daniel

[Robert Klemme]

In fact I recommend to use that switch regardless of OS used: it's more
portable and also serves as documentation hint that the file is actually
binary.

Kind regards

robert

[ara.t....@noaa.gov]

ah. indeed.

[Wolfgang]

Thank you all,

this worked perfectly!

Wolfgang

ara.t....@noaa.gov schrieb: