reading binary data from a file

amigero has asked for the wisdom of the Perl Monks concerning the following question:

Hello, I'm new to perlmonks and this is my first post! I've only coded in matlab, so I will reference matlab functions to see if you guys can help me find the equivalent in perl.
I"m trying to read binary data from a file. The structure of the file contains data that has information in 158 byte increments, meaning every new 158 bytes is independent from the rest. so it goes like this:
$file = $ARGV_[0]
open(fileFID,$file);
for (my $ii = 1; $ii<=10;$ii++){
read(fileFID,my $data,158);
#now everytime I loop I want to put the new information of
#the file into a new row. In matlab you would simply say:
# $new_data(ii,:)=$data; and the info from read would go
#into a new row. I want to keep the $new_data variable for
#later use because I will be calling it more than once and
#I dont' want to read the information from the file
#everytime. Please help!
}

Comment on reading binary data from a file

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Re: reading binary data from a file by ikegami (Patriarch) on Oct 28, 2009 at 21:16 UTC
push `my $fn = @ARGV[0]; open(my $fh, '<', $fn) or die("Can't open \"$fn\": $!\n"); binmode($fh); my @recs; while (read($fh, my $rec, 158)) { push @recs, $rec; }` [download] Don't forget binmode on binary files!	[reply] [d/l]
Re^2: reading binary data from a file by amigero (Novice) on Oct 29, 2009 at 00:12 UTC
Thank you. Now, how do I access the 23rd to 35th byte for every row and put it into another array? In Matlab it would be: `new_array = array(:,23:35);` and you're done Second, how do I access a row and column in that array and put it into a scalar? In Matlab it would be: `scalar = array(3,4)` meaning I'm grabing the 3rd row and 4th column byte.	[reply] [d/l] [select]
Re^3: reading binary data from a file by ikegami (Patriarch) on Oct 29, 2009 at 00:38 UTC
`substr` can be used to extract portions of other strings. It returns a string, though. If you want to treat each character of that string as a byte, you can then use `unpack 'C'`. Square brackets can be used to create anonymous arrays. The contents of the brackets are assigned to the array. The operator returns a reference to the anonymous array. `$array[2][3]` gets the 4th element of the array referenced by the 2nd element of `@array`. perllol All together: `my @recs; while (read($fh, my $rec, 158)) { push @recs, [ unpack 'C', substr($rec, 22, 13) ]; } print $recs[2][3];` [download] Since you're using `unpack` anyway, you can just skip over the unwanted bytes instead of using `substr`. `push @recs, [ unpack 'x22 C13', $rec ];` [download] Update: Ooops, was missing the "*" in the unpack format.	[reply] [d/l] [select]