Print and save data in binary format in Perl - perl

Question

My script generates some very very huge files, and I am trying to print/save the output in a binary format to reduce the file size as much as possible!
Each time that script generates five values, like:
$a1 = 1.64729
$a2 = 4.33329
$a3 = 3.55724
$a4 = 1.45759
$a5 = 7.474700
It prints in the output like:
A:1.64729,4.33329,3.55724,1.45759,7.474700
I am not sure whether this is the best way, but I thought to pack each row when it is printing to the output! I used pack/unpack built-in function in Perl!
I had a look at perldoc, but I did not understand which format specifiers were proper (???)!
#!/usr/bin/perl
...
#A = ($a1,$a2,$a3,$a4,$a5);
print pack ("???", ("A:", join(",", map { sprintf "%.1f", $_ } #A)), "\n";

Answer 1

If you compress the file (instead of trying to write binary bytes) you will get a small file. That's because your entire file will have mostly the ten digit characters, plus a decimal point, and a comma.
You can compress a file as you write it via IO::Zlib. This will use either the Zlib library, or the gzip command.
However, if you want to use pack, go ahead. Get the Camel Book which gives much clearer documentation than the standard Perldoc.
It's not all that difficult:
my $output = "A:1.64729,4.33329,3.55724,1.45759,7.474700";
$output =~ s/^A://; #Remove the 'A:'
my #numbers = split /,/, $output # Make into an array
my $packed = pack "d5", #numbers; # Pack five inputs as floating point numbers
say join ",", "d5", $packed; # Unpacks those five decimal encoded numbers
You'll probably have to use syswrite and sysread since aren't reading and writing strings. This is unbuffered reading and writing, and you have to specify the number of bytes you're reading or writing.
One more thing: If you know where the decimal point is in the number (that is, it's always a number between 1 and up to 10) you can convert the number into an integer which will allow you to pack the number into an even smaller number of bytes:
my $output = "A:1.64729,4.33329,3.55724,1.45759,7.474700";
$output =~ s/^A://; #Remove the 'A:'
$output =~ s/,//g; #Remove all the decimal points
my #numbers = split /,/, $output # Make into an array
my $packed = pack "L5", #numbers; # Pack five inputs as unsigned long numbers