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

Hi monks
Could you please explain the below code, I didn't get much idea about pack function.
I got this code from internet,
$integer = pack("i", 171);
$twoints = pack("i2", 103, 241);

[download]
The output for pack("i", 171) is 1/2 and if I change pack("i", 172), output is 1/4 and for pack("i", 173) some special character.... please explain this function.
Thanks

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Re: Please explain the pack function
by Corion (Patriarch) on Apr 19, 2007 at 17:30 UTC

    The output was not 1/2, but ½, and it also wasn't 1/4, but ¼.

    Let's look at what the documentation for the pack function says:

    Takes a LIST of values and converts it into a string using the rules given by the TEMPLATE.

    So, the function takes a list of values (in our case, the number 171, or the number 172), and converts it into a string, according to our template. The template "i" says that pack is to take the values and pack them into integers in machine representation. Perl then returns the packed values as a string. If you look at the ASCII chart, you will find that at place 171, there is "½", at least for the character set you use. That's all there is to it.

    If you want to read some more information on pack, see the perlpacktut tutorial on pack.

[reply]
[d/l]
      Corion, Thanks a lot for your help.
[reply]
Re: Please explain the pack function
by BrowserUk (Patriarch) on Apr 19, 2007 at 18:06 UTC

    Maybe this will help? 

    $integer = pack( 'i', 171 );; #The [pack] template 'i' converts a inte
+ger

print length( $integer );; # to a 4-byte string
4

# The first byte is ascii value 171
print ord( substr( $integer, $_, 1 ) ) for 0 .. 3;; 
171
0
0
0

print chr( 171 );; ## Which displays as '½'. (On windows console,code 
+pages 437 & 850).
½

print chr( 172 );; ## Which displays as '¼'. (On windows console, code
+ pages 437 & 850).
¼

    [download]
    Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
    "Science is about questioning the status quo. Questioning authority".
    In the absence of evidence, opinion is indistinguishable from prejudice.
[reply]
[d/l]
      BrowserUk,Thanks a lot for your help.
[reply]
Re: Please explain the pack function
by andye (Curate) on Apr 20, 2007 at 08:43 UTC
[reply]

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