print packed string

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

Dear Monks

I have a question regarding the pack function.

 $string = pack("H*",15);  
 print "$string\n";                 #prints some weird symbol
 print unpack("H*",$string),"\n";   #prints 15
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If I try to print the packed string, what would I expect? How does this packed string associate with the value to be packed?

Thank you.

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Re: print packed string by Grimy (Pilgrim) on Jun 01, 2013 at 01:25 UTC
A packed string is basically a base 256 integer. For example, the string 'AB' would represent the integer 65 + 66 * 256 (65 and 66 being the ASCII codes of A and B respectively). The H format asks `pack` to treat the next argument as an hexadecimal string. Since 15 is an integer, it is first converted to the string '15', which is then parsed as an hex-string, giving the number 21 (1 * 16 + 5). This number is then encoded in base 256, giving the character with ASCII code 21. `pack('H*', 15) eq chr 21` Since `chr 21` isn't a printable character (it's actually Negative Acknowledge), it is rendered with this weird symbol you saw. This is largely system-dependant.	[reply] [d/l] [select]
Re^2: print packed string by lightoverhead (Pilgrim) on Jun 01, 2013 at 15:07 UTC
Thank you Grimy.This is exactly what I want to know!	[reply]
Re: print packed string by johngg (Canon) on May 31, 2013 at 23:18 UTC
I find that pack is easier to understand with code examples that pipe the packed string into hexdump. The last snippet is just packing the ASCII character set. `$ perl -e 'print pack q{H}, sprintf q{%2x}, 0x3a;' \| hexdump -C 00000000 3a \|:\| 00000001` [download] `$ perl -e ' print pack q{(H)}, map { sprintf q{%2x}, $_ } 0b111010, 072, 58, 0x3a;' \| hexdump -C 00000000 3a 3a 3a 3a \|::::\| 00000004` [download] $ perl -e ' print pack q{(H)}, map { sprintf q{%2x}, $_ } 0 .. 127;' \| hexdump -C 00000000 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f \|......... +.......\| 00000010 10 11 12 13 14 15 16 17 18 19 1a 1b 1c 1d 1e 1f \|......... +.......\| 00000020 20 21 22 23 24 25 26 27 28 29 2a 2b 2c 2d 2e 2f \| !"#$%&'( +)+,-./\| 00000030 30 31 32 33 34 35 36 37 38 39 3a 3b 3c 3d 3e 3f \|012345678 +9:;<=>?\| 00000040 40 41 42 43 44 45 46 47 48 49 4a 4b 4c 4d 4e 4f \|@ABCDEFGH +IJKLMNO\| 00000050 50 51 52 53 54 55 56 57 58 59 5a 5b 5c 5d 5e 5f \|PQRSTUVWX +YZ[\]^_\| 00000060 60 61 62 63 64 65 66 67 68 69 6a 6b 6c 6d 6e 6f \|`abcdefgh +ijklmno\| 00000070 70 71 72 73 74 75 76 77 78 79 7a 7b 7c 7d 7e 7f \|pqrstuvwx +yz{\|}~.\| 00000080 [download] I hope that you also find these examples helpful. Cheers, JohnGG	[reply] [d/l] [select]
Re: print packed string by toolic (Bishop) on May 31, 2013 at 21:08 UTC
The Basic Principle	[reply]