Ruby's pack and unpack explained

Question

Even after reading the standard documentation, I still can't understand how Ruby's Array#pack and String#unpack exactly work. Here is the example that's causing me the most trouble:

irb(main):001:0> chars = ["61","62","63"]
=> ["61", "62", "63"]
irb(main):002:0> chars.pack("H*")
=> "a"
irb(main):003:0> chars.pack("HHH")
=> "```"

I expected both these operations to return the same output: "abc". Each of them "fails" in a different manner (not really a fail since I probably expect the wrong thing). So two questions:

1. What is the logic behind those outputs?
2. How can I achieve the effect I want, i.e. transforming a sequence of hexadecimal numbers to the corresponding string. Even better - given an integer n, how to transform it to a string identical to the text file that when is considered as a number (say, in a hex editor) equals n?

Answer 1

We were working on a similar problem this morning. If the array size is unknown, you can use:

ary = ["61", "62", "63"]
ary.pack('H2' * ary.size)
=> "abc"

You can reverse it using:

str = "abc"
str.unpack('H2' * str.size)
=> ["61", "62", "63"]