Ruby's String#hex confusion

Question

I've found it weird that String#hex in Ruby doesn't return the right hex value for a given char. I might be misunderstanding the method, but take the following example:

'a'.hex
=> 10

Whereas the right hex value for 'a' would be 61:

'a'.unpack('H*')
=> 61

Am I missing something? What's hex for? Any hints appreciated!

Thanks

Solution

String#hex doesn't give you the ASCII index of a character, it's for transforming a base-16 number (hexadecimal) from a string to an integer:

% ri String\#hex
String#hex

(from ruby site)
------------------------------------------------------------------------------
  str.hex   -> integer


------------------------------------------------------------------------------

Treats leading characters from str as a string of hexadecimal digits
(with an optional sign and an optional 0x) and returns the
corresponding number. Zero is returned on error.

  "0x0a".hex     #=> 10
  "-1234".hex    #=> -4660
  "0".hex        #=> 0
  "wombat".hex   #=> 0

So it uses the normal mapping:

'0'.hex #=> 0
'1'.hex #=> 1
...
'9'.hex #=> 9
'a'.hex #=> 10 == 0xA
'b'.hex #=> 11
...
'f'.hex   #=> 15 == 0xF == 0x0F
'10'.hex  #=> 16 == 0x10
'11'.hex  #=> 17 == 0x11
...
'ff'.hex  #=> 255 == 0xFF

It's very similar to String#to_i when using base 16:

'0xff'.to_i(16) #=> 255
'FF'.to_i(16)   #=> 255
'-FF'.to_i(16)  #=> -255

From the docs:

% ri String\#to_i
String#to_i

(from ruby site)
------------------------------------------------------------------------------
  str.to_i(base=10)   -> integer


------------------------------------------------------------------------------

Returns the result of interpreting leading characters in str as an
integer base base (between 2 and 36). Extraneous characters past the
end of a valid number are ignored. If there is not a valid number at the start
of str, 0 is returned. This method never raises an exception
when base is valid.

  "12345".to_i             #=> 12345
  "99 red balloons".to_i   #=> 99
  "0a".to_i                #=> 0
  "0a".to_i(16)            #=> 10
  "hello".to_i             #=> 0
  "1100101".to_i(2)        #=> 101
  "1100101".to_i(8)        #=> 294977
  "1100101".to_i(10)       #=> 1100101
  "1100101".to_i(16)       #=> 17826049

Comments

[Chris Heald]

"a" is hexadecimal for 10. "f" is hex for 16.

[robnasby]

Actually, "f" is hexadecimal for 15.

[rampion]

What do you mean by "hex value" for a character? Because 'a' is hexadecimal for 16. Do you mean ASCII code?

[FullOfCaffeine]

Very comprehensive answer. I deleted my previous answer as it didn't make any sense whatsoever. Thanks for making it all clear!