packing to byte string using ("C") directive in Ruby

Question

I'm trying to pack a byte array into a string using array.pack("C") but I'm getting some weird values pack. For example, when I say [7].pack("C"), the return value is "\a", whereas when I say "\x07".unpack("C"), the return value is [7], which is the desired behavior.

[6].pack("C") for example returns "\x06", again the desired outcome, whereas [33].pack("C") returns "!".

I'm new to bytes and hex in general. What is happening here? In my understanding, [33].pack("C") should return "\x21".

tadman · Accepted Answer · 2017-04-18 18:36:28Z

Why would it return "\x21"? The \x notation is reserved for non-printable characters, but since \x21 is equivalent to ! then that's what's displayed.

The inspect version of a string is generally the most readable version of it, not the most literal.

Likewise:

"!".ord
# => 33
"\x21".ord
# => 33
"\x21".ord.to_s(16)
# => "21"
33.chr
# => "!"

There's a number of special characters documented in the Strings section:

\a             bell, ASCII 07h (BEL)
\b             backspace, ASCII 08h (BS)
\t             horizontal tab, ASCII 09h (TAB)
\n             newline (line feed), ASCII 0Ah (LF)
\v             vertical tab, ASCII 0Bh (VT)
\f             form feed, ASCII 0Ch (FF)
\r             carriage return, ASCII 0Dh (CR)
\e             escape, ASCII 1Bh (ESC)
\s             space, ASCII 20h (SPC)
\\             backslash, \
\nnn           octal bit pattern, where nnn is 1-3 octal digits ([0-7])
\xnn           hexadecimal bit pattern, where nn is 1-2 hexadecimal digits ([0-9a-fA-F])
\unnnn         Unicode character, where nnnn is exactly 4 hexadecimal digits ([0-9a-fA-F])
\u{nnnn ...}   Unicode character(s), where each nnnn is 1-6 hexadecimal digits ([0-9a-fA-F])
\cx or \C-x    control character, where x is an ASCII printable character

Holger Just · Accepted Answer · 2017-04-18 18:37:05Z

2

These strings are fully equivalent, they are just different representation if exactly the same bytes:

"\a" == "\x07"
# => true

"\x21" == "!"
# => true

If you have a look at the ASCII table, you'll see that the hex 21 (decimal 33) is represented by the exclamation mark character. Similarly, the hex 7 (and decimal 7) character is represented by a control character called BEL (bell) which us usually represented as "\a".

In general, Ruby choses the most generic representation of strings when printing them. There will always be other (equivalent) representations you can use in your code. However, the will always map back to the exact same bytes in memory.

answered Apr 18, 2017 at 18:37

Holger Just

56.5k15 gold badges124 silver badges137 bronze badges

Collectives™ on Stack Overflow

packing to byte string using ("C") directive in Ruby

2 Answers 2

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related