Differences between revisions 9 and 10
Revision 9 as of 2009-10-14 23:48:06
Size: 2397
Comment: Added a faster version of chr
Revision 10 as of 2009-10-16 07:47:19
Size: 1931
Editor: pgas
Comment: move the faster solution with the other examples, using printf for more portability
Deletions are marked like this. Additions are marked like this.
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   # another faster version
   # the octal conversion is done using maths, potentially avoiding a subshell and a fork
   chr() {
     printf "\0$(( $1 % 8 + 10 * ( $1 / 8 ) + 20 ))";
   }
Line 47: Line 53:

This version of {{{chr}}} executes much faster than the {{{printf}}} version above (about 1/40 to less than 1/150 the time when run in a loop):

{{{
   chr() { echo -en "\0$(( $1 % 8 + 10 * ( $1 / 8 ) + 20 ))"; }
}}}

{{{
   for p in chr newchr; do time for i in {1..4000}; do $p 65 >/dev/null; done; done

   System1 System2
   real 0m46.824s real 1m33.814s
   user 0m4.624s user 0m8.540s
   sys 0m33.290s sys 1m23.978s

   real 0m1.340s real 0m0.512s
   user 0m1.096s user 0m0.389s
   sys 0m0.124s sys 0m0.096s
}}}
 

How do I convert an ASCII character to its decimal (or hexadecimal) value and back?

If you have a known octal or hexadecimal value (at script-writing time), you can just use printf:

   # POSIX
   printf '\x27\047\n'

This prints two literal ' characters (27 is the hexadecimal ASCII value of the character, and 47 is the octal value) and a newline.

If you need to convert characters (or numeric ASCII values) that are not known in advance (i.e., in variables), you can use something a little more complicated:

   # POSIX
   # chr() - converts decimal value to its ASCII character representation
   # ord() - converts ASCII character to its decimal value
 
   chr() {
     printf \\$(printf '%03o' $1)
   }
 
   # another faster version 
   # the octal conversion is done using maths, potentially avoiding a subshell and a fork
   chr() { 
     printf "\0$(( $1 % 8 + 10 * ( $1 / 8 ) + 20 ))"; 
   }

   ord() {
     printf '%d' "'$1"
   }

   # hex() - converts ASCII character to a hexadecimal value
   # unhex() - converts a hexadecimal value to an ASCII character

   hex() { 
      printf '%x' "'$1"
   }

   unhex() {
      printf \\x"$1"
   }

   # examples:
 
   chr $(ord A)    # -> A
   ord $(chr 65)   # -> 65

The ord function above is quite tricky.

  • Tricky? Rather, it's using a feature that I can't find documented anywhere -- putting a single quote in front of an integer. Neat effect, but how on earth did you find out about it? Source diving? -- GreyCat

    • It validates The Single Unix Specification: "If the leading character is a single-quote or double-quote, the value shall be the numeric value in the underlying codeset of the character following the single-quote or double-quote." (see printf() to know more) -- mjf

BashFAQ/071 (last edited 2021-02-08 16:03:51 by GreyCat)