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(gawk.info)Translate Program

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Transliterating Characters
--------------------------

   The system `tr' utility transliterates characters.  For example, it
is often used to map uppercase letters into lowercase for further
processing:

     GENERATE DATA | tr 'A-Z' 'a-z' | PROCESS DATA ...

   `tr' requires two lists of characters.(1)  When processing the
input, the first character in the first list is replaced with the first
character in the second list, the second character in the first list is
replaced with the second character in the second list, and so on.  If
there are more characters in the "from" list than in the "to" list, the
last character of the "to" list is used for the remaining characters in
the "from" list.

   Some time ago, a user proposed that a transliteration function should
be added to `gawk'.  The following program was written to prove that
character transliteration could be done with a user-level function.
This program is not as complete as the system `tr' utility but it does
most of the job.

   The `translate' program demonstrates one of the few weaknesses of
standard `awk': dealing with individual characters is very painful,
requiring repeated use of the `substr', `index', and `gsub' built-in
functions (Note: String Manipulation Functions.).(2)

   There are two functions.  The first, `stranslate', takes three
arguments:

`from'
     A list of characters to translate from.

`to'
     A list of characters to translate to.

`target'
     The string to do the translation on.

   Associative arrays make the translation part fairly easy. `t_ar'
holds the "to" characters, indexed by the "from" characters.  Then a
simple loop goes through `from', one character at a time.  For each
character in `from', if the character appears in `target', `gsub' is
used to change it to the corresponding `to' character.

   The `translate' function simply calls `stranslate' using `$0' as the
target.  The main program sets two global variables, `FROM' and `TO',
from the command line, and then changes `ARGV' so that `awk' reads from
the standard input.

   Finally, the processing rule simply calls `translate' for each
record:

     # translate.awk --- do tr-like stuff
     # Bugs: does not handle things like: tr A-Z a-z, it has
     # to be spelled out. However, if `to' is shorter than `from',
     # the last character in `to' is used for the rest of `from'.
     
     function stranslate(from, to, target,     lf, lt, t_ar, i, c)
     {
         lf = length(from)
         lt = length(to)
         for (i = 1; i <= lt; i++)
             t_ar[substr(from, i, 1)] = substr(to, i, 1)
         if (lt < lf)
             for (; i <= lf; i++)
                 t_ar[substr(from, i, 1)] = substr(to, lt, 1)
         for (i = 1; i <= lf; i++) {
             c = substr(from, i, 1)
             if (index(target, c) > 0)
                 gsub(c, t_ar[c], target)
         }
         return target
     }
     
     function translate(from, to)
     {
         return $0 = stranslate(from, to, $0)
     }
     
     # main program
     BEGIN {
         if (ARGC < 3) {
             print "usage: translate from to" > "/dev/stderr"
             exit
         }
         FROM = ARGV[1]
         TO = ARGV[2]
         ARGC = 2
         ARGV[1] = "-"
     }
     
     {
         translate(FROM, TO)
         print
     }

   While it is possible to do character transliteration in a user-level
function, it is not necessarily efficient, and we (the `gawk' authors)
started to consider adding a built-in function.  However, shortly after
writing this program, we learned that the System V Release 4 `awk' had
added the `toupper' and `tolower' functions (Note: String Manipulation
Functions.).  These functions handle the vast
majority of the cases where character transliteration is necessary, and
so we chose to simply add those functions to `gawk' as well and then
leave well enough alone.

   An obvious improvement to this program would be to set up the `t_ar'
array only once, in a `BEGIN' rule. However, this assumes that the
"from" and "to" lists will never change throughout the lifetime of the
program.

   ---------- Footnotes ----------

   (1) On some older System V systems, `tr' may require that the lists
be written as range expressions enclosed in square brackets (`[a-z]')
and quoted, to prevent the shell from attempting a file name expansion.
This is not a feature.

   (2) This program was written before `gawk' acquired the ability to
split each character in a string into separate array elements.