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Transliterating Characters

The system tr utility transliterates characters. For example, it is often used to map upper-case letters into lower-case, for further processing.

generate data | tr '[A-Z]' '[a-z]' | process data ...

You give tr two lists of characters enclosed in square brackets. Usually, the lists are quoted to keep the shell from attempting to do a filename expansion.(23) 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 to us that we add a transliteration function to gawk. Being opposed to "creeping featurism," I wrote the following program 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 will do 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 (see section Built-in Functions for String Manipulation).(24)

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

A list of characters to translate from.
A list of characters to translate to.
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 will read from the standard input.

Finally, the processing rule simply calls translate for each record.

# translate --- do tr like stuff
# Arnold Robbins, [email protected], Public Domain
# August 1989

# 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
    if (ARGC < 3) {
        print "usage: translate from to" > "/dev/stderr"
    FROM = ARGV[1]
    TO = ARGV[2]
    ARGC = 2
    ARGV[1] = "-"

    translate(FROM, TO)

While it is possible to do character transliteration in a user-level function, it is not necessarily efficient, and we 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. 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.

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