Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!swrinde!elroy.jpl.nasa.gov!jpl-devvax!lwall
From: lwall@jpl-devvax.jpl.nasa.gov (Larry Wall)
Newsgroups: comp.lang.perl
Subject: Re: Manipulating strings
Message-ID: <1991May25.060211.3468@jpl-devvax.jpl.nasa.gov>
Date: 25 May 91 06:02:11 GMT
References: <SETZER.91May23110013@matagh.ncsu.edu>
Reply-To: lwall@jpl-devvax.JPL.NASA.GOV (Larry Wall)
Organization: Jet Propulsion Laboratory, Pasadena, CA
Lines: 68

In article <SETZER.91May23110013@matagh.ncsu.edu> setzer@matagh.ncsu.edu (Th'PoC) writes:
: The problem: I have four files of identical length (< 5k).  Each
: contains 8 bit data, but do not contain any of `A'..`Z'.  At any fixed
: offset into each file, either 1) all four characters are identical, or
: 2) one or more of the characters has the eighth bit set.  I want to
: generate an "overlay", i.e., if (1), output the character; if (2),
: output a letter in `A'..`P' that will tell me which files had the
: eighth bit set.
: 
: The [*ugly*] solution:  I `open'ed the four files, slurped each
: of them into an array, and looped over the length of the array:
: 
: --
: open(A, "< a") || die "can't open 'a'";  @a=split(//,join("",<A>));
: # do the same for files "b", "c", and "d"
: 
: for ($i=0; $i < $#a+1; $i++) { # do the `if's on `ord($a[$i])&0x80' etc.
: --
: 
: The question: Is there a "prettier" way to do this?  (There must be.
: Isn't there a perl axiom that if you have to use an indexing variable,
: you're doing it the "wrong" way?)  It comes down to simple string
: manipulation, but I can't see a way to use any of the implicit looping
: constructs (`for (@a)', `foreach ...').  I would appreciate insight
: into an alternate way to solve the problem.  Thanks.

Well, here's one way:

#!/usr/bin/perl

vec($foo,0,0);

$a = `cat a`;
$b = `cat b`;
$c = `cat c`;
$d = `cat d`;

$string = $a;

$a =~ tr/\200-\377/A/;
$a =~ tr/A/\0/c;
$b =~ tr/\200-\377/B/;
$b =~ tr/B/\0/c;
$c =~ tr/\200-\377/D/;
$c =~ tr/D/\0/c;
$d =~ tr/\200-\377/H/;
$d =~ tr/H/\0/c;

$highs |= $a;
$highs |= $b;
$highs |= $c;
$highs |= $d;

($mask = $highs) =~ tr/A-Z/\177/c;
$mask =~ tr/A-Z/\0/;
$string &= $mask;
$string |= $highs;

print $string;

: If the fundamentalists don't hate you, you have the wrong lifestyle.
:   -- James Nicoll

If the fundamentalists hate you when they should hate your lifestyle,
they've lost sight of the fundamental lifestyle.
  -- me

Larry