in reply to Re: should this backspace removal code be done better?
in thread should this backspace removal code be done better?

Small bug:

#!/usr/bin/perl -wl

$s = "this is an\b\b not correct usage";
$s =~ s[.\cH][]g;

print $s;

print "and one's still there" if grep { ord == ord"\b" } split //,$s;
__END__
output:
this is  not correct usage
and one's still there

[download]

It looks correct at first glance, but the second backspace is still there.

Update: Much better. Only problem is in the case of a leading \b, you can get into an infinite loop. However only the regex needs alteration.

$s =~ s/(?:[^\cH]\cH|^\cH+)//g while ...

[download]

antirice    
The first rule of Perl club is - use Perl
The ith rule of Perl club is - follow rule i - 1 for i > 1

Replies are listed 'Best First'.
Re: Re: Re: should this backspace removal code be done better?
by smackdab (Pilgrim) on Oct 05, 2003 at 06:46 UTC
    AWESOME, it sinks in slowly ;-)

    This solves the problem from the poster below... 
    $s = "\bthis is an\b correct\b\b\b usage\b";
do 1  while ($s =~ s/(?:[^\cH]\cH|^\cH+)//g);

    [download]
[reply]
[d/l]

      A matter of style, but I don't care for do 1 while ... since you could just do a while with an empty block. Anyhow, I decided to benchmark all three ways:

      #!/usr/bin/perl -w

use Benchmark qw(cmpthese);
$s = "\bthis is an\b correct\b\b\b usage\b";

sub uk {
  $a = $s;
  $a =~ s[(?:[^\cH]\cH|^\cH)][]g while 1+index $a, chr(8);
  $a;
}

sub new1 {
  $a = $s;
  while ($a =~ s/(?:[^\cH]\cH|^\cH+)//g) {}
  $a;
}

sub smack {
  $a = $s;
  do 1 while ($a =~ s/(?:[^\cH]\cH|^\cH+)//g);
  $a;
}

cmpthese(-5,{uk=>\&uk,new1=>\&new1,smack=>\&smack});
$s x= 100;
cmpthese(-5,{uk=>\&uk,new1=>\&new1,smack=>\&smack});

__END__
Benchmark: running new1, smack, uk for at least 5 CPU seconds...
      new1:  5 wallclock secs ( 5.23 usr +  0.00 sys =  5.23 CPU) @ 31
+024.33/s (n=162393)
     smack:  6 wallclock secs ( 2.16 usr +  3.05 sys =  5.22 CPU) @ 29
+16.79/s (n=15222)
        uk:  5 wallclock secs ( 5.19 usr +  0.00 sys =  5.19 CPU) @ 36
+570.60/s (n=189710)
         Rate smack  new1    uk
smack  2917/s    --  -91%  -92%
new1  31024/s  964%    --  -15%
uk    36571/s 1154%   18%    --
Benchmark: running new1, smack, uk for at least 5 CPU seconds...
      new1:  6 wallclock secs ( 5.48 usr +  0.00 sys =  5.48 CPU) @ 36
+8.11/s (n=2016)
     smack:  6 wallclock secs ( 5.01 usr +  0.41 sys =  5.41 CPU) @ 32
+6.56/s (n=1768)
        uk:  5 wallclock secs ( 5.23 usr +  0.00 sys =  5.23 CPU) @ 46
+2.14/s (n=2419)
       Rate smack  new1    uk
smack 327/s    --  -11%  -29%
new1  368/s   13%    --  -20%
uk    462/s   42%   26%    --

      [download]

      For whatever reason, your version hits sys very hard. However, BrowserUK's version is the fastest of the pack.

      Hope this helps.

      antirice    
      The first rule of Perl club is - use Perl
      The       ith rule of Perl club is - follow rule i - 1 for i > 1

[reply]
[d/l]
[select]

        Not any more. This new smack is a lot faster by eliminating the alternation.

        #!/usr/bin/perl -w

use Benchmark qw(cmpthese);
$s = "\bthis is an\b correct\b\b\b usage\b";

sub uk {
  $a = $s;
  $a =~ s[(?:[^\cH]\cH|^\cH)][]g while 1+index $a, chr(8);
  $a;
}

sub new1 {
  $a = $s;
  while ($a =~ s/(?:[^\cH]\cH|^\cH+)//g) {}
  $a;
}

sub smack {
  $a = $s;
  $a =~ s/^\cH+//;
  1 while ($a =~ s/[^\cH]\cH//g);
  $a;
}

cmpthese(-5,{uk=>\&uk,new1=>\&new1,smack=>\&smack});
$s x= 100;
cmpthese(-5,{uk=>\&uk,new1=>\&new1,smack=>\&smack});

Benchmark: running new1, smack, uk, each for at least 5 CPU seconds...
      new1:  6 wallclock secs ( 5.39 usr +  0.03 sys =  5.42 CPU) @ 11
+344.28/s (n=61486)
     smack:  6 wallclock secs ( 5.24 usr +  0.02 sys =  5.26 CPU) @ 36
+814.64/s (n=193645)
        uk:  4 wallclock secs ( 5.26 usr +  0.06 sys =  5.32 CPU) @ 13
+261.09/s (n=70549)
         Rate  new1    uk smack
new1  11344/s    --  -14%  -69%
uk    13261/s   17%    --  -64%
smack 36815/s  225%  178%    --
Benchmark: running new1, smack, uk, each for at least 5 CPU seconds...
      new1:  5 wallclock secs ( 5.22 usr +  0.05 sys =  5.27 CPU) @ 14
+0.04/s (n=738)
     smack:  6 wallclock secs ( 5.26 usr +  0.07 sys =  5.33 CPU) @ 87
+3.55/s (n=4656)
        uk:  5 wallclock secs ( 5.18 usr +  0.04 sys =  5.22 CPU) @ 17
+6.44/s (n=921)
       Rate  new1    uk smack
new1  140/s    --  -21%  -84%
uk    176/s   26%    --  -80%
smack 874/s  524%  395%    --

        [download]

        /-\

[reply]
[d/l]

      Catering for multiline input and assuming backspaces at beginning of lines should simply be removed:

      $s = "\b\bthis is an\b correct\b\b\b usage\b\n\b2nd\n\b\b3rd\n";
$s =~ s/^[\b]+//mg;
1 while $s =~ s/[^\b][\b]//g;

      [download]

      /-\

[reply]
[d/l]

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