in reply to Re: Removing commas and dollar signs from a variable.
in thread Removing commas and dollar signs from a variable.

Why use all that regex processing power that you don't need? tr is a much better solution in this case.

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  • Comment on Re: Re: Removing commas and dollar signs from a variable.

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by 2501 (Pilgrim) on Dec 22, 2001 at 00:48 UTC
    I am missing the benefits/drawbacks of s/// vs. tr/// .
    is it something to truly consider or is it just "more correct" ?

    thanks!
[reply]
      tr/// is faster, but s/// can be used for more complex matches.
      I'll support my answer with a Benchmarked example:

      #!/usr/bin/perl -w
use strict;
use Benchmark;

# Generate 1 MB of random data

my $data;
for (my $i = 0; $i <= 1048576; $i++){
    $data .= chr int rand 256;
}
my $copy = $data;

study $data;
study $copy;

# Remove X'es

Benchmark::cmpthese(-10, {
    's///'  => sub { (my $dummy = $data) =~ s/X//g; },
    'tr///' => sub { (my $dummy = $copy) =~ tr/X//d; }
});

print $copy eq $data ? "OK\n" : "NOT OK\n";

      [download]

      This script's output: 
      Benchmark: running s///, tr///, each for at least 10 CPU seconds...
      s///: 12 wallclock secs (10.80 usr +  0.02 sys = 10.82 CPU) @ 37
+.89/s (n=410)
     tr///: 15 wallclock secs (10.08 usr +  0.08 sys = 10.16 CPU) @ 42
+.81/s (n=435)
        Rate  s/// tr///
s///  37.9/s    --  -11%
tr/// 42.8/s   13%    --
OK

      [download]

      Update 200112212151: I forgot to copy the string before removing the X'es. After the first iteration, there'd be no X'es left. For your entertainment, I present the previous version of my post:
      Hi,

      I was going to answer that tr/// was faster, and s/// can be used for more complex matches.
      I was going to support my answer with a Benchmarked example, which would show that tr/// was a lot faster.

      BUT my benchmark told me s/// is the winner. On 1 MB of random data, s/X//g is faster than tr/X//d. If anyone can tell me why this is, or what I'm doing wrong, I'd really appreciate that.

      #!/usr/bin/perl -w
use strict;
use Benchmark;

# Generate 1 MB of random data

my $data;
for (my $i = 0; $i < 1048576; $i++){
    $data .= chr int rand 256;
}
my $copy = $data;

# Remove X'es

Benchmark::cmpthese(-10, {
    's///'  => sub { $data =~ s/X//g; },
    'tr///' => sub { $copy =~ tr/X//d; }
});

print $copy eq $data ? "OK\n" : "NOT OK\n";

      [download]
      This script's output: 
      Benchmark: running s///, tr///, each for at least 10 CPU seconds...
      s///: 11 wallclock secs (10.52 usr +  0.01 sys = 10.53 CPU) @ 72
+.08/s (n=759)
     tr///: 10 wallclock secs (10.53 usr +  0.01 sys = 10.54 CPU) @ 61
+.29/s (n=646)
        Rate tr///  s///
tr/// 61.3/s    --  -15%
s///  72.1/s   18%    --
OK

      [download]
       

      2;0 juerd@ouranos:~$ perl -e'undef christmas'
Segmentation fault
2;139 juerd@ouranos:~$

      [download]

[reply]
[d/l]
[select]
      yes, tr/// is not a regex, where s/// is. This means that tr/// should move faster. Let's find out! I'm using 3 sets of randomly generated data, and removing any occurence of the letter 'e' in the string. The results? A winner is tr///! 

      use Benchmark;
$reps=500000;

$x.=("a".."z")[rand 26] for (1..256); 
Benchmark::cmpthese($reps, {
'sub256' => '$_=$x;s/e//g;',
'trn256' => '$_=$x;tr/e//d;',
});
print "-"x40,"\n";
$x="";
$x.=("a".."z")[rand 26] for (1..1024); 
Benchmark::cmpthese($reps, {
'sub1024' => '$_=$x;s/e//g;',
'trn1024' => '$_=$x;tr/e//d;',
});
print "-"x40,"\n";
$x="";
$x.=("a".."z")[rand 26] for (1..5120); 
Benchmark::cmpthese($reps, {
'sub5k' => '$_=$x;s/e//g;',
'trn5k' => '$_=$x;tr/e//d;',
});
print "-"x40,"\n";

      [download]
      Benchmark: timing 500000 iterations of sub256, trn256...
    sub256:  3 wallclock secs ( 3.84 usr +  0.00 sys =  3.84 CPU) @ 130208.33/s (n=500000)
    trn256:  2 wallclock secs ( 1.81 usr +  0.00 sys =  1.81 CPU) @ 276243.09/s (n=500000)
           Rate sub256 trn256
sub256 130208/s     --   -53%
trn256 276243/s   112%     --
----------------------------------------
Benchmark: timing 500000 iterations of sub1024, trn1024...
   sub1024: 15 wallclock secs (14.56 usr +  0.00 sys = 14.56 CPU) @ 34340.66/s (n=500000)
   trn1024:  6 wallclock secs ( 6.81 usr +  0.00 sys =  6.81 CPU) @ 73421.44/s (n=500000)
           Rate sub1024 trn1024
sub1024 34341/s      --    -53%
trn1024 73421/s    114%      --
----------------------------------------
Benchmark: timing 500000 iterations of sub5k, trn5k...
     sub5k: 66 wallclock secs (65.36 usr +  0.00 sys = 65.36 CPU) @ 7649.94/s (n=500000)
     trn5k: 31 wallclock secs (30.64 usr +  0.00 sys = 30.64 CPU) @ 16318.54/s (n=500000)
         Rate sub5k trn5k
sub5k  7650/s    --  -53%
trn5k 16319/s  113%    --
----------------------------------------
[reply]
[d/l]
      tr///; uses straight substitution.
      s///; uses the regex engine.
      Straight substitution is relatively faster and 'cheaper'.
[reply]
[d/l]
[select]
      tr/// is for characters, s/// is for strings. tr is generally a better choice where it works because it's more efficent. s/// can do everything tr can because it can work with strings of length 1 (ie - one character), it's just slightly slower. I guess it's more about taste and speed than anything else
[reply]
Re: Re: Re: Removing commas and dollar signs from a variable.
by Rich36 (Chaplain) on Dec 22, 2001 at 00:35 UTC
    Good point. tr's not something I use much, but probably should. Thanks for the reminder.
    Rich36
    There's more than one way to screw it up...
[reply]

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