in reply to programming languages and immutable data types

Hmm can you plz show me the practical difference to perl?

lanx@nc10-ubuntu:~$ python
Python 2.5.2 (r252:60911, Jan 20 2010, 23:16:55) 
...
>>> str="123"
>>> str+="4"
>>> str
'1234'

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The only thing I can think of are runtime issues :

I've read that in JS  str+="a";str+="b" takes 40% longer than str=str+"a"+"b", because strings have to be copied...

But in perl the second version needs much longer...

  DB<1> $str="";$t=time;for (1..100000) {$str=$str."a"."b"};print time
+-$t
13
  DB<2> $str="";$t=time;for (1..1000000) {$str.="a";$str.="b"};print t
+ime-$t
1

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Actually the same code in python has similar benchmarks

>>> for i in range(1, 1000000):
...   str+="a"+"b"
... 
>>> str=""
>>> for i in range(1, 100000):
...   str=str+"a"+"b"
...

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So where is the practical difference?

Cheers Rolf

UPDATE: Maybe not a good benchmark, since the growing of $str plays a too important role ...

  DB<1> $t=time;for (1..10000000) {$str="";$str.="a"."b";};print time-
+$t
10
  DB<2> $t=time;for (1..10000000) {$str="";$str=$str."a";$str.="b";};p
+rint time-$t
17
  DB<3> $t=time;for (1..10000000) {$str="";$str=$str."a"."b";};print t
+ime-$t
15

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Replies are listed 'Best First'.
Re^2: programming languages and immutable data types
by BrowserUk (Patriarch) on May 03, 2010 at 22:55 UTC

    There are other ways of mutating a string than just appending to the end. For example: 

    $n=1e3; 
cmpthese -1,{
    a=>q[
        my$s = chr(0)x$n; 
        for my $i ( 0..length($s)-1){         
            substr $s, $i, 1, chr(1);
         }                                 
     ],
    b=>q[
        my$s = chr(0)x$n; 
        for my $i ( 0..length($s)-1){ 
             $s = substr($s,0,$i-1) . chr(1) . substr $s,$i+1;
        } 
    ]
};;
    Rate    b    a
b  811/s   -- -85%
a 5525/s 581%   --

$n=1e4;
cmpthese -1,{
    a=>q[
        my$s = chr(0)x$n; 
        for my $i ( 0..length($s)-1){        
             substr $s, $i, 1, chr(1);
        }                                  
    ],
    b=>q[
        my$s = chr(0)x$n; 
        for my $i ( 0..length($s)-1){ 
            $s = substr($s,0,$i-1) . chr(1) . substr $s,$i+1;
        } 
    ]
};;
    Rate     b     a
b 13.0/s    --  -98%
a  556/s 4173%    --

$n=1e5; 
cmpthese -1,{
    a=>q[
        my$s = chr(0)x$n; 
        for my $i ( 0..length($s)-1){         
            substr $s, $i, 1, chr(1);
        }                                  
    ],
    b=>q[
        my$s = chr(0)x$n; 
        for my $i ( 0..length($s)-1){ 
            $s = substr($s,0,$i-1) . chr(1) . substr $s,$i+1;
        } 
    ]
};;
            (warning: too few iterations for a reliable count)

     s/iter      b      a
b      12.5     --  -100%
a 1.77e-002 70277%     --

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    Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
    "Science is about questioning the status quo. Questioning authority".
    In the absence of evidence, opinion is indistinguishable from prejudice.
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