cutlass2006 has asked for the wisdom of the Perl Monks concerning the following question:

Dear monkists,

I note here that the concept of an automated counter variable in looping structures have been discussed before.

My question is why does this output the current loop count ? 

for (1..10){
print
}

could this not be subverted as a loop counter ? tia.

update: slapping forehead: thanks....

Replies are listed 'Best First'.
Re: perl loop counter
by Fletch (Bishop) on Jul 18, 2008 at 15:31 UTC

    Because unless otherwise specified a foreach loop (which can also be spelled for as you've done) uses the usual default variable $_, and likewise if not given an explicit argument print prints the contents of said default variable.

    The cake is a lie.
    The cake is a lie.
    The cake is a lie.

[reply]
[d/l]
Re: perl loop counter
by zentara (Cardinal) on Jul 18, 2008 at 15:38 UTC
    Additionally, you will probably be suffering from buffering. You need to put 
    $|++;

    [download]
    at the top of your script, so it will print immediately.

    If you put a newline "\n" in your print statement, it will print immediately in your loop; but with no newline, you won't get a print until 2k is ready to print, and it will all come out in 2k chunks.

    I'm not really a human, but I play one on earth CandyGram for Mongo
[reply]
[d/l]
      Only if you're printing to a file. If STDOUT is connected to a terminal (as it is most of the time) it won't be buffered.

      Update: I must be drinking the stupid juice today. Ttys are line-buffered, duh.

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        ...it won't be buffered

        Actually, STDOUT is line buffered by default when connected to a tty, i.e. - as zentara said - output will be written either upon encountering a newline, or when the buffer has filled up (or when the program terminates, and the buffer gets flushed).

        Try playing with something like 

        for (1..3000){
    print "$_ ";
    sleep 5 unless $_ % 1000;
}

        [download]
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[d/l]
        Are you sure? On my linux xterm, this prints out in roughly 2k chunks. 
        #!/usr/bin/perl

for (1..6000){
  print $_;
  select (undef,undef,undef,.001); 
 #small delay to see buffering effects
}

        [download]
        I'm not really a human, but I play one on earth CandyGram for Mongo
[reply]
[d/l]

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