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

Hi,

I have a simple, general question regarding a real small issue that bothers me:

I'm printing a list of elements on the fly, so I don't have prior knowledge about the number of printed elements. I want a simple format where the elements are separated by a comma (elem1, elem2...) or something similar. Now, if I use a simple loop like:

while(elements to check) {
  if (elem should be printed) {
     print elem . ","
  }
}

[download]
I get a comma after the last element...

I know this sounds quite stupid, but is there a way to handle this?

Replies are listed 'Best First'.
Re: printing lists...
by Ratazong (Monsignor) on Jul 27, 2010 at 10:39 UTC

    You don't know the last element ... but you know the first. Using that knowledge, you can enhance your logic as follows:

    my $first = 0;                         # a flag to remember the first 
+element

while(elements to check) {             # *
  if (elem should be printed) {        # *

     if ($first == 0) { $first = 1; }  # don't print a comma before th
+e first element
     else             { print ",";  }  # print a comma

     print elem                        # *
  }
}

    [download]

    HTH, Rata

    Update: marked the pseudocode of the original poster with # * to reduce confusion
[reply]
[d/l]
[select]
      Clever approach. Minor glitch in implementation.
      Update: that should be "glitch in my interpretation." Neither Ratazong's nor OP's data is necessarsarily formatted as I did it.

      my $first=0;

while (<DATA>) {
    chomp ($_);
    my $element = $_;
    if ($first == 0) { 
        $first = 1;   # don't print a comma before the first element
    } else {
         print ",";    # print a comma
    }
     print $element;
}

print "\n";

__DATA__
foo
bar
blivitz
this is a line
1
2
a
b
cde

      [download]

      Ratazzong produces:

      foo
,bar
,blivitz
,this is a line
,1
,2
,a
,b
,cde

      [download]

      instead of (what I understand to be) OP's desire:

      foo,bar,blivitz,this is a line,1,2,a,b,cde
[reply]
[d/l]
[select]
Re: printing lists...
by cdarke (Prior) on Jul 27, 2010 at 12:01 UTC 
    my @elements = qw(The quick brown fox);
local $, = ',';
print @elements;

    [download]
    Gives: 
    The,quick,brown,fox

    [download]
    Or you could use $" with interpolation.
[reply]
[d/l]
[select]
Re: printing lists...
by johngg (Canon) on Jul 27, 2010 at 13:09 UTC

    Some examples.

    $ perl -E '
> @arr = qw{ apple pear plum };
> say @arr;
> say qq{@arr};
> say do {
>    local $" = q{, };
>    qq{@arr};
>    };
> say join q{, }, @arr;
> print join qq{\n}, @arr;
> say q{OOPS!! no trailing newline :-(};
> print join qq{\n}, @arr, q{};
> say q{BETTER!! empty string joined on the end :-)};'
applepearplum
apple pear plum
apple, pear, plum
apple, pear, plum
apple
pear
plumOOPS!! no trailing newline :-(
apple
pear
plum
BETTER!! empty string joined on the end :-)
$

    [download]

    Cheers,

    JohnGG

[reply]
[d/l]
Re: printing lists...
by Anonymous Monk on Jul 27, 2010 at 10:24 UTC
[reply]

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