What I want to do is create a string from an array, joining each element with a different string depending on the values of the elements being joined.

For example, I want to join this list

0,1,0,1

to get this result:

__RESULT__
0:10:1

[download]

My first thought was to assume that join worked iteratively on the elements of the array, and so if I had a matching array I could do this

my @a = (':','',':','');
my $i=0;
print join $a[$i++], 0,1,0,1; # Wrong, $i isn't incremented

[download]

But $i is not incremented during the execution of join, and so the first element of @a is used to concatenate all my list items:

__RESULT__
0:1:0:1

[download]

If join did work iteratively, I would expect it to set $_ for each iteration, and if this was true I could do this

print join $_ ? '' : ':', 0,1,0,1; # Wrong, $_ isn't set

__RESULT__
0:1:0:1

[download]

I also realised that I might be thinking naively about the iteration - that each element is not handled in isolation, but in pairs like this

(start) 
1. join $a[0] and $a[1]
2. join $a[1] and $a[2]
3. join $a[2] and $a[3] 
(finished)

[download]

And that if indeed join works on pairs of elements, setting $_ doesn't make sense - which element would be used for each iteration? (As a side thought I wonder if it would make sense to set $a and $b, like in a sort.)

But that doesn't seem to be how join works. I suppose it could also be that join does work iteratively, and on pairs, but that it only evaluates the EXPR part of join just once.

I can just forget about join and rewrite like this 

my $j = '';
$j .= $_ ? length $j ? ":$_" : $_ : $_ for (0,1,0,1);
print $j;

__RESULT__
0:10:1

[download]

But I'm surprised that there isn't some join trick that would do the job more succinctly.

Am I thinking straight? Please tell me what I'm overlooking!

 

In reply to Surprised by join by EdwardG

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