2005/06/12 Perl Range Operator (.. and ...)

In a list context, this operator is easy to use and understand.

It is much more confusing in a scalar context, and is often badly explained in books and webpages. It is very easy to misunderstand how it works at all, never mind the difference between using ".." vs. "...", which can trip you up badly. Let's look at the simplest cases first and then move to the more difficult:


#!/usr/bin/perl
foreach(1 .. 10) {
 print;
}
foreach(A .. B) {
 print;
}
 

That's the simplest case. It just prints "12345678910AB". That's useful in some places, of course.

#!/usr/bin/perl
while (<>) {
  print "$. $_" if 3 .. 10;
}
 

That prints lines 3 to 10 of any input or file given to it. If we change it to "print "$. $_" if 3 ... 10;" it works the same way. But we aren't limited to line numbers. Let's create a file "g":

# stuff
# stuff
# end on same line as start
START foo  END
#
# end by itself
END
# more stuff
# more stuff
# another start
START
# and more stuff
END
# trailing stuff
 

And a Perl script to read it:

#!/usr/bin/perl   
while (<>) {
  print if (/START/ .. /END/);
}
 

Running that against our file produces:

START foo  END
START
# and more stuff
END
 

But with the three dot version:

#!/usr/bin/perl   
while (<>) {
  print if (/START/ ... /END/);
}
 

the results are different:

START foo  END
# 
# end by itself
END
START
# and more stuff
END
 

The difference is how the operator treats its left side and right side when they appear on the same line or out of order. Here's what's going on: with the two dot ".." version, the operator returns false until the left hand side is true. It then switches to returning true and keeps doing that until its right hand expression evaluates to true. But the operator REMAINS true until the next evaluation even so, so it only switches back to false when the next line is read. With the three dot "..." version, the right hand expression will not be evaluated while the left hand is false, and vice versa. Confusing? You bet.. be very careful when using these operators and test your logic against sample input very carefully. Personally, I'll usually write out a much longer loop maintaining my own flip-flops than use this, just because I often get tricked by its nuances. If your brain works better than mine, you might find these handy in quite a few situations.



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