Re: arrays: shifting in while loop

This is how Perl is interpreting the critical line of your code:

C:\test>perl -MO=Deparse,p -e" print (shift @a), (shift @a), "\n";"
print(shift @a), shift @a, \'n';
-e syntax OK
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As you can see, only the first parameter is being passed to print.

If you add another set of parens:

print( (shift @a), (shift @a), "\n" );
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Or drop the parens entirely:

print shift @a, shift @a, "\n";
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Your code will (probably) work as you expect.

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Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.

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In the absence of evidence, opinion is indistinguishable from prejudice.

The start of some sanity?

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Re^2: arrays: shifting in while loop by ivanthemad (Initiate) on Mar 06, 2012 at 14:47 UTC
Thank you, you are correct! Re-examining my code in the interim between posting and your timely response, I noticed that I was in fact receiving a warning: `print (...) interpreted as function at - line 7.` [download] Thank you for the clear explanation which has rectified my misunderstanding! I will also examine further your use of `-MO=Deparse,p -e`, which looks to be very useful indeed.	[reply] [d/l] [select]
Re^3: arrays: shifting in while loop by kcott (Archbishop) on Mar 06, 2012 at 15:09 UTC
When you get a message like "print (...) interpreted as function ...", you can typically disambiguate your statement for Perl by simply adding a `+` in front of the opening parenthesis. Your original code runs like this: `ken@ganymede: ~/local/bin $ perl -Mstrict -Mwarnings -E 'my $s = "foo bar baz qux 3 3 1 3"; my @ +a = split /\s/, $s; splice(@a, 0, 4); while (@a) { print (shift @a), +(shift @a), "\n"; }' print (...) interpreted as function at -e line 1. Useless use of a constant ( ) in void context at -e line 1. 31ken@ganymede: ~/local/bin $` [download] Adding a single `+` (changing `... print (...` to `... print +(...`), you get the output you were after: `ken@ganymede: ~/local/bin $ perl -Mstrict -Mwarnings -E 'my $s = "foo bar baz qux 3 3 1 3"; my @ +a = split /\s/, $s; splice(@a, 0, 4); while (@a) { print +(shift @a), + (shift @a), "\n"; }' 33 13 ken@ganymede: ~/local/bin $` [download] This is discussed in more detail in perlop - Symbolic Unary Operators. -- Ken	[reply] [d/l] [select]
Re^3: arrays: shifting in while loop by Anonymous Monk on Mar 06, 2012 at 14:50 UTC
which looks to be very useful indeed. Its so useful, its part of the Basic debugging checklist Tutorial.	[reply]
Re^3: arrays: shifting in while loop by JavaFan (Canon) on Mar 06, 2012 at 15:07 UTC
Do note you only get said warning because you use exactly one space between `print` and the parenthesis. Drop the space, use 2 spaces, or a tab, and the warning disappears. Replace print by warn, and the warning disappears as well.	[reply] [d/l]