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

i am sorry if some has already posted this. why grep behaves differently , when we use expression and using the output of the expression directly in the place of expression 

my $t = 1;
print grep {$t^=1} qw( a b c d e );

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and using 0 directly instead using expression $t^=1 below 

my $t = 1;

print grep {0} qw( a b c d e );

my $t = 1;

print grep {$t} qw( a b c d e );

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20090717 Janitored by Corion: Added formatting, code tags, as per Writeup Formatting Tips

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Re: grep function usage
by cdarke (Prior) on Jul 13, 2009 at 13:35 UTC
    grep returns all elements for which the code block returns true. Like most languages, in Perl zero is considered false and one (non-zero) is true.
    {$t ^= 1} is carrying out an XOR operation, which produces a 1 if either of its inputs is 1 and a 0 if both 0 or 1. Therefore 1 (true) is produced for the second (b) and forth (d) iterations.

    Using just the number 1 means the result is true for all elements, so all are printed, while using zero (false) mean none are. Using $t directly is just another way of saying true, since its value is 1.
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Re: grep function usage
by moritz (Cardinal) on Jul 13, 2009 at 12:55 UTC
    Note that the block that's passed to grep is executed for every list element.

    $t^=1 changes the value of $t (and thus the return value) on each execution, 0 or 1 don't.

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Re: grep function usage
by Marshall (Canon) on Jul 13, 2009 at 13:56 UTC
    Perl grep does what Perl grep does. Perl grep is not like command line grep. Think of grep{} like a "filter" that operates on a list. If the thing within grep{$something} is TRUE, then the input is passed to the output, ie, a "filter"...THAT'S IT!

    Ok, look at this: 

    my $t = 1;
print grep {$t} qw( a b c d e ); 
What do you think that means?

this is the same as:
grep{1}qw( a b c d e );
and yields the exact same output as input list (a b c d e) 

grep {0} qw( a b c d e ); 
yields nothing! 
because this means foreach(a b c d e), pass each
item to the output (to the left) if the thing within
grep{} is true. BUT {0} is NOT true! So nothing is
passed to your print.


my $True_false = "False";
foreach (qw ( a b c d e ))
{
   print if ($True_false eq "True");
   print "" && $_; #another way
   #either way prints nothing!!!
}


Your very cute use of XOR is well, cute.

my $t = 1;
print grep {$t^=1} qw( a b c d e );

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    The above code "toggles" the value within the grep{} between TRUE and FALSE. So you get every other list element. Odd or even depending upon whether $t starts at 0 or starts at 1. This "how do I get the even things in the list" is cute, but maybe a bit too cute?

    Anyway the code works exactly as expected.

    Update: Maybe the problem is that you don't know what $t^=1 means?
    $t+=1; means $t = $t + 1; #better as $t++, but no matter here..
    $t^=1; means $t = $t ^ 1;

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