Re: Scalar joining, concatenation, involving varying text

use strict;
use warnings;

my $combined = '';
my ( $a, $b, $c ) = ( 1, 0, 1 );

eval qq(if ( \$$_ ) { \$combined .= "$_=\$$_ " }) for qw( a b c );

print "$combined\n";
__END__
a=1 c=1
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Update: Quotation bug fixed. (In the original version of my solution the RHS of the concat/assignment in the eval was the single-quoted '$_=\$$_ ', resulting in $combined ending up with the value 'a=$a c=$c' instead of 'a=1 c=1').

Also, to clarify what the eval is doing, at each iteration, only the $_ get interpolated; the remaining $'s are quoted verbatim. Hence, in the first iteration, the argument to eval is the string

'if ( $a ) { $combined .= "a=$a " }'
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All the a's in this string come from interpolating $_, whereas the $'s come from the \$'s in the original double-quoted expression.

The fuss with double quotes and backslashes is a way to selectively interpolate certain values (i.e. $_'s); without all the backslashing, perl would try to interpolate $$, for example.

Perhaps this is a clearer alternative:

my $template = 'if ( $%s ) { $combined .= "%s=$%s " }';
eval sprintf $template, $_, $_, $_ for qw( a b c );
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Of course, in the last snippet, $template can alternatively be set to

'$combined .= "%s=$%s " if $%s'
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'$%s and $combined .= "%s=$%s "'
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the lowliest monk

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Re^2: Scalar joining, concatenation, involving varying text by mhearse (Chaplain) on Jun 27, 2005 at 20:26 UTC
Thanks for the reply. Could you elaborate on the logic here. Does `\` prevent interpolation until later evaluation? I just want to make sure I understand, `\$$_`allow us to substitute a variable name as the name for an implied variable within a loop.	[reply] [d/l] [select]
Re^3: Scalar joining, concatenation, involving varying text by ikegami (Patriarch) on Jun 27, 2005 at 20:32 UTC
`\` in string literals causes the next character as ordinary, so yes, it will will prevent interpolation: `$cow = 'moo!'; $_ = 'cow'; print("$cow"); # Prints moo! print("\$cow"); # Prints $cow print("\\\$cow"); # Prints \$cow print("\$$_"); # Prints $cow print(eval "$cow"); # Error executing "moo!". print(eval "\$cow"); # Prints moo! print(eval "\$$_"); # Prints moo!` [download] `qq{...}` is the same thing as `"..."`	[reply] [d/l] [select]