Re: Scalar joining, concatenation, involving varying text

[tlm]

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

---

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 " }'

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 );

Of course, in the last snippet, $template can alternatively be set to

'$combined .= "%s=$%s " if $%s'

or

'$%s and $combined .= "%s=$%s "'

[mhearse]

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.

[ikegami]

\ 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!

qq{...} is the same thing as "..."