Perl5 = (equals) operator precedence

Question

 $a,$b,$c = 1,2,3;
 print "$a, $b, $c\n";

returns

 , , 1

So does = (equals) take higher precedence than the tuple construction - doing this?

 $a,$b,($c=1),2,3;

Answer 1

Yes. There's a precedence table in perlop. Assignment operators are level 19, and comma is level 20. In general, Perl's operators have the same precedence as the corresponding C operators (for those operators that have a corresponding C operator).

If you meant ($a,$b,$c) = (1,2,3); you have to use the parens.

Answer 2

The comma operator as you used it (in scalar context) is not for tuple construction, it's for evaluating several expressions and returning the last one.

Perl does things differently depending on context, it decides what to do depending on if it's expecting a scalar value, a list, nothing at all, ... See perldoc perldata's section on Context for an introduction.

So, if you do:

perl -e '$a = (1 and 4,2,0); print"$a\n"'

You get 0, because 4,2,0 is evaluated in scalar context, and behaves like C's comma operator, evaluating expressions between commas and returning the result of the last one.

If you force 4,2,0 to be evaluated in list context:

perl -e '$a = (1 and @a=(4,2,0)); print"$a\n"'

You get 3, because assigning to an array forces list context (the additional parenthesis are there to solve the precedence issue cjm mentioned), and the value of a list in scalar context (forced by being the RHS of an and in scalar context) is the number of elements it has (logical and in Perl returns the last expression evaluated, instead of a boolean value as in other programming languages).

So, as cjm said, you need to do:

($a,$b,$c) = (1,2,3);

to deal with precedence and force list context.

Notice the difference between:

$ perl -e '$a,$b,$c = (7,6,8); print "$a $b $c\n"'
8

The comma operator is evaluated in scalar context, and returns 8.

$ perl -e '($a,$b,$c) = (7,6,8); print "$a $b $c\n"'
7 6 8

The comma operator is evaluated in list context, and returns a list.

$ perl -e '$a,$b,$c = () = (7,6,8); print "$a $b $c\n"'
3

The comma operator is evaluated in list context, returning a list, then the assignment to $c forces scalar context, returning the number of elements in the list.