If perl is invocation by reference, why is this happening?
I read that perl uses call-by-reference when executing routines. I made a simple piece of code to test this property, but it behaves as if perl was the default:
$x=50;
$y=70;
sub interchange {
($x1, $y1) = @_;
$z1 = $x1;
$x1 = $y1;
$y1 = $z1;
print "x1:$x1 y1:$y1\n";
}
&interchange ($x, $y);
print "x:$x y:$y\n";
This leads to the following output:
$ perl example.pl
x1:70 y1:50
x:50 y:70
If the arguments were processed by a callback method, shouldn't x be equal to x1 and y equal to y1?
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To change values outside of sub, you will need to change the values @_
.
The following sub interchange
makes the values change:
sub interchange {
($x1, $y1) = @_; # this line copies the values to 2 new variables
$z1 = $x1;
$x1 = $y1;
$y1 = $z1;
$_[0] = $x1; # this line added to change value outside sub
$_[1] = $y1; # this line added to change value outside sub
print "x1:$x1 y1:$y1\n";
}
This gives the result:
x1:70 y1:50
x:70 y:50
More information here: http://www.cs.cf.ac.uk/Dave/PERL/node51.html
But, quoting the article:
You can see that the function may have affected the @array variable in the main program. This is generally considered bad programming practice because it does not isolate what the function does from the rest of the program.
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Perl is always definitely referenced by reference. You are declaring ($x1, $y1) = @_
copying the original argument values, since @_ contains aliases to the original parameters.
From the perlsub manpage:
Any arguments passed are displayed in the @_ array. Therefore, if you call a function with two arguments, they will be stored in $ [0] and $ [1]. The @_ array is a local array, but its elements are aliases for real scalar parameters. In particular, if the $ _ [0] element is updated, the corresponding argument is updated (or an error is raised if it is not updatable).
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I'm just getting started with Perl, and I believe you are misunderstanding what you are passing to a subroutine. When you pass $ x and $ y, you are passing the scalars $ x and $ y. You need to explicitly pass a reference that is also a scalar (the only thing ever allowed to pass to subroutines). I understand where you came from thinking that this is a call by reference, as for arrays and hashes, as you need to pass references to them.
This code should do what you are looking for:
#!/usr/bin/perl
$x=50;
$y=70;
sub interchange {
($x1, $y1) = @_;
$z1 = $$x1; # Dereferencing $x1
$$x1 = $$y1; # Dereferencing $x1 and $y1
$$y1 = $z1; # Dereferencing $y1
print "x1:$$x1 y1:$$y1\n";
}
&interchange (\$x, \$y); # Passing references to $x and $y, not their values
print "x:$x y:$y\n";
I am passing references to $ x and $ y using \ $ x and \ $ y. Then I use $$ x and $$ y to dereference them inside the subroutine.
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