In Perl, I've always liked the key-value pair style of argument passing,
fruit( apples => red );
I do this a lot:
sub fruit {
my %args = #_;
$args{apples}
}
Purely for compactness and having more than one way to do it, is there a way to either:
access the key-value pairs without assigning #_ to a hash? I.e. in a single statement?
have the subroutine's arguments automatically become a hash reference, perhaps via a subroutine prototype?
Without:
assigning to a temp variable my %args = #_;
having the caller pass by reference i.e. fruit({ apples => red }); purely for aesthetics
Attempted
${%{\#_}}{apples}
Trying to reference #_, interpret that as a hash ref, and access a value by key.
But I get an error that it's not a hash reference. (Which it isn't ^.^ ) I'm thinking of C where you can cast pointers, amongst other things, and avoid explicit reassignment.
I also tried subroutine prototypes
sub fruit (%) { ... }
...but the arguments get collapsed into #_ as usual.
You can't perform a hash lookup (${...}{...}) without having a hash. But you could create an anonymous hash.
my $apples = ${ { #_ } }{apples};
my $oranges = ${ { #_ } }{oranges};
You could also use the simpler post-dereference syntax
my $apples = { #_ }->{apples};
my $oranges = { #_ }->{oranges};
That would be very inefficient though. You'd be creating a new hash for each parameter. That's why a named hash is normally used.
my %args = #_;
my $apples = $args{apples};
my $oranges = $args{oranges};
An alternative, however, would be to use a hash slice.
my ($apples, $oranges) = #{ { #_ } }{qw( apples oranges )};
The following is the post-derefence version, but it's only available in 5.24+[1]:
my ($apples, $oranges) = { #_ }->#{qw( apples oranges )};
It's available in 5.20+ if you use the following:
use feature qw( postderef );
no warnings qw( experimental::postderef );
If you're more concerned about compactness than efficiency, you can do it this way:
sub fruit {
print( +{#_}->{apples}, "\n" );
my $y = {#_}->{pears};
print("$y\n");
}
fruit(apples => 'red', pears => 'green');
The reason +{#_}->{apples} was used instead of {#_}->{apples} is that it conflicts with the print BLOCK LIST syntax of print without it (or some other means of disambiguation).
Related
I'm writing some code where I am using a subroutine as both an lvalue and an rvalue to read and write database values. The problem is, I want it to react differently based on whether it is being used as an lvalue or an rvalue.
I want the subroutine to write to the database when it is used as an lvalue, and read from the database when it is used as an rvalue.
Example:
# Write some data
$database->record_name($subscript) = $value;
# Read some data
my $value = $database->record_name($subscript);
The only way I can think of the make this work is to find a way for the subroutine to recognize whether it is being used as an lvalue or an rvalue and react differently for each case.
Is there a way to do this?
Deciding how to behave on whether it was called as an lvalue or not is a bad idea since foo(record_name(...)) would call it as an lvalue.
Instead, you should decide how to behave on whether it is used as an lvalue or not.
You can do that by returning a magical value.
use Variable::Magic qw( cast wizard );
my $wiz = wizard(
data => sub { shift; \#_ },
get => sub { my ($ref, $args) = #_; $$ref = get_record_name(#$args); },
set => sub { my ($ref, $args) = #_; set_record_name(#$args, $$ref); },
);
sub record_name :lvalue {
cast(my $rv, $wiz, #_);
return $rv;
}
A little test:
use Data::Dumper;
sub get_record_name { print("get: #_\n"); return "val"; }
sub set_record_name { print("set: #_\n"); }
my $x = record_name("abc", "def"); # Called as rvalue
record_name("abc", "def") = "xyz"; # Called as lvalue. Used as lvalue.
my $y_ref = \record_name("abc", "def"); # Called as lvalue.
my $y = $$y_ref; # Used as rvalue.
$$y_ref = "xyz"; # Used as lvalue.
Output:
get: abc def
set: abc def xyz
get: abc def
set: abc def xyz
After seeing this, you've surely learned that you should abandon the idea of using an lvalue sub. It's possible to hide all that complexity (such as by using sentinel), but the complexity remains. The fanciness is not worth all the complexity. Use separate setters and getters or use an accessor whose role is based on the number of parameters passed to it ($s=acc(); vs acc($s)) instead.
For this situation you might like to try my Sentinel module.
It provides a function you can use in the accessor, to turn it into a more get/set style approach. E.g. you could
use Sentinel qw( sentinel );
sub get_record_name { ... }
sub set_record_name { ... }
sub record_name
{
sentinel get => \&get_record_name,
set => \&set_record_name,
obj => shift;
}
At this point, the following pairs of lines of code are equivalent
$name = $record->record_name;
$name = $record->get_record_name;
$record->record_name = $new_name;
$record->set_record_name( $new_name );
Of course, if you're not needing to provide the specific get_ and set_ prefixed versions of the methods as well, you could inline them as closures.
See the module docs also for further ideas.
In my opinion, lvalue subroutines in Perl were a dumb idea. Just support ->record_name($subscript, $value) as a setter and ->record_name($subscript) as a getter.
That said, you can use the Want module, like this
use Want;
sub record_name:lvalue {
if ( want('LVALUE') ) {
...
}
else {
...
}
}
though that will also treat this as an LVALUE:
foo( $database->record_name($subscript) );
If you want only assignment statements to be treated specially, use want('ASSIGN') instead.
This might seem to be an odd thing to do, but how do I reference a hash while 'inside' the hash itself? Here's what I'm trying to do:
I have a hash of hashes with a sub at the end, like:
my $h = { A => [...], B => [...], ..., EXPAND => sub { ... } };
. I'm looking to implement EXPAND to see if the key C is present in this hash, and if so, insert another key value pair D.
So my question is, how do I pass the reference to this hash to the sub, without using the variable name of the hash? I expect to need to do this to a few hashes and I don't want to keep having to change the sub to reference the name of the hash it's currently in.
What you've got there is some nested array references, not hashes. Let's assume you actually meant that you have something like this:
my $h = { A => {...}, B => {...}, ..., EXPAND() };
In that case, you can't reference $h from within its own definition, because $h does not exist until the expression is completely evaluated.
If you're content to make it two lines, then you can do this:
my $h = { A=> {...}, B => {...} };
$h = { %$h, EXPAND( $h ) };
The general solution is to write a function that, given a hash and a function to expand that hash, returns that hash with the expansion function added to it. We can close over the hash in the expansion function so that the hash's name doesn't need to be mentioned in it. That looks like this:
use strict;
use warnings;
use 5.010;
sub add_expander {
my ($expanding_hash, $expander_sub) = #_;
my $result = { %$expanding_hash };
$result->{EXPAND} = sub { $expander_sub->($result) };
return $result;
}
my $h = add_expander(
{
A => 5,
B => 6,
},
sub {
my ($hash) = #_;
my ($maxkey) = sort { $b cmp $a } grep { $_ ne 'EXPAND' } keys %$hash;
my $newkey = chr(ord($maxkey) + 1);
$hash->{$newkey} = 'BOO!';
}
);
use Data::Dumper;
say Dumper $h;
$h->{EXPAND}->();
say Dumper $h;
Notice that we are creating $h but that the add_expander call contains no mention of $h. Instead, the sub passed into the call expects the hash it is meant to expand as its first argument. Running add_expander on the hash on the sub creates a closure that will remember which hash the expander is associated with and incorporates it into the hash.
This solution assumes that what should happen when a hash is expanded can vary by subject hash, so add_expander takes an arbitrary sub. If you don't need that degree of freedom, you can incorporate the expansion sub into add_expander.
The hash being built (potentially) happens after EXPAND() runs. I would probably use something like this:
$h = EXPAND( { A=>... } )
Where EXPAND(...) returns the modified hashref or a clone if the original needs to remain intact.
Need help figuring out how to do this. My code:
my %hash;
$hash{'1'}= {'Make' => 'Toyota','Color' => 'Red',};
$hash{'2'}= {'Make' => 'Ford','Color' => 'Blue',};
$hash{'3'}= {'Make' => 'Honda','Color' => 'Yellow',};
&printInfo(%hash);
sub printInfo{
my (%hash) = %_;
foreach my $key (keys %_{
my $a = $_{$key}{'Make'};
my $b = $_{$key}{'Color'};
print "$a $b\n";
}
}
The easy way, which may lead to problems when the code evolves, is simply by assigning the default array #_ (which contains all key-value-pairs as an even list) to the %hash which then rebuilds accordingliy. So your code would look like this:
sub printInfo {
my %hash = #_;
...
}
The better way would be to pass the hash as reference to the subroutine. This way you could still pass more parameters to your subroutine.
printInfo(\%hash);
sub PrintInfo {
my %hash = %{$_[0]};
...
}
An introduction to using references in Perl can be found in the perlreftut
You're so very, very close. There is no %_ for passing hashes, it must be passed in #_. Luckily, Hashes are assigned using a list context, so
sub printInfo {
my %hash = #_;
...
}
will make it work!
Also note, using the & in front of the subroutine call has been, in most cases, unnecessary since at least Perl 5.000. You can call Perl subroutines just like in other languages these days, with just the name and arguments. (As #mob points out in the comments, there are some instances where this is still necessary; see perlsub to understand this more, if interested.)
The best way to pass hashes and arrays is by reference. A reference is simply a way to talk about a complex data structure as a single data point -- something that can be stored in a scalar variable (like $foo).
Read up on references, so you understand how to create a reference and dereference a reference in order to get your original data back.
The very basics: You precede your data structure with a backslash to get the reference to that structure.
my $hash_ref = \%hash;
my $array_ref = \#array;
my $scalar_ref = \$scalar; #Legal, but doesn't do much for you...
A reference is a memory location of the original structure (plus a clue about the structure):
print "$hash_ref\n";
Will print something like:
HASH(0x7f9b0a843708)
To get the reference back into a useable format, you simply put the reference into the correct sigil in front:
my %new_hash = %{ $hash_ref };
You should learn about using references since this is the way you can create extremely complex data structures in Perl, and how Object Oriented Perl works.
Let's say you want to pass three hashes to your subroutine. Here are the three hashes:
my %hash1 = ( this => 1, that => 2, the => 3, other => 4 );
my %hash2 = ( tom => 10, dick => 20, harry => 30 );
my %hash3 = ( no => 100, man => 200, is => 300, an => 400, island => 500 );
I'll create the references for them
my $hash_ref1 = \%hash1;
my $hash_ref2 = \%hash2;
my $hash_ref3 = \%hash3;
And now just pass the references:
mysub ( $hash_ref1, $hash_ref2, $hash_ref3 );
The references are scalar data, so there's no problem passing them to my subroutine:
sub mysub {
my $sub_hash_ref1 = shift;
my $sub_hash_ref2 = shift;
my $sub_hash_ref3 = shift;
Now, I just dereference them, and my subroutine can use them.
my %sub_hash1 = %{ $sub_hash_ref1 };
my %sub_hash2 = %{ $sub_hash_ref2 };
my %sub_hash3 = %{ $sub_hash_ref3 };
You can see what a reference is a reference to by using the ref command:
my $ref_type = ref $sub_hash_ref; # $ref_type is now equal to "HASH"
This is useful if you want to make sure you're being passed the correct type of data structure.
sub mysub {
my $hash_ref = shift;
if ( ref $hash_ref ne "HASH" ) {
croak qq(You need to pass in a hash reference);
}
Also note that these are memory references, so modifying the reference will modify the original hash:
my %hash = (this => 1, is => 2, a => 3 test => 4);
print "$hash{test}\n"; # Printing "4" as expected
sub mysub ( \%hash ); # Passing the reference
print "$hash{test}\n"; # This is printing "foo". See subroutine:
sub mysub {
my $hash_ref = shift;
$hash_ref->{test} = "foo"; This is modifying the original hash!
}
This can be good -- it allows you to modify data passed to the subroutine, or bad -- it allows you to unintentionally modify data passed to the original subroutine.
I believe you want
my %hash;
$hash{'1'}= {'Make' => 'Toyota','Color' => 'Red',};
$hash{'2'}= {'Make' => 'Ford','Color' => 'Blue',};
$hash{'3'}= {'Make' => 'Honda','Color' => 'Yellow',};
printInfo(%hash);
sub printInfo{
my %hash = #_;
foreach my $key (keys %hash){
my $a = $hash{$key}{'Make'};
my $b = $hash{$key}{'Color'};
print "$a $b\n";
}
}
In the line printInfo(%hash) the %hash is expanded to a list with the alternating key-value pairs.
In printInfo, the #_ is this list that, and assigned to %hash it creates again the keys with their corresponding value from the alternating elements in the list.
You can pass them as
The argument list do_hash_thing( %hash )
A reference to the hash in the argument list
`do_hash_thing( #args_before, \%hash, #args_after )
As a reference by prototype, working like keys and other hash operators.
The list works like so:
sub do_hash_thing {
my %hash = #_;
...
}
do_hash_thing( %hash );
This also allows you to "stream" hash arguments as well:
do_hash_thing( %hash_1, %hash_2, parameter => 'green', other => 'pair' );
By reference works like this:
sub do_hash_thing {
my $hash_ref = shift;
...
}
do_hash_thing( \%hash, #other_args );
Here by prototype (\%#). The prototype makes perl look for a hash in the first argument and pass it by reference.
sub do_hash_thing (\%#) {
my $hash_ref = shift;
...
}
do_hash_thing( %hash => qw(other args) );
# OR
do_hash_thing %hash => qw(other args);
Caveat: prototypes don't work on methods.
I hava a perl subroutine where i would like to pass parameters as a hash
(the aim is to include a css depending on the parameter 'iconsize').
I am using the call:
get_function_bar_begin('iconsize' => '32');
for the subroutine get_function_bar_begin:
use strict;
...
sub get_function_bar_begin
{
my $self = shift;
my %template_params = %{ shift || {} };
return $self->render_template('global/bars /tmpl_incl_function_bar_begin.html',%template_params);
}
Why does this yield the error message:
Error executing run mode 'start': undef error - Can't use string ("iconsize") as a HASH ref while "strict refs" in use at CheckBar.pm at line 334
Am i doing something wrong here?
Is there an other way to submit my data ('iconsize') as a hash?
(i am still new to Perl)
EDIT: Solution which worked for me. I didn't change the call, but my function:
sub get_function_bar_begin
{
my $self = shift;
my $paramref = shift;
my %params = (ref($paramref) eq 'HASH') ? %$paramref : ();
my $iconsize = $params{'iconsize'} || '';
return $self->render_template('global/bars/tmpl_incl_function_bar_begin.html',
{
'iconsize' => $iconsize,
}
);
}
You are using the hash-dereferencing operator ( %{ } ) on the first argument of your parameter list. But that argument is not a hash reference, it's just the string 'iconsize'. You can do what you want by one of two ways:
Pass an anonymous hash reference:
get_function_bar_begin( { 'iconsize' => '32' } );
Or continue to pass a normal list, as you are right now, and change your function accordingly:
sub get_function_bar_begin {
my $self = shift;
my %template_params = #_;
}
Notice in this version that we simply assign the argument list directly to the hash (after extracting $self). This works because a list of name => value pairs is just syntactic sugar for a normal list.
I prefer the second method, since there's no particularly good reason to construct an anonymous hashref and then dereference it right away.
There's also some good information on how this works in this post: Object-Oriented Perl constructor syntax.
You're violating strict refs by trying to use the string iconsize as a hash reference.
I think you just want:
my( $self, %template_params ) = #_;
The first argument will go into $self and the rest create the hash by taking pairs of items from the rest of #_.
Passing hash with parameters as list
You need to use #_ variable instead of shift. Like this:
my %template_params = #_; ## convert key => value pairs into hash
There is different between hashes and references to hash in perl. Then you pass 'iconsize' => '32' as parameter this means list to perl, which can be interpreited as hash.
Passing hash with parameters as hash reference
But when you try %{ shift || {} } perl expect second parameter to be a hash references. In this case you can fix it in following way:
get_function_bar_begin({ 'iconsize' => '32' }); ## make anonymous hash for params
The problem is this line:
get_function_bar_begin('iconsize' => '32');
This does not pass a hash reference, as you seem to think, but a hash, which appears as a list to the callee. So when you do %{ shift }, you're only shifting the key 'iconsize', not the entire list. The solution is actually to make the second line of your function simpler:
my %template_params = #_;
I'm a little confused about what is going on in Perl constructors. I found these two examples perldoc perlbot.
package Foo;
#In Perl, the constructor is just a subroutine called new.
sub new {
#I don't get what this line does at all, but I always see it. Do I need it?
my $type = shift;
#I'm turning the array of inputs into a hash, called parameters.
my %params = #_;
#I'm making a new hash called $self to store my instance variables?
my $self = {};
#I'm adding two values to the instance variables called "High" and "Low".
#But I'm not sure how $params{'High'} has any meaning, since it was an
#array, and I turned it into a hash.
$self->{'High'} = $params{'High'};
$self->{'Low'} = $params{'Low'};
#Even though I read the page on [bless][2], I still don't get what it does.
bless $self, $type;
}
And another example is:
package Bar;
sub new {
my $type = shift;
#I still don't see how I can just turn an array into a hash and expect things
#to work out for me.
my %params = #_;
my $self = [];
#Exactly where did params{'Left'} and params{'Right'} come from?
$self->[0] = $params{'Left'};
$self->[1] = $params{'Right'};
#and again with the bless.
bless $self, $type;
}
And here is the script that uses these objects:
package main;
$a = Foo->new( 'High' => 42, 'Low' => 11 );
print "High=$a->{'High'}\n";
print "Low=$a->{'Low'}\n";
$b = Bar->new( 'Left' => 78, 'Right' => 40 );
print "Left=$b->[0]\n";
print "Right=$b->[1]\n";
I've injected the questions/confusion that I've been having into the code as comments.
To answer the main thrust of your question, since a hash can be initialized as a list of key => value pairs, you can send such a list to a function and then assign #_ to a hash. This is the standard way of doing named parameters in Perl.
For example,
sub foo {
my %stuff = #_;
...
}
foo( beer => 'good', vodka => 'great' );
This will result in %stuff in subroutine foo having a hash with two keys, beer and vodka, and the corresponding values.
Now, in OO Perl, there's some additional wrinkles. Whenever you use the arrow (->) operator to call a method, whatever was on the left side of the arrow is stuck onto the beginning of the #_ array.
So if you say Foo->new( 1, 2, 3 );
Then inside your constructor, #_ will look like this: ( 'Foo', 1, 2, 3 ).
So we use shift, which without an argument operates on #_ implicitly, to get that first item out of #_, and assign it to $type. After that, #_ has just our name/value pairs left, and we can assign it directly to a hash for convenience.
We then use that $type value for bless. All bless does is take a reference (in your first example a hash ref) and say "this reference is associated with a particular package." Alakazzam, you have an object.
Remember that $type contains the string 'Foo', which is the name of our package. If you don't specify a second argument to bless, it will use the name of the current package, which will also work in this example but will not work for inherited constructors.
.1. In Perl, the constructor is just a subroutine called new.
Yes, by convention new is a constructor. It may also perform initialization or not. new should return an object on success or throw an exception (die/croak) if an error has occurred that prevents object creation.
You can name your constructor anything you like, have as many constructors as you like, and even build bless objects into any name space you desire (not that this is a good idea).
.2. I don't get what my $type = shift; does at all, but I always see it. Do I need it?
shift with no arguments takes an argument off the head of #_ and assigns it to $type. The -> operator passes the invocant (left hand side) as the first argument to the subroutine. So this line gets the class name from the argument list. And, yes, you do need it.
.3. How does an array of inputs become the %params hash? my %params = #_;
Assignment into a hash is done in list context, with pairs of list items being grouped into as key/value pairs. So %foo = 1, 2, 3, 4;, creates a hash such that $foo{1} == 2 and $foo{3} == 4. This is typically done to create named parameters for a subroutine. If the sub is passed an odd number of arguments, an warning will be generated if warnings are enabled.
.4. What does 'my $self = {};` do?
This line creates an anonymous hash reference and assigns it to the lexically scoped variable $self. The hash reference will store the data for the object. Typically, the keys in the hash have a one-to-one mapping to the object attributes. So if class Foo has attributes 'size' and 'color', if you inspect the contents of a Foo object, you will see something like $foo = { size => 'm', color => 'black' };.
.5. Given $self->{'High'} = $params{'High'}; where does $params{'High'} come from?
This code relies on the arguments passed to new. If new was called like Foo->new( High => 46 ), then the hash created as per question 3 will have a value for the key High (46). In this case it is equivalent to saying $self->{High} = 46. But if the method is called like Foo->new() then no value will be available, and we have $self->{High} = undef.
.6. What does bless do?
bless takes a reference and associates with a particular package, so that you can use it to make method calls. With one argument, the reference is assoicated with the current package. With two arguments, the second argument specifies the package to associate the reference with. It is best to always use the two argument form, so that your constructors can be inherited by a sub class and still function properly.
Finally, I'll rewrite your hash based object accessor as I would write it using classical OO Perl.
package Foo;
use strict;
use warnings;
use Carp qw(croak);
sub new {
my $class = shift;
croak "Illegal parameter list has odd number of values"
if #_ % 2;
my %params = #_;
my $self = {};
bless $self, $class;
# This could be abstracted out into a method call if you
# expect to need to override this check.
for my $required (qw{ name rank serial_number });
croak "Required parameter '$required' not passed to '$class' constructor"
unless exists $params{$required};
}
# initialize all attributes by passing arguments to accessor methods.
for my $attrib ( keys %params ) {
croak "Invalid parameter '$attrib' passed to '$class' constructor"
unless $self->can( $attrib );
$self->$attrib( $params{$attrib} );
}
return $self;
}
Your question is not about OO Perl. You are confused about data structures.
A hash can be initialized using a list or array:
my #x = ('High' => 42, 'Low' => 11);
my %h = #x;
use Data::Dumper;
print Dumper \%h;
$VAR1 = {
'Low' => 11,
'High' => 42
};
When you invoke a method on a blessed reference, the reference is prepended to the argument list the method receives:
#!/usr/bin/perl
package My::Mod;
use strict;
use warnings;
use Data::Dumper;
$Data::Dumper::Indent = 0;
sub new { bless [] => shift }
sub frobnicate { Dumper(\#_) }
package main;
use strict;
use warnings;
my $x = My::Mod->new;
# invoke instance method
print $x->frobnicate('High' => 42, 'Low' => 11);
# invoke class method
print My::Mod->frobnicate('High' => 42, 'Low' => 11);
# call sub frobnicate in package My::Mod
print My::Mod::frobnicate('High' => 42, 'Low' => 11);
Output:
$VAR1 = [bless( [], 'My::Mod' ),'High',42,'Low',11];
$VAR1 = ['My::Mod','High',42,'Low',11];
$VAR1 = ['High',42,'Low',11];
Some points that haven't been dealt with yet:
In Perl, the constructor is just a
subroutine called new.
Not quite. Calling the constructor new is just a convention. You can call it anything you like. There is nothing special about that name from perl's point of view.
bless $self, $type;
Both of your examples don't return the result of bless explicitly. I hope that you know that they do so implicitly anyway.
If you assign an array to a hash, perl treats alternating elements in the array as keys and values. Your array is look at like
my #array = (key1, val1, key2, val2, key3, val3, ...);
When you assign that to %hash, you get
my %hash = #array;
# %hash = ( key1 => val1, key2 => val2, key3 => val3, ...);
Which is another way of saying that in perl list/hash construction syntax, "," and "=>" mean the same thing.
In Perl, all arguments to subroutines are passed via the predefined array #_.
The shift removes and returns the first item from the #_ array. In Perl OO, this is the method invocant -- typically a class name for constructors and an object for other methods.
Hashes flatten to and can be initialized by lists. It's a common trick to emulate named arguments to subroutines. e.g.
Employee->new(name => 'Fred Flintstone', occupation => 'quarry worker');
Ignoring the class name (which is shifted off) the odd elements become hash keys and the even elements become the corresponding values.
The my $self = {} creates a new hash reference to hold the instance data. The bless function is what turns the normal hash reference $self into an object. All it does is add some metadata that identifies the reference as belonging to the class.
Yes, I know that I'm being a bit of a necromancer here, but...
While all of these answers are excellent, I thought I'd mention Moose. Moose makes constructors easy (package Foo;use Moose; automatically provides a constructor called new (although the name "new" can be overridden if you'd like)) but doesn't take away any configurability if you need it.
Once I looked through the documentation for Moose (which is pretty good overall, and there are a lot more tutorial snippets around if you google appropriately), I never looked back.