Am trying to make a subroutine that replaces data depending on datatype: the problem is i can't get the datatype of the parameter, i used this:
sub replace {
my ($search, $replacement, $subject) = #_;
if (ref($search) eq "HASH") {
print "r is a reference to a HASH.\n";
}
elsif (ref($search) eq "SCALAR") {
print "r is a reference to a SCALAR.\n";
}
elsif (ref($search) eq "ARRAY") {
print "r is a reference to a ARRAY.\n";
}
}
my $str = "Foo";
my #arr = ("Foo");
replace($str);
replace(#arr);
But none works. am really new to perl
ref() takes a reference to something, not the something itself. Here:
replace($str);
replace(#arr);
...you are sending in the something directly. Send in a reference to the something instead, by putting a \ in front of it (which says, "take a reference to this something"):
replace(\$str);
replace(\#arr);
Output:
r is a reference to a SCALAR.
r is a reference to a ARRAY.
Note also that in your replace() function, in this line:
my ($search, $replacement, $subject) = #_;
You are effectively asking for a scalar value as the thing to search, so passing in a list (array, hash etc) will clobber $replacement and $subject if the passed in list has more than one element, so you may want to do something like this to ensure you're getting the proper params, and nothing is clobbered unexpectedly:
sub replace {
my ($search, $replacement, $subject) = #_;
die "first arg must be a ref\n" if ! ref $search;
Of course, you can do further argument checking, but this'll ensure that the first parameter can only be a reference to something. Instead of die(), you can also just return so the program doesn't crash, or print or warn and then return.
It is not stated what you want to do with it, but here's what is wrong with what you show.
The ref function shows the datatype of the reference subtmitted to it, or it returns an empty string if its argument isn't a reference at all.
So to get the expected behavior you should do
replace(\$str);
replace(\#arr);
Also, you need to add the test to your function
else (not ref $search)
for when a submitted string is not a reference.
For completeness, I should also point out an issue, explained in the answer by stevieb. When you pass an array to a function, it receives it as a flat list of arguments. With your function you clearly do not want replace(#arr). They are assigned to your list of scalar variables in order, one element to each. (As soon as there is an array variable it all goes into it.) See, for example, this post.
Related
I want to create a hash of optional query parameters that are sometimes passed to my subroutine. Sometimes a query parameter called welcome is passed in, and the value is either 1 or 0.
If that variable exists, I want to add it to a hash.
I've created a configuration value called OPTIONAL_URL_PARAMS which is a list of expected parameter names that can be passed in:
use constant OPTIONAL_URL_PARAMS => ("welcome")
So far I have:
my $tempParams = {};
if ( $optionalParam ) {
foreach my $param (#{&OPTIONAL_URL_PARAMS}) {
if ($optionalParam eq $self->{$param}) {
$tempParams->{$param} = $optionalParam;
$tempParams->{$param} =~ s/\s+//g; # strip whitespace
}
}
}
But this tries to use the value of $self->{$param} instead of its name. i.e. I want welcome to match welcome, but it's trying to match 1 instead.
I know when it comes to keys in a hash you can use keys %hash, but is there a way you can do this with regular variables?
Edit
My subroutine is being called indirectly:
my $url_handler = URL::Handler->new($param, $optionalParam);
sub new {
my $class = shift;
my $args = #_;
my $self = {
param => $args{param},
optionalParams => $args{optionalParam}
};
}
If $optionalParam's variable name is 'welcome', then I want to try and map it to the constant welcome.
This is not an answer any more, but I cannot remove it yet as there is still a discussion going on to clarify the question.
foreach my $param (#{&OPTIONAL_URL_PARAMS}) {
# ...
}
The return value of OPTIONAL_URL_PARAMS (you already got an error here and that's why you have the &, that should have told you something...) is simply a list, not an array ref. Actually at this point it should throw an error because you cannot use 1 as an array reference.
Edit
In Perl, when you pass arguments to a subroutine, all the values are flattened into a single list (reference). Specifically, if you are passing parameters to a sub, the sub doesn't know the names of the variables you originally used. It only knows their values. Therefore, if you want names as well as values, you have to pass them separately. An easy way is using a hash. E.g., in new():
my $class = shift;
my $param = shift; # The first, required parameter
my %therest = (#_); # Optional parameters, if any
Then you can say URL::Handler->new($param, 'welcome' => 1), and $therest{welcome} will have the value 1. You can also say
URL::Handler->new($param, 'welcome'=>1, 'another'=>42);
and %therest will have the two keys welcome and another.
See also some further discussion of passing whole hashes as parameters
Original
This also probably doesn't answer the question!
Some thoughts on the code from your comment.
my $url_handler = URL::Handler->new($param, $optionalParam);
sub new {
my $class = shift; # OK; refers to URL::Handler
my $args = #_; # Problematic: now $args is the _number_ of args passed (list #_ evaluated in scalar context).
my $self = {
# There is no %args hash, so the next two lines are not doing what you expect.
# I actually don't know enough perl to know what they do! :)
param => $args{param},
optionalParams => $args{optionalParam}
};
}
Some thoughts:
use strict; and use warnings; at the top of your source file, if you haven't yet.
I can think of no languages other than Algol 60 that support this idea. It goes against the idea of encapsulation, and prevents you from using an array or hash element, a function call, a constant, or an expression as the actual parameter to a call
Variable names are purely for the convenience of the programmer and have no part in the functionality of any running program. If you wished so, you could write your code using a single array #memory and have "variables" $memory[0], $memory[1] etc. But you would be bypassing the most useful part of compiler technology that allows us to relate readable text to memory locations. It is best to consider those names to be lost once the program is running
The called code should be interested only in the values passed, and it would be a nightmare if the name of a variable passed as an actual parameter were significant within the subroutine
If you were able to access the name of a variable passed as a parameter, what do you suppose would be provided to subroutine stats if the call looked like this
stats( ( $base[$i] + 0.6 + sum(#offset{qw/ x y /}) + sum(#aa) ) / #aa )
In summary, it cannot be done in general. If you need to associate a value with a name then you should probably be looking at hashes
Your code
my $url_handler = URL::Handler->new($param, $optionalParam);
sub new {
my $class = shift;
my $args = #_;
my $self = {
param => $args{param},
optionalParams => $args{optionalParam}
};
}
has a number of problems
You correctly shift the name of the class from parameter array #_, but then set my $args = #_, which sets $args to the number of elements remaining in #_. But the value of $args is irrelevant because you never use it again
You then set $self to a new anonymous hash, which is created with two elements, using the values from hash %args. But %args doesn't exist, so the value of both elements will be undef. Had you put use strict and use warnings 'all' in place you would have been alerted to this
The keys that you're using to access this non-existent hash are param and optionalParam, and I think it's more than a coincidence that they match the names of the actual parameters of the call to new
While Perl is unusual in that it allows programmatic access to its symbol tables, it is an arcane and unrecommended method. Those names are essentially hidden from the program and the programmer and while modules like Exporter must manipulate symbol tables to do their job, any such behaviour inside base-level software is very much to be avoided
Finally, you never use $self again after defining it. You should be blessing it into a class according to the $class variable (which contains the string URL::Handler) and returning it from the constructor
I hope this helps
I have a module called News (original name, I know) with a method called get_fields, this method returns all the fields that belong to the module like this
sub get_fields {
my $self = shift;
return $self;
}
Now when I call it like this in a different module where I need to do stuff to the fields
my %fields = %{ $news->get_fields };
I discovered doing it like this prevented this issue
Type of argument to keys on reference must be unblessed hashref or
arrayref
when I iterate other fields like this
foreach my $key ( keys %fields ) {
%pairs->{$key} = %fields->{$key} if %fields->{$key};
}
in order to use the values of the fields, I get this warning
Using a hash as a reference is deprecated
which is pointing back to the foreach loop.
How can I avoid this error message without getting the unbless warning back?
I think you're getting mixed up between objects and hashes. get_fields will return $self - which whilst I can't tell for certain, looks like it'll be returning a blessed object.
Now, blessed objects are quite similar to hashes, but they're not the same. You can test the difference with the ref function.
So the question is more - why are you doing this? Why are you trying to cast an object reference into a hash? Because that's what you're doing with:
my %fields = %{ $news->get_fields };
Because pretty fundamentally - even if that worked, it would be a horrible thing to do. The point, purpose and reason for objects is encapsulation - e.g. things outside the module don't meddle with stuff inside.
So why not instead have get_fields return a list of fields, which you can then iterate on and make method calls? This would really be the 'right' way to do something like this.
sub get_fields {
my ( $self ) = #_;
return keys %$self;
}
Or if you really must, embed a method within your object that returns as hash - rather than an object reference - that you can then manipulate externally.
Generally - you don't refer to hashes with a % prefix, unless you're manipulating the whole hash.
To extract a single element from %pairs you should do:
foreach my $key ( keys %pairs ) {
print $pairs{$key},"\n";
}
If the contents of $pairs{$key} is a reference, then you can use the -> to indicate that you should dereference, e.g. $pairs -> {$key}.
I have the following sub-routine.
sub Function{
my $ref = \($_[0]);
if(ref($ref) eq 'SCALAR'){
. . .
}
}
I am trying to get it to work such that passing a list, scalar or hash to it, it converts that into a reference and depending upon whether that reference is of type ARRAY, SCALAR or HASH, different actions are performed.
It could be called as
Function(%a)
Function($a)
Function(#a)
How can I make $ref be a reference to whatever is passed to the Function? My current approach isn't working.
Function(%a)
Function($a)
Function(#a)
You can't do what you are asking. There is no way inside Function to determine whether an array or a hash was passed, because, in fact, you can't pass an array or a hash to a sub in the first place. You can only pass a list; the hashes and arrays are converted to lists in the sub calls above.
You should pass a reference to the function in the first place:
Function(\%a)
Function($a)
Function(\#a)
Then you can check what it is easily:
sub Function {
my $param = shift;
if (not ref $param) {
...
} elsif (ref $param eq 'HASH') {
...
} elsif (ref $param eq 'ARRAY') {
...
}
You can use a prototyped function. Although Perl 5 prototypes are a total mess, they can be used quite well in this case. As I understand them the prototype
sub Function (\[$#%]) {...}
should solve your problem, and the reference be in $_[0].
The big disadvantage is that the variable you pass has to start with the $, #, or % character, so you can't call Function with a constant argument directly:
Function(1)
fails, but
my $temp = 1;
Function($temp)
works.
Your approach failes, because #_ is a list, and all elements are scalars.
Why not just check to see if the argument passed in is a reference and act accordingly?
sub gimme_a_ref
{
my $arg=shift;
if(ref $arg ne ref "")
{
return $arg;
}
else
{
return \$arg;
}
}
I'm passing some arguments to a function, one of which might be undefined.
$a = ($config->function(param('a'),'int'));
my module contains a function which looks like this:
sub function{
my $self = $_[0];
my $t = $_[1];
my $type = $_[2];
print "$self,$t,$type<br/>";
}
I've tried with shift instead of the #_ syntax, but there's no change.
The problem is that if $config->function is called with an undefined param('a') it prints like this:
MY_FUNC=HASH(0x206e9e0),name,
it seems that $t is being set to the value of what $type should be and the undef is being ignored completely.
undef is perfectly valid in a parameter list. I suspect that the problem here is that the param function is returning an empty list, rather than undef.
sub function {
no warnings 'uninitialized';
print join "/", #_;
}
function(undef,"foo"); # outputs "/foo"
function((), "foo"); # outputs "foo"
In the argument list to function, the param function is evaluated in list context.
sub param1 {
return; # undef in scalar context, but empty list in list context
}
sub param2 {
return undef; # scalar ctx: undef, list ctx: list with one undef elem
}
function(param1(), "foo"); # param1() -> () ... outputs "foo"
function(param2(), "foo"); # param2() -> (undef) ... outputs "/foo"
A workaround is to make sure that your param function is evaluated in scalar context.
function(scalar param1(), "foo"); # now outputs "/foo"
Note that actually saying return undef in your Perl subroutines is considered by some to be a code smell.
The CGI param method is correctly returning an empty list in the case of an error. This is standard behaviour for Perl subroutines and methods, which return a false value in the case of an erorr.
As #mob says, an explicit return undef is wrong precisely because it returns a one-element list in list context. This is a true value and implies success, which would break a lot of code if it was changed
All you need to do is change your code to extract the parameter separately
my $param = param('a');
$config->function($param, 'int'));
or if you're really keen to put the call in the parameter list then you can write
$config->function(scalar param('a'), 'int'));
or
$config->function(param('a') // undef, 'int'));
[I started this as a comment but it got too long].
Inside an argument list, arguments are evaluated in list context. Two answers have already noted that, but I think you missed it for all of the other things they were talking about.
Beyond that, param has to handle at least three cases, and the interface Lincoln chose, although sometimes annoying, is a good enough way to handle them.
The query parameter a does not appear at all:
b=1
The query parameter a appears, but with no value:
a=&b=1
The query parameter a appears with a value:
a=1&b=1
How do I tell what type of value is in a Perl variable?
$x might be a scalar, a ref to an array or a ref to a hash (or maybe other things).
ref():
Perl provides the ref() function so that you can check the reference type before dereferencing a reference...
By using the ref() function you can protect program code that dereferences variables from producing errors when the wrong type of reference is used...
$x is always a scalar. The hint is the sigil $: any variable (or dereferencing of some other type) starting with $ is a scalar. (See perldoc perldata for more about data types.)
A reference is just a particular type of scalar.
The built-in function ref will tell you what kind of reference it is. On the other hand, if you have a blessed reference, ref will only tell you the package name the reference was blessed into, not the actual core type of the data (blessed references can be hashrefs, arrayrefs or other things). You can use Scalar::Util 's reftype will tell you what type of reference it is:
use Scalar::Util qw(reftype);
my $x = bless {}, 'My::Foo';
my $y = { };
print "type of x: " . ref($x) . "\n";
print "type of y: " . ref($y) . "\n";
print "base type of x: " . reftype($x) . "\n";
print "base type of y: " . reftype($y) . "\n";
...produces the output:
type of x: My::Foo
type of y: HASH
base type of x: HASH
base type of y: HASH
For more information about the other types of references (e.g. coderef, arrayref etc), see this question: How can I get Perl's ref() function to return REF, IO, and LVALUE? and perldoc perlref.
Note: You should not use ref to implement code branches with a blessed object (e.g. $ref($a) eq "My::Foo" ? say "is a Foo object" : say "foo not defined";) -- if you need to make any decisions based on the type of a variable, use isa (i.e if ($a->isa("My::Foo") { ... or if ($a->can("foo") { ...). Also see polymorphism.
A scalar always holds a single element. Whatever is in a scalar variable is always a scalar. A reference is a scalar value.
If you want to know if it is a reference, you can use ref. If you want to know the reference type,
you can use the reftype routine from Scalar::Util.
If you want to know if it is an object, you can use the blessed routine from Scalar::Util. You should never care what the blessed package is, though. UNIVERSAL has some methods to tell you about an object: if you want to check that it has the method you want to call, use can; if you want to see that it inherits from something, use isa; and if you want to see it the object handles a role, use DOES.
If you want to know if that scalar is actually just acting like a scalar but tied to a class, try tied. If you get an object, continue your checks.
If you want to know if it looks like a number, you can use looks_like_number from Scalar::Util. If it doesn't look like a number and it's not a reference, it's a string. However, all simple values can be strings.
If you need to do something more fancy, you can use a module such as Params::Validate.
I like polymorphism instead of manually checking for something:
use MooseX::Declare;
class Foo {
use MooseX::MultiMethods;
multi method foo (ArrayRef $arg){ say "arg is an array" }
multi method foo (HashRef $arg) { say "arg is a hash" }
multi method foo (Any $arg) { say "arg is something else" }
}
Foo->new->foo([]); # arg is an array
Foo->new->foo(40); # arg is something else
This is much more powerful than manual checking, as you can reuse your "checks" like you would any other type constraint. That means when you want to handle arrays, hashes, and even numbers less than 42, you just write a constraint for "even numbers less than 42" and add a new multimethod for that case. The "calling code" is not affected.
Your type library:
package MyApp::Types;
use MooseX::Types -declare => ['EvenNumberLessThan42'];
use MooseX::Types::Moose qw(Num);
subtype EvenNumberLessThan42, as Num, where { $_ < 42 && $_ % 2 == 0 };
Then make Foo support this (in that class definition):
class Foo {
use MyApp::Types qw(EvenNumberLessThan42);
multi method foo (EvenNumberLessThan42 $arg) { say "arg is an even number less than 42" }
}
Then Foo->new->foo(40) prints arg is an even number less than 42 instead of arg is something else.
Maintainable.
At some point I read a reasonably convincing argument on Perlmonks that testing the type of a scalar with ref or reftype is a bad idea. I don't recall who put the idea forward, or the link. Sorry.
The point was that in Perl there are many mechanisms that make it possible to make a given scalar act like just about anything you want. If you tie a filehandle so that it acts like a hash, the testing with reftype will tell you that you have a filehanle. It won't tell you that you need to use it like a hash.
So, the argument went, it is better to use duck typing to find out what a variable is.
Instead of:
sub foo {
my $var = shift;
my $type = reftype $var;
my $result;
if( $type eq 'HASH' ) {
$result = $var->{foo};
}
elsif( $type eq 'ARRAY' ) {
$result = $var->[3];
}
else {
$result = 'foo';
}
return $result;
}
You should do something like this:
sub foo {
my $var = shift;
my $type = reftype $var;
my $result;
eval {
$result = $var->{foo};
1; # guarantee a true result if code works.
}
or eval {
$result = $var->[3];
1;
}
or do {
$result = 'foo';
}
return $result;
}
For the most part I don't actually do this, but in some cases I have. I'm still making my mind up as to when this approach is appropriate. I thought I'd throw the concept out for further discussion. I'd love to see comments.
Update
I realized I should put forward my thoughts on this approach.
This method has the advantage of handling anything you throw at it.
It has the disadvantage of being cumbersome, and somewhat strange. Stumbling upon this in some code would make me issue a big fat 'WTF'.
I like the idea of testing whether a scalar acts like a hash-ref, rather that whether it is a hash ref.
I don't like this implementation.