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my $var1=[{'a'=>'1','b'=>'2'},1];
print #$var1[0]->{a};
it will print 1
but, if i print like below:
print #$var1->{a};
it will print error like below
Can't use an undefined value as a HASH reference;
Can anyone explain diff between both print statement?
#$var1[0]->{a}
is usually written as
$var1->[0]{a}
The second syntax, though, is different.
#$var1->{a}
is equivalent to
#{$var1}->{a};
You can't dereference an array (#{$var1}) as a hash. Another question is why undef is reported, to which I don't know the answer.
In the first statement you print the value of key 'a' of the first element in your array (which is $var1)
In the second statement you print the value of key 'a' of your array (and get an error as array doesn't have keys)
Hope this helps
my $var1=[{'a'=>'1','b'=>'2'},1];
$var1 is array reference which contains hash reference at index 0 and scalar at index 1
to derefer $var1 to array, we have to use #$var1.(which gives the 2-element array)
And for accessing single element we have to use $$var1[0] or $var1->[0].
And again $var1->[0] is a hash reference.
To derefer it, we have to use $var1->[0]{'a'}.
But the statement "#$var1->{'a'}" is invalid, since
Hash reference is present at 0 index of the array "#$var1".
All references are scalar, Array cannot be used to derefer at hash reference.
For more information, please refer
Perl Data Structures Cookbook
Bless my Referents
what is the difference between -> and $ in Perl
where -> is infix dereference operator.
$ is also dereference operator.
what exactly meaning of this operator?
I cannot recommend strongly enough the core documentation references tutorial, perldoc perlreftut, written by the well-respected perler Mark Jason Dominus. It is concise and teaches a few simple rules for creating and using references.
I'm not saying RTFM, I'm saying, there is a great doc on this point, seek it out, it will help you!
Yes, both $ and -> are dereference operators, even though they are quite different.
Perl allows you to use references to other data. These are roughly similar to pointers in some languages. To get the original data structure, we have to dereference them. This generally involves curly braces (which can be omitted in trivial cases) and the sigil of the type we are dereferencing to. (Most values can only be dereferenced to one type, else an error is thrown).
${ $scalar_ref };
#{ $array_ref };
%{ $hash_ref };
*{ $glob_ref };
&{ $code_ref }(#args);
If the $type_ref is just a variable, the curlies can be omitted, but they are practical when we have more complex expressions.
Now the problem is to access fields in hashes or arrays, without assigning to an intermediate hash. This is where the -> operator is used:
# instead of
my %hash = %{ $hashref };
my $field = $hash{field};
# we can do this and avoid unneccessary copying :)
my $field = ${$hashref}{field}; # curlies around $hashref optional
my $field = $hashref->{field};
Similar for arrays, coderefs, and method calls on objects:
$array_ref->[$index]; $$array_ref[$index];
$code_ref->(#args); &$coderef(#args);
$object->method(#args);
Actually, what the -> operator does in the context of method calls (or what looks like them) is a bit more complex, but that doesn't seem the issue of your question.
For arrayrefs and hashrefs, you can just imagine that the $$ref[$i]-like dereference simply replaces $ref with the name of the array, which it does in a symbolic sense: $array[$i]. So the first $ sigil is that of the array element, and the second $ that of the scalar holding the reference.
My perl is a little rusty but $ always means the following is the name of a scalar. The arrow operator (->) is a very different beast. It means dereference the name and then apply the following operators. The following is an example from perlref(1):
Because of being able to omit the curlies for the simple case of $$x, people often
make the mistake of viewing the dereferencing symbols as proper operators, and wonder
about their precedence. If they were, though, you could use parentheses instead of
braces. That's not the case. Consider the difference below; case 0 is a short-hand
version of case 1, not case 2:
$$hashref{"KEY"} = "VALUE"; # CASE 0
${$hashref}{"KEY"} = "VALUE"; # CASE 1
${$hashref{"KEY"}} = "VALUE"; # CASE 2
${$hashref->{"KEY"}} = "VALUE"; # CASE 3
Case 2 is also deceptive in that you're accessing a variable called %hashref,
not dereferencing through $hashref to the hash it's presumably referencing.
That would be case 3.
They can be different ways of doing the same thing.
See http://perlmonks.org/?node=References+quick+reference; rule 3 there shows how some $ expressions can be changed to use -> instead.
I'm having troubles with a Perl script. I try to store a hash in a hash. The script is trivial:
use Data::Dumper;
my %h1=();
$h1{name}="parent";
my %h2=();
$h2{name}="child";
$h1{nested}=%h2; # store hash h2 in hash h1
print "h2:\n";
print Dumper(%h2); # works
print "h1{nested}:\n";
print Dumper($h1{nested}); # fails
The results:
h2:
$VAR1 = 'name';
$VAR2 = 'child';
h1{nested}:
$VAR1 = '1/8';
Why is the $h1{nested} not dumped as a hash, but as some kind of weird scalar (1/8)?
PS: even if this question sounds trivial - I searched SO but did not find that it was asked before.
PPS: my Perl is v5.10.1 (*) built for x86_64-linux-gnu-thread-multi
(with 53 registered patches, see perl -V for more detail)
You can only store a hashref in a hash:
$h1{nested}=\%h2;
and then you would access %h2's name by doing
$h1{nested}->{name}
In your code, %h2 is forced to scalar context, which shows you that "1/8" value, and stores that.
In perl the values stored in a list (hash or array) are always scalars. Given this, the only way to store a hash inside another hash is to store a reference to it.
$h1{'nested'} = \%h2;
or also
$h1{'nested'} = { 'name'=>'child' };
(the braces in the right hand side is a reference to an anonymous hash).
BTW, to not quote the literals in the keys is usually considered bad practice, see here
Why is the $h1{nested} not dumped as a hash, but as some kind of weird scalar (1/8)?
Because you're storing it in a scalar context!
When you do this:
$h1{nested} = %h2;
You're storing a scalar. Since %h2 is a hash, you're given the ol' fraction string. According to the Perldoc website
If you evaluate a hash in scalar context, it returns false if the hash is empty. If there are any key/value pairs, it returns true; more precisely, the value returned is a string consisting of the number of used buckets and the number of allocated buckets, separated by a slash.
That explains the 1/8 you're getting.
What you need to do is store the hash as a reference in the other hash. As others pointed out, it should be:
$h1{nested} = \%h2;
The backslash before the hash's name gives you the memory location where the hash is stored. You can use the curly braces, but I prefer the backslash notation.
Take a look at perldoc prelreftut on your computer (or on the webpage I've linked to). This will tell you how to make such things as a list of lists, hashes or hashes, lists of hashes, and hashes of lists. Just a word o` warning: If you get too complex, it'll be hard to maintain, so once you've had your fun, take a look at perldoc's Perl Object Orientation Programming Tutorial.
The perldoc command contains lots of Perl documentation including for all Perl function, Perl modules installed on your system, and even basic information about the Perl language.
I'm trying to figure out Perl subroutines and how they work.
From perlsub I understand that subroutines are call-by-reference and that an assignment (like my(#copy) = #_;) is needed to turn them into call-by-value.
In the following, I see that change is called-by-reference because "a" and "b" are changed into "x" and "y". But I'm confused about why the array isn't extended with an extra element "z"?
use strict;
use Data::Dumper;
my #a = ( "a" ,"b" );
change(#a);
print Dumper(\#a);
sub change
{
#_[0] = "x";
#_[1] = "y";
#_[2] = "z";
}
Output:
$VAR1 = [
'x',
'y'
];
In the following, I pass a hash instead of an array. Why isn't the key changed from "a" to "x"?
use strict;
use Data::Dumper;
my %a = ( "a" => "b" );
change(%a);
print Dumper(\%a);
sub change
{
#_[0] = "x";
#_[1] = "y";
}
Output:
$VAR1 = {
'a' => 'y'
};
I know the real solution is to pass the array or hash by reference using \#, but I'd like to understand the behaviour of these programs exactly.
Perl always passes by reference. It's just that sometimes the caller passes temporary scalars.
The first thing you have to realise is that the arguments of subs can be one and only one thing: a list of scalars.* One cannot pass arrays or hashes to them. Arrays and hashes are evaluated, returning a list of their content. That means that
f(#a)
is the same** as
f($a[0], $a[1], $a[2])
Perl passes by reference. Specifically, Perl aliases each of the arguments to the elements of #_. Modifying the elements #_ will change the scalars returned by $a[0], etc. and thus will modify the elements of #a.
The second thing of importance is that the key of an array or hash element determines where the element is stored in the structure. Otherwise, $a[4] and $h{k} would require looking at each element of the array or hash to find the desired value. This means that the keys aren't modifiable. Moving a value requires creating a new element with the new key and deleting the element at the old key.
As such, whenever you get the keys of an array or hash, you get a copy of the keys. Fresh scalars, so to speak.
Back to the question,
f(%h)
is the same** as
f(
my $k1 = "a", $h{a},
my $k2 = "b", $h{b},
my $k2 = "c", $h{c},
)
#_ is still aliased to the values returned by %h, but some of those are just temporary scalars used to hold a key. Changing those will have no lasting effect.
* — Some built-ins (e.g. grep) are more like flow control statements (e.g. while). They have their own parsing rules, and thus aren't limited to the conventional model of a sub.
** — Prototypes can affect how the argument list is evaluated, but it will still result in a list of scalars.
Perl's subroutines accept parameters as flat lists of scalars. An array passed as a parameter is for all practical purposes a flat list too. Even a hash is treated as a flat list of one key followed by one value, followed by one key, etc.
A flat list is not passed as a reference unless you do so explicitly. The fact that modifying $_[0] modifies $a[0] is because the elements of #_ become aliases for the elements passed as parameters. Modifying $_[0] is the same as modifying $a[0] in your example. But while this is approximately similar to the common notion of "pass by reference" as it applies to any programming language, this isn't specifically passing a Perl reference; Perl's references are different (and indeed "reference" is an overloaded term). An alias (in Perl) is a synonym for something, where as a reference is similar to a pointer to something.
As perlsyn states, if you assign to #_ as a whole, you break its alias status. Also note, if you try to modify $_[0], and $_[0] happens to be a literal instead of a variable, you'll get an error. On the other hand, modifying $_[0] does modify the caller's value if it is modifiable. So in example one, changing $_[0] and $_[1] propagates back to #a because each element of #_ is an alias for each element in #a.
Your second example is a little tricky. Hash keys are immutable. Perl doesn't provide a way to modify a hash key, aside from deleting it. That means that $_[0] is not modifiable. When you attempt to modify $_[0] Perl cannot comply with that request. It probably ought to throw a warning, but doesn't. You see, the flat list passed to it consists of unmodifiable-key followed by modifiable-value, etc. This is mostly a non-issue. I cannot think of any reason to modify individual elements of a hash in the way you're demonstrating; since hashes have no particular order you wouldn't have simple control over which elements in #_ propagate back to which values in %a.
As you pointed out, the proper protocol is to pass \#a or \%a, so that they can be referred to as $_[0]->{element} or $_[0]->[0]. Even though the notation is a little more complicated, it becomes second nature after awhile, and is much clearer (in my opinion) as to what is going on.
Be sure to have a look at the perlsub documentation. In particular:
Any arguments passed in show up in the array #_. Therefore, if you called a function with two arguments, those would be stored in $_[0] and $_[1]. The array #_ is a local array, but its elements are aliases for the actual scalar parameters. In particular, if an element $_[0] is updated, the corresponding argument is updated (or an error occurs if it is not updatable). If an argument is an array or hash element which did not exist when the function was called, that element is created only when (and if) it is modified or a reference to it is taken. (Some earlier versions of Perl created the element whether or not the element was assigned to.) Assigning to the whole array #_ removes that aliasing, and does not update any arguments.
(Note that use warnings is even more important than use strict.)
#_ itself isn't a reference to anything, it is an array (really, just a view of the stack, though if you do something like take a reference to it, it morphs into a real array) whose elements each are an alias to a passed parameter. And those passed parameters are the individual scalars passed; there is no concept of passing an array or hash (though you can pass a reference to one).
So shifts, splices, additional elements added, etc. to #_ don't affect anything passed, though they may change the index of or remove from the array one of the original aliases.
So where you call change(#a), this puts two aliases on the stack, one to $a[0] and one to $a[1]. change(%a) is more complicated; %a flattens out into an alternating list of keys and values, where the values are the actual hash values and modifying them modifies what's stored in the hash, but where the keys are merely copies, no longer associated with the hash.
Perl does not pass the array or hash itself by reference, it unfurls the entries (the array elements, or the hash keys and values) into a list and passes this list to the function. #_ then allows you to access the scalars as references.
This is roughly the same as writing:
#a = (1, 2, 3);
$b = \$a[2];
${$b} = 4;
#a now [1, 2, 4];
You'll note that in the first case you were not able to add an extra item to #a, all that happened was that you modified the members of #a that already existed. In the second case, the hash keys don't really exist in the hash as scalars, so these need to be created as copies in temporary scalars when the expanded list of the hash is created to be passed into the function. Modifying this temporary scalar will not modify the hash key, as it is not the hash key.
If you want to modify an array or hash in a function, you will need to pass a reference to the container:
change(\%foo);
sub change {
$_[0]->{a} = 1;
}
Firstly, you are confusing the # sigil as indicating an array. This is actually a list. When you call Change(#a) you are passing the list to the function, not an array object.
The case with the hash is slightly different. Perl evaluates your call into a list and passes the values as a list instead.
Since arrays and hashes can only contain scalars in Perl, why do you have to use the $ to tell the interpreter that the value is a scalar when accessing array or hash elements? In other words, assuming you have an array #myarray and a hash %myhash, why do you need to do:
$x = $myarray[1];
$y = $myhash{'foo'};
instead of just doing :
$x = myarray[1];
$y = myhash{'foo'};
Why are the above ambiguous?
Wouldn't it be illegal Perl code if it was anything but a $ in that place? For example, aren't all of the following illegal in Perl?
#var[0];
#var{'key'};
%var[0];
%var{'key'};
I've just used
my $x = myarray[1];
in a program and, to my surprise, here's what happened when I ran it:
$ perl foo.pl
Flying Butt Monkeys!
That's because the whole program looks like this:
$ cat foo.pl
#!/usr/bin/env perl
use strict;
use warnings;
sub myarray {
print "Flying Butt Monkeys!\n";
}
my $x = myarray[1];
So myarray calls a subroutine passing it a reference to an anonymous array containing a single element, 1.
That's another reason you need the sigil on an array access.
Slices aren't illegal:
#slice = #myarray[1, 2, 5];
#slice = #myhash{qw/foo bar baz/};
And I suspect that's part of the reason why you need to specify if you want to get a single value out of the hash/array or not.
The sigil give you the return type of the container. So if something starts with #, you know that it returns a list. If it starts with $, it returns a scalar.
Now if there is only an identifier after the sigil (like $foo or #foo, then it's a simple variable access. If it's followed by a [, it is an access on an array, if it's followed by a {, it's an access on a hash.
# variables
$foo
#foo
# accesses
$stuff{blubb} # accesses %stuff, returns a scalar
#stuff{#list} # accesses %stuff, returns an array
$stuff[blubb] # accesses #stuff, returns a scalar
# (and calls the blubb() function)
#stuff[blubb] # accesses #stuff, returns an array
Some human languages have very similar concepts.
However many programmers found that confusing, so Perl 6 uses an invariant sigil.
In general the Perl 5 compiler wants to know at compile time if something is in list or in scalar context, so without the leading sigil some terms would become ambiguous.
This is valid Perl: #var[0]. It is an array slice of length one. #var[0,1] would be an array slice of length two.
#var['key'] is not valid Perl because arrays can only be indexed by numbers, and
the other two (%var[0] and %var['key']) are not valid Perl because hash slices use the {} to index the hash.
#var{'key'} and #var{0} are both valid hash slices, though. Obviously it isn't normal to take slices of length one, but it is certainly valid.
See the slice section of perldata perldocfor more information about slicing in Perl.
People have already pointed out that you can have slices and contexts, but sigils are there to separate the things that are variables from everything else. You don't have to know all of the keywords or subroutine names to choose a sensible variable name. It's one of the big things I miss about Perl in other languages.
I can think of one way that
$x = myarray[1];
is ambiguous - what if you wanted a array called m?
$x = m[1];
How can you tell that apart from a regex match?
In other words, the syntax is there to help the Perl interpreter, well, interpret!
In Perl 5 (to be changed in Perl 6) a sigil indicates the context of your expression.
You want a particular scalar out of a hash so it's $hash{key}.
You want the value of a particular slot out of an array, so it's $array[0].
However, as pointed out by zigdon, slices are legal. They interpret the those expressions in a list context.
You want a lists of 1 value in a hash #hash{key} works
But also larger lists work as well, like #hash{qw<key1 key2 ... key_n>}.
You want a couple of slots out of an array #array[0,3,5..7,$n..$n+5] works
#array[0] is a list of size 1.
There is no "hash context", so neither %hash{#keys} nor %hash{key} has meaning.
So you have "#" + "array[0]" <=> < sigil = context > + < indexing expression > as the complete expression.
The sigil provides the context for the access:
$ means scalar context (a scalar
variable or a single element of a hash or an array)
# means list context (a whole array or a slice of
a hash or an array)
% is an entire hash
In Perl 5 you need the sigils ($ and #) because the default interpretation of bareword identifier is that of a subroutine call (thus eliminating the need to use & in most cases ).