So I have an object that contains an array:
package MyObject;
sub new {
my($type) = #_;
my $self->{Params}{Status}{Packages} = [];
}
I have a add new package sub which appends onto this "Package" array like:
sub add_package {
my($self, $package_obj) = #_;
push $self->{Params}{Status}{Packages}, $package;
}
Now when I go to find all the packages in my array I have issues. Whenever I try and pull out the packages like this:
foreach my $package($self->{Params}{Status}{Packages}) {
# do something with $package.
}
This only loops through one time. Now from what I understand the hash actually stores a pointer to the array so I tried to do:
foreach my $package(#$self->{Params}{Status}{Packages}) {
# do something with $package.
}
But then there is an error saying that $self is not an array. I did notice when I do:
scalar $self->{Params}{Status}{Packages};
It returns:
#ARRAY(0xSome Address);
What am I missing? And how can I use a foreach loop to go through my array?
$self->{Params}{Status}{Packages} is a reference to an array, in Perl terminology. When you have a reference to something, put the right character in front of it to dereference it. If the reference is more than just a name with possibly some sigils in front, you need to surround it with braces. It's a matter of precedence: #$self->{Params}{Status}{Packages} is parsed as (#$self)->{Params}{Status}{Packages}, but you need
#{$self->{Params}{Status}{Packages}}
i.e. the array referenced by the expression $self->{Params}{Status}{Packages}.
In this case, you need to wrap it all in the array dereference block #{} so perl knows which portion you're trying to dereference...
for my $package (#{ $self->{Params}{Status}{Packages} }){
print "$package\n";
}
Also, just to keep things consistent, I prefer to always deref the array with the block when extracting, or inserting:
push #{ $self->{Params}{Status}{Packages} }, $package;
UPDATE: As of 5.24.0+, autoderef (using keys(), values() or each() with a reference) will almost certainly be removed, and replaced with postfix references. However, using the #{} and %{} will continue to be supported, and is backwards compatible, so I'd recommend using them at all times.
In my view, the clearest way to do this is to extract the array reference to a temporary scalar variable, which makes accessing the array very straightforward
my $packages = $self->{Params}{Status}{Packages};
for my $package ( #$packages ) {
# do something with $package.
}
Also, if you have use strict and use warnings enabled as you should, your add_package subroutine will produce the message
push on reference is experimental
This isn't something you can safely ignore. Experimental features may change their behaviour or disappear completely in later versions of Perl, and it is unwise to make use of them in production code. You can fix your subroutine in a similar way, like this
sub add_package {
my ($self, $package_obj) = #_;
my $packages = $self->{Params}{Status}{Packages};
push #$packages, $package;
}
Related
I am newbie to perl. And have been working with CSV files, JSON strings, arrays and hashes.
I have written this code, but it is giving me error. I want to use $header_copy in foreach loop.
1. my #headers=qw/January February March April May June/;
2. my $header_copy=\#headers;
3. print("$header_copy->[2]"); # Prints "March" Correctly
4. print("$header_copy[2]"); #Gives Error.
Error:
Global symbol "#header_copy" requires explicit package name at line 4
And I want to use $header_copy in for loop: like:
foreach $i ($header_copy){ **code..** }
You are taking the reference of #headers array using \#headers into $header_copy. So, before accessing it, you need to dereference it.
There are two ways(actually more than that) for it:
Using Arrow operator(->) - Most suitable for accesing a single item from arrayref
Using # { } - Suitable for iterating over arrayrefs .
$header_copy[2] will give error because you are accessing an element from arrayref without dereferencing it. The interpreter assumes it as an array #header_copy not an arrayref because the syntax says it.
Below program summarizes both approach:
#!/usr/bin/perl
use strict;
use warnings;
# define # headers
my #headers = qw/January February March April May June/;
# taken refrence of #headers array into $header_copy
my $header_copy = \#headers;
# dereferencing using arrow(->) operator
print $header_copy -> [2],"\n";
# derefrencing for iteration using #{...}
foreach(#{ $header_copy }) {
print $_,"\n";
}
The error 'Global symbol "..." requires explicit package name' is Perl-speak for "you're trying to use an undeclared variable". It even tells you the name of the undeclared variable - in this case it's #header_copy.
And if you look in your code, you'll see there's no declaration for an array called #header_copy. Oh, you have a scalar variable called $header_copy. And that contains a reference to an array (the array #headers). But that has no connection to an array called #header_copy.
So why does Perl think you want to use the array #header_copy? Well in the last line of your code, you use $header_copy[2] - which means "the third element in array #header_copy". And that generates an error because (as I've already pointed out) you don't have that array.
On the previous line you use $header_copy->[2]. And that works fine because ->[...] is the correct way to look up an element in an array that you have a reference to.
The important thing to realise is that $header_copy->[2] and $header_copy[2] mean two completely different things and refer to two completely different variables.
You also ask how you can get back to an array that you have a reference to (this is called "dereferencing"). That is simple. In general you use:
#{ expression that returns an array reference }
So, in your case, that would be:
#{ $header_copy }
But in cases where your expression is a scalar variable, you can simplify it by omitting the braces, so it becomes:
#$header_copy
So you want:
foreach my $i (#$header_copy) {
...
}
# reads data
my #headers=qw/January February March April May June/;
# use a foreach loop and store local copy of item nr in $headeritem_copy
foreach my $headeritem_copy (#headers) {
print("$headeritem_copy\n");
}
my #headers=qw/January February March April May June/;
my $header_copy=\#headers;
print("$header_copy->[2]"); # Prints "March" Correctly
De-reference the particular index
print("#{$header_copy}->[2]"); #Gives Error.
I need a data structure to keep metadata about a field in a database, which I'm going to access to write dynamic SQL.
I'm using a hash to store things like the name, maybe data type, etc. And most importantly, an array of hashes containing information about the values I want to query out of the field, and the name I want to alias them with.
When I try to access elements of that array, I get:
Global symbol "%elem" requires explicit package name at test.pl line 18.
It sounds like maybe it's having trouble registering the fact that the loop variable representing the array elements is a hash, not a scalar. If I try:
foreach my %elem
then I get:
Missing $ on loop variable at test.pl line 17 (#1)
So far I can't find the relevant Perl documentation that addresses this.
#!/usr/local/bin/perl
use warnings;
use strict;
use diagnostics;
use POSIX 'strftime';
my %struct = (
#"field" = "foobar",
"values" => [
{value => "Y", name => "FOO"}
, {value => "N", name => "BAR"}
]
);
foreach my $elem (#{$struct->{'values'}}) {
print $elem->{'value'};
}
I expect the program to print "YN" to the console.
UPDATE, as someone pointed out I needed to use %hash->{'ref'} in the loop addressing. I added it. Now, I get a notification saying that using a hash as a reference is deprecated (?) but it is printing to the console now!
When I tried running your code, I got a different error than you reported:
Global symbol "$struct" requires explicit package name
This is because you've defined a hash %struct, not a hashref $struct, so you don't need to dereference it. Thus, I changed the line
foreach my $elem (#{$struct->{'values'}}) {
to
foreach my $elem (#{$struct{'values'}}) {
(note no -> to dereference) and it ran perfectly, no errors or warnings, and emitted the output
YN
as expected.
%struct is a hash, not a hash reference. Therefore, $struct->{'values'} is not the correct way to access the values key.
for my $elem (#{$struct{values}}) {
print "$elem->{value}\n";
}
will the following code always work in perl ?
for loop iterating over #array {
# do something
if ($condition) {
remove current element from #array
}
}
Because I know in Java this results in some Exceptions, The above code is working for me for now, but I want to be sure that it will work for all cases in perl. Thanks
Well, it's said in the doc:
If any part of LIST is an array, foreach will get very confused if you
add or remove elements within the loop body, for example with splice.
So don't do that.
It's a bit better with each:
If you add or delete a hash's elements while iterating over it,
entries may be skipped or duplicated--so don't do that. Exception: In
the current implementation, it is always safe to delete the item most
recently returned by each(), so the following code works properly:
while (($key, $value) = each %hash) {
print $key, "\n";
delete $hash{$key}; # This is safe
}
But I suppose the best option here would be just using grep:
#some_array = grep {
# do something with $_
some_condition($_);
} #some_array;
I have the following code :
set arr1(a1) t1
set arr2(a2) t2
set l1 {}
lappend l1 arr1
lappend l1 arr2
set arr3(a3) $l1
foreach names [array names arr3] {
set value $arr3($names)
puts "names = $names, value = $value"
foreach ar $value {
if {[array exists $ar]} {
puts "$ar is an array"
foreach {key val} [array get $ar] {
set d1 $ar($key)
puts "ar key = $key value = $val "
}
}
}
}
but when I run the tcl script it fails for the line "set d1 $ar($key)" . The error msg is 'can't read "ar(a1)": variable isn't array' . Can you please suggest what is causing the error and how do I resolve the same.
When you use the syntax $ar($key), you are looking up the key $key in the array ar and returning its value. This is how Tcl is defined to work, it's in the basic language syntax. However, you're using the ar variable to hold a scalar value, not an array (the two are completely separate; arrays aren't values, though lists and dictionaries are). That's why you're getting the error message.
To read from an array that is named in a variable, you either need to use a longer piece of syntax so that you substitute the variable name and then read from that variable (Tcl doesn't do this for you by default, since it's quite dangerous if you're not prepared for it) or you need to make an alias to the named array variable.
Double-Substitution via set
set d1 [set ${ar}($key)]
This works because $… is really (under the hood) an alias for set with a single argument. (Well, except that it doesn't actually call the command; they both call the same C API.) We use the ${...} form to limit what the initial $ uses as its variable name. Be aware that if you put an array element name in ar, you'll get odd results from this.
Aliasing an Array or an Element
upvar 0 $ar theAlias
set d1 $theAlias($key)
The upvar command links variables together, and in particular when used with 0 as its first argument, it aliases variables in the current scope. By establishing theAlias as a fixed alias to the actual array (the one named by $ar), we can then access it just like a normal array. You could also alias directly to an element:
upvar 0 ${ar}($key) theAlias
set d1 $theAlias
Note the same syntax as used with the set solution above; we want the name of the element, not to read it. (Warning: do not alias to elements of the global env array; the code that couples to the system environment variables does not work in a friendly way with aliases.)
The main issue with using upvar is that you can't turn theAlias back into a non-aliased variable (though you can retarget the alias by calling upvar again) other than by throwing away the current stack frame (trivial for a procedure body, not too hard for a namespace via namespace delete, but problematic with the global namespace as deleting that terminates the whole Tcl interpreter).
I am trying to build a Hash that has an array as one value; this array will then contain hashes. Unfortunately, I have coded it wrong and it is being interpreted as a psuedo-hash. Please help!
my $xcHash = {};
my $xcLine;
#populate hash header
$xcHash->{XC_HASH_LINES} = ();
#for each line of data
$xcLine = {};
#populate line hash
push(#{$xcHash->{XC_HASH_LINES}}, $xcLine);
foreach $xcLine ($xcHash->{XC_HASH_LINES})
#psuedo-hash error occurs when I try to use $xcLine->{...}
$xcHash->{XC_HASH_LINES} is an arrayref and not an array. So
$xcHash->{XC_HASH_LINES} = ();
should be:
$xcHash->{XC_HASH_LINES} = [];
foreach takes a list. It can be a list containing a single scalar (foreach ($foo)), but that's not what you want here.
foreach $xcLine ($xcHash->{XC_HASH_LINES})
should be:
foreach my $xcLine (#{$xcHash->{XC_HASH_LINES}})
foreach $xcLine ($xcHash->{XC_HASH_LINES})
should be
foreach $xcLine ( #{ $xcHash->{XC_HASH_LINES} } )
See http://perlmonks.org/?node=References+quick+reference for easy to remember rules for how to dereference complex data structures.
Golden Rule #1
use strict;
use warnings;
It might seem like a fight at the beginning, but they will instill good Perl practices and help identify many syntactical errors that might otherwise go unnoticed.
Also, Perl has a neat feature called autovivification. It means that $xcHash and $xcLine need not be pre-defined or constructed as references to arrays or hashes.
The issue faced here is to do with the not uncommon notion that a scalar can hold an array or hash; it doesn't. What it holds is a reference. This means that the $xcHash->{XC_HASH_LINES} is an arrayref, not an array, which is why it needs to be dereferenced as an array using the #{...} notation.
Here's what I would do:
my %xcHash;
for each line of data:
push #{$xcHash{XC_HASH_LINES}},$xcLine;