In code that uploads excel spreadsheets it gives me the data in array ref:
for( #{$listref} ){...
I access it with $_->{'whateverthehashkeyis'} and have no problem.
What I need to do is pass the hash I am accessing in the current iteration of the loop to another subroutine.
This is where I am having problems. I have tried different things with no luck.
This DOES NOT work, but it should be an example of what I need to do
%args = #{$_};
$results = &format_trading_card_preview_item(\%args);
....
sub format_trading_card_preview_item
{
my %args = shift;
I think what I need to do is dereference the hash to send it over. Is that right?
Thanks in advance for any help
It looks like $listref is a reference to an array of hash references.
If you need to use the variable holding the hash references then it is better if you name that variable instead of using the default scalar $_
There is also no point in dereferencing the hash and copying it to %args, only to take a reference to that hash and pass it as a parameter to your subroutine
And it is wrong to call a subroutine with an ampersand & character, and has been so ever since Perl v5.5 landed over seventeen years ago
Your loop should look like this
for my $item ( #$listref ) {
format_trading_card_preview_item($item);
}
Within the subroutine, it depends a lot on what you want to do with the hash passed in, but you don't say anything about that, so it's probably best to leave it as a reference and write
sub format_trading_card_preview_item {
my ($item) = #_;
...
}
or you could use the statement modifier form of for, like this
format_trading_card_preview_item($_) for #$listref;
To answer your question, you don't need to dereference the hash reference in order to pass it to another subroutine. Creating a shallow copy and then taking a reference to that new hash is inefficient, but it would technically work just fine.
However, your problem is that you're confusing hashes and arrays by using the syntax to dereference an array reference on something that is actually a hash reference. In fact, you should have gotten an error message basically saying the same thing:
Not an ARRAY reference at foo.pl line ...
What you actually want to do is something like this:
for my $href (#$listref) { # variable names could be better
# do something
my $results = format_trading_card_preview_item($href);
# do something else
}
sub format_trading_card_preview_item {
my $args = shift;
print $args->{foo};
return 42;
}
Check out perlreftut and perlref for more information on Perl references and nested data structures.
Related
I am calling a perl subroutine
&ProcessInput($line, \%txcountershash, \%txhash);
I have this definition:
sub ProcessInput() {
my $word;
my $counterkey;
my $line = $_[0];
my $countershash = $_[1];
my $outputhash = $_[2];
# remove all the blanks from the line
$line =~ s/\s+//g;
my ($port,$counter,$value,$datetime) = split('\|',$line,4);
my $counterdesc = $countershash{$counter};
the line that causes the problem is the last line. It says that global symbol %countershash requires explicit package name. I'm not sure why it's giving me the error. There aren't any problems otherwise, if I comment that line out the script runs. The hash is set up as an error code for a key and a description that is the value. I'm trying to get the value at a particular key in $countershash so I can add the error description to the output hash.
The problem is dereferencing. you should dereference the hash inside the subroutine
my $counterdesc = $countershash->{$counter};
-> this is called arrow operator which is used to deference the array and hashes.
The code $counterhash{$counter} means "look up the key $counter in the hash %counterhash". You don't have a hash called %counterhash, you have a hash reference in a scalar variable called $counterhash. %counterhash and $counterhash are two completely different variables.
In order to look up an element in a hash reference, you need to use different syntax.
my $counterdesc = $countershash->{$counter};
Please don't use the $$countershash{$counter} syntax (previously) used in another answer. That will only confuse whoever needs to maintain your code in six months time (which might well be you).
I am looking for a solution to Perl's warning
"keys on reference is experimental at"
I get this from code like this:
foreach my $f (keys($normal{$nuc}{$e})) {#x, y, and z
I found something similar on StackOverflow here:
Perl throws "keys on reference is experimental"
but I don't see how I can apply it in my situation.
How can I get the keys to multiple keyed hashes without throwing this error?
keys %{$normal{$nuc}{$e}}
E.g. dereference it first.
If you had a reference to start off with, you don't need {} E.g.:
my $ref = $normal{$nuc}{$e};
print keys %$ref;
The problem is that $normal{$nuc}{$e} is a hash reference, and keys will officially only accept a hash. The solution is simple—you must dereference the reference—and you can get around this by writing
for my $f ( keys %{ $normal{$nuc}{$e} } ) { ... }
but it may be wiser to extract the hash reference into an intermediate variable. This will make your code much clearer, like so
my $hr = $normal{$nuc}{$e};
for my $f ( keys %$hr ) { ... }
I would encourage you to write more meaningful variable names. $f and $e may tell you a lot while you're writing it, but it is sure to cause others problems, and it may even come back to hit you a year or two down the line
Likewise, I am sure that there is a better identifier than $hr, but I don't know the meaning of the various levels of your data structure so I couldn't do any better. Please call it something that's relevant to the data that it points to
keys $hash_ref
is the reason for the warning keys on references is experimental
Solution:
keys %{ $hash_ref }
Reference:
https://www.perl.com/pub/2005/07/14/bestpractices.html/
I'm using a subroutine to make a few different hash maps. I'm currently passing the hashmap by reference, but this conflicts when doing it multiple times. Should I be passing the hash by value or passing the hash reference?
use strict;
use warnings;
sub fromFile($){
local $/;
local our %counts =();
my $string = <$_[0]>;
open FILE, $string or die $!;
my $contents = <FILE>;
close FILE or die $!;
my $pa = qr{
( \pL {2} )
(?{
if(exists $counts{lc($^N)}){
$counts{lc($^N)} = $counts{lc($^N)} + 1;
}
else{
$counts{lc($^N)} = '1';
}
})
(*FAIL)
}x;
$contents =~ $pa;
return %counts;
}
sub main(){
my %english_map = &fromFile("english.txt");
#my %german_map = &fromFile("german.txt");
}
main();
When I run the different txt files individually I get no problems, but with both I get some conflicts.
Three comments:
Don't confuse passing a reference with passing by reference
Passing a reference is passing a scalar containing a reference (a type of value).
The compiler passes an argument by reference when it passes the argument without making a copy.
The compiler passes an argument by value when it passes a copy of the argument.
Arguments are always passed by reference in Perl
Modifying a function's parameters (the elements of #_) will change the corresponding variable in the caller. That's one of the reason the convention to copy the parameters exists.
my ($x, $y) = #_; # This copies the args.
Of course, the primary reason for copying the parameters is to "name" them, but it saves us from some nasty surprises we'd get by using the elements of #_ directly.
$ perl -E'sub f { my ($x) = #_; "b"=~/(.)/; say $x; } "a"=~/(.)/; f($1)'
a
$ perl -E'sub f { "b"=~/(.)/; say $_[0]; } "a"=~/(.)/; f($1)'
b
One cannot pass an array or hash as an argument in Perl
The only thing that can be passed to a Perl sub is a list of scalars. (It's also the only thing that can be returned by one.)
Since #a evaluates to $a[0], $a[1], ... in list context,
foo(#a)
is the same as
foo($a[0], $a[1], ...)
That's why we create a reference to the array or hash we want to pass to a sub and pass the reference.
If we didn't, the array or hash would be evaluated into a list of scalars, and it would have to be reconstructed inside the sub. Not only is that expensive, it's impossible in cases like
foo(#a, #b)
because foo has no way to know how many arguments were returned by #a and how many were returned by #b.
Note that it's possible to make it look like an array or hash is being passed as an argument using prototypes, but the prototype just causes a reference to the array/hash to be created automatically, and that's what actually passed to the sub.
For a couple of reasons you should use pass-by-reference, but the code you show returns the hash by value.
You should use my rather than local except for built-in variables like $/, and then for only as small a scope as possible.
Prototypes on subroutines are almost never a good idea. They do something very specific, and if you don't know what that is you shouldn't use them.
Calling subroutines using the ampersand sigil, as in &fromFile("english.txt"), hasn't been correct since Perl 4, about twenty years ago. It affects the parameters delivered to a subroutine in at least two different ways and is a bad idea.
I'm not sure why you are using a file glob with my $string = <$_[0]>. Are you expecting wildcards in the filename passed as the parameter? If so then you will be opening and reading only the first matching file, otherwise the glob is unnecessary.
Lexical file handles like $fh are better than bareword file handles like FILE, and will be closed implicitly when they are destroyed - usually at the end of the block where they are declared.
I am not sure how your hash %counts gets populated. No regex on its own can fill a hash, but I will have to trust you!
Try this version. People familiar with Perl will thank you (ironically!) for not using camel-case variable names. And it is rare to see a main subroutine declared and called. That is C, this is Perl.
Update I have changed this code to do what your original regex did.
use strict;
use warnings;
sub from_file {
my ($filename) = #_;
my $contents = do {
open my $fh, '<', $filename or die qq{Unable to open "$filename": $!};
local $/;
my $contents = <$fh>;
};
my %counts;
$counts{lc $1}++ while $contents =~ /(?=(\pL{2}))/g;
return \%counts;
}
sub main {
my $english_map = from_file('english.txt');
my $german_map = from_file('german.txt');
}
main();
You can use either a reference or pass the entire hash or array. Your choice. There are two issues that might make you choose one over the other:
Passing other parameters
Memory Management
Perl doesn't really have subroutine parameters. Instead, you're simply passing in an array of parameters. What if you're subroutine is seeing which array has more elements. I couldn't do this:
foo(#first, #second);
because all I'll be passing in is one big array that combines all the members of both. This is true with hashes too. Imagine a program that takes two hashes and finds the ones with common keys:
#common_keys = common(%hash1, %hash1);
Again, I'm combining all the keys and their values in both hashes into one big array.
The only way around this issue is to pass a reference:
foo(\#first, \#second);
#common_keys = common(\%hash1, \%hash2);
In this case, I'm passing the memory location where these two hashes are stored in memory. My subroutine can use those hash references. However, you do have to take some care which I'll explain with the second explanation.
The second reason to pass a reference is memory management. If my array or hash is a few dozen entries, it really doesn't matter all that much. However, imagine I have 10,000,000 entries in my hash or array. Copying all those members could take quite a bit of time. Passing by reference saves me memory, but with a terrible cost. Most of the time, I'm using subroutines as a way of not affecting my main program. This is why subroutines are suppose to use their own variables and why you're taught in most programming courses about variable scope.
However, when I pass a reference, I'm breaking that scope. Here's a simple program that doesn't pass a reference.
#! /usr/bin/env perl
use strict;
use warnings;
my #array = qw(this that the other);
foo (#array);
print join ( ":", #array ) . "\n";
sub foo {
my #foo_array = #_;
$foo_array[1] = "FOO";
}
Note that the subroutine foo1 is changing the second element of the passed in array. However, even though I pass in #array into foo, the subroutine doesn't change the value of #array. That's because the subroutine is working on a copy (created by my #foo_array = #_;). Once the subroutine exists, the copy disappears.
When I execute this program, I get:
this:that:the:other
Now, here's the same program, except I'm passing in a reference, and in the interest of memory management, I use that reference:
#! /usr/bin/env perl
use strict;
use warnings;
my #array = qw(this that the other);
foo (\#array);
print join ( ":", #array ) . "\n";
sub foo {
my $foo_array_ref = shift;
$foo_array_ref->[1] = "FOO";
}
When I execute this program, I get:
this:FOO:the:other
That's because I don't pass in the array, but a reference to that array. It's the same memory location that holds #array. Thus, changing the reference in my subroutine causes it to be changed in my main program. Most of the time, you do not want to do this.
You can get around this by passing in a reference, then copying that reference to an array. For example, if I had done this:
sub foo {
my #foo_array = #{ shift() };
I would be making a copy of my reference to another array. It protects my variables, but it does mean I'm copying my array over to another object which takes time and memory. Back in the 1980s when I first was programming, this was a big issue. However, in this age of gigabyte memory and quadcore processors, the main issue isn't memory management, but maintainability. Even if your array or hash contained 10 million entries, you'll probably not notice any time or memory issues.
This also works the other way around too. I could return from my subroutine a reference to a hash or the entire hash. Many people like returning a reference, but this could be problematic.
In object oriented Perl programming, I use references to keep track of my objects. Normally, I'll have a reference to a hash I can use to store other values, arrays, and hashes.
In a recent program, I was counting IDs and how many times they are referenced in a log file. This was stored in an object (which is just a reference to a hash). I had a method that would return the entire hash of IDs and their counts. I could have done this:
return $self->{COUNT_HASH};
But, what happened, if the user started modifying that reference I passed? They would be actually manipulating my object without using my methods to add and subtract from the IDs. Not something that I want them to do. Instead, I create a new hash, and then return a reference to that hash:
my %hash_counts = % { $self-{COUNT_HASH} };
return \%hash_count;
This copied my reference to an array, and then I passed the reference to the array. This protects my data from outside manipulation. I could still return a reference, but the user would no longer have access to my object without going through my methods.
By the way, I like using wantarray which gives the caller a choice on how they want their data:
my %hash_counts = %{ $self->{COUNT_HASH} };
return want array ? %hash_counts : \%hash_counts;
This allows me to return a reference or a hash depending how the user called my object:
my %hash_counts = $object->totals(); # Returns a hash
my $hash_counts_ref = $object->totals(); # Returns a reference to a hash
1 A footnote: The #_ array is pointing to the same memory location as the parameters of your calling subroutine. Thus, if I pass in foo(#array) and then did $_[1] = "foo";, I would be changing the second element of #array.
I am trying to pass reference of a hash into a subroutine defined in another module in order to increase performance. In the subroutine in this other module reference is dereferenced as:
sub subRoutine{
my $hash_ref = $_[0];
my %hash = %$hash_ref;
$hash{$a_key} = $a_value;
}
So this changes the value of that key in this module BUT it doesn't change the value in the perl program where this subroutine is called. What is the reason and how can I avoid this? Is it correct to use references instead of passing hashed/returning hashed in order to improve performance?
Thanks in advance!
Best wishes!
Passing references is fine if you want to manipulate the original variable, as you do here. The problem is that you are creating a copy of the referenced hash right away, and operating on the copy.
The line my %hash = %$hash_ref; is creating a new hash and copying all the key/value pairs from the original hash. When you change this copy it has no effect on the original.
Just get rid of that line, and replace the line $hash{$a_key} = $a_value; with $hash_ref->{$a_key} = $a_value;. You are now referring to an element of the original hash, and changing it.
Don't dereference into a local copy, just use the reference:
$hash_ref->{$a_key} = $a_value;
When you say %hash = %$hash_ref, you are dereferencing it to a local copy (subroutine scope). If you want to change the value of the hash you passed into the subroutine, use
$hash_ref->{$a_key} = $a_value.
I think I have misunderstood some aspects of argument passing to functions in Perl. What's the difference between func(\#array) and func(#array)?
AFAIK, in both functions, arguments are passed by reference and in both functions we can change the elements of #array in the main program. So what's the difference? When should we use which?
#array = (1,2,3);
func(#array);
func(\#array);
sub func {
...
}
Also, how do I imitate pass-by-value in Perl? Is using #_ the only way?
It's impossible to pass arrays to subs. Subs take a list of scalars for argument. (And that's the only thing they can return too.)
You can pass a reference to an array:
func(\#array)
You can pass the elements of an array:
func(#array)
When should we use which?
If you want to pass more than just the elements of the array (e.g. pass $x, $y and #a), it can become tricky unless you pass a reference.
If you're going to process lists (e.g. sum mysub grep { ... } ...), you might not want to pass a reference.
If you want to modify the array (as opposed to just modifying the existing elements of the array), you need to pass a reference.
It can be more efficient to pass a reference for long arrays, since creating and putting one reference on the stack is faster than creating an alias for each element of a large array. This will rarely be an issue, though.
It's usually decided by one of the first two of the above. Beyond that, it's mostly a question of personal preference.
Also, how do I imitate pass-by-value in Perl?
sub foo {
my ($x) = #_; # Changing $x doesn't change the argument.
...
}
sub foo {
my #a = #_; # Changing #a or its contents
... # doesn't change the arguments.
}
AFAIK, in both functions, arguments are passed by reference and in both functions we can change the elements of #array in the main program.
"change the elements of", yes. However, in the func(#array) case, the sub has no means to make other changes to the array (truncating it, pushing, popping, slicing, passing a reference to something else, even undef'ing it).
I would avoid using the term "passed by reference", since the mechanism is completely different than Perl's references. It is less overloaded :) to say that in the sub, #_'s elements start off aliased to the elements passed to the sub.
func(\#array) passes a reference. func(#array) passes a list (of the elements in #array). As Keith pointed out, these elements are passed by reference. However, you can make a copy inside the sub in order to pass by value.
What you are after is this:
sub func {
my #array = #_;
}
This will pass a copy of the arguments of func to #array, which is a local variable within the scope of the subroutine.
Documentation here