I was glancing through some code I had written in my Perl class and I noticed this.
my ($string) = #_;
my #stringarray = split(//, $string);
I am wondering two things:
The first line where the variable is in parenthesis, this is something you do when declaring more than one variable and if I removed them it would still work right?
The second question would be what does the #_ do?
The #_ variable is an array that contains all the parameters passed into a subroutine.
The parentheses around the $string variable are absolutely necessary. They designate that you are assigning variables from an array. Without them, the #_ array is assigned to $string in a scalar context, which means that $string would be equal to the number of parameters passed into the subroutine. For example:
sub foo {
my $bar = #_;
print $bar;
}
foo('bar');
The output here is 1--definitely not what you are expecting in this case.
Alternatively, you could assign the $string variable without using the #_ array and using the shift function instead:
sub foo {
my $bar = shift;
print $bar;
}
Using one method over the other is quite a matter of taste. I asked this very question which you can check out if you are interested.
When you encounter a special (or punctuation) variable in Perl, check out the perlvar documentation. It lists them all, gives you an English equivalent, and tells you what it does.
Perl has two different contexts, scalar context, and list context. An array '#_', if used in scalar context returns the size of the array.
So given these two examples, the first one gives you the size of the #_ array, and the other gives you the first element.
my $string = #_ ;
my ($string) = #_ ;
Perl has three 'Default' variables $_, #_, and depending on who you ask %_. Many operations will use these variables, if you don't give them a variable to work on. The only exception is there is no operation that currently will by default use %_.
For example we have push, pop, shift, and unshift, that all will accept an array as the first parameter.
If you don't give them a parameter, they will use the 'default' variable instead. So 'shift;' is the same as 'shift #_;'
The way that subroutines were designed, you couldn't formally tell the compiler which values you wanted in which variables. Well it made sense to just use the 'default' array variable '#_' to hold the arguments.
So these three subroutines are (nearly) identical.
sub myjoin{
my ( $stringl, $stringr ) = #_;
return "$stringl$stringr";
}
sub myjoin{
my $stringl = shift;
my $stringr = shift;
return "$stringl$stringr";
}
sub myjoin{
my $stringl = shift #_;
my $stringr = shift #_;
return "$stringl$stringr";
}
I think the first one is slightly faster than the other two, because you aren't modifying the #_ variable.
The variable #_ is an array (hence the # prefix) that holds all of the parameters to the current function.
Related
I use #_ in a subroutine to get a parameter which is assigned as a reference of an array, but the result dose not showing as an array reference.
My code is down below.
my #aar = (9,8,7,6,5);
my $ref = \#aar;
AAR($ref);
sub AAR {
my $ref = #_;
print "ref = $ref";
}
This will print 1 , not an array reference , but if I replace #_ with shift , the print result will be a reference.
can anyone explain why I can't get a reference using #_ to me ?
This is about context in Perl. It is a crucial aspect of the language.
An expression like
my $var = #ary;
attempts to assign an array to a scalar.
That doesn't make sense as it stands and what happens is that the right-hand side is evaluated to the number of elements of the array and that is assigned to $var.
In order to change that behavior you need to provide the "list context" to the assignment operator.† In this case you'd do
my ($var) = #ary;
and now we have an assignment of a list (of array elements) to a list (of variables, here only $var), where they are assigned one for one. So here the first element of #ary is assigned to $var. Please note that this statement plays loose with the elusive notion of the "list."
So in your case you want
my ($ref) = #_;
and the first element from #_ is assigned to $ref, as needed.
Alternatively, you can remove and return the first element of #_ using shift, in which case the scalar-context assignment is fine
my $ref = shift #_;
In this case you can also do
my $ref = shift;
since shift by default works on #_.
This is useful when you want to remove the first element of input as it's being assigned so that the remaining #_ is well suited for further processing. It is often done in object-oriented code.
It is well worth pointing out that many operators and builtin facilities in Perl act differently depending on what context they are invoked in.
For some specifics, just a few examples: the regex match operator returns true/false (1/empty string) in scalar context but the actual matches in list context,‡ readdir returns a single entry in scalar context but all of them in list context, while localtime shows a bit more distinct difference. This context-sensitive behavior is in every corner of Perl.
User level subroutines can be made to behave that way via wantarray.
†
See Scalar vs List Assignment Operator
for a detailed discussion
‡
See it in perlretut and in perlop for instance
When you assign an array to a scalar, you're getting the size of the array. You pass one argument (a reference to an array) to AAR, that's why you get 1.
To get the actual parameters, place the local variable in braces:
sub AAR {
my ($ref) = #_;
print "ref = $ref\n";
}
This prints something like ref = ARRAY(0x5566c89a4710).
You can then use the reference to access the array elements like this:
print join(", ", #{$ref});
Other thread that I read: What does assigning 'shift' to a variable mean?
I also used perldoc -f shift:
shift ARRAY
shift
Shifts the first value of the array off and returns it, shortening the array by 1 and moving everything down. If there are no elements in the array, returns the undefined value. If ARRAY is omitted, shifts the #_ array within the lexical scope of subroutines and formats, and the #ARGV array outside of a subroutine and also within the lexical scopes established by the eval STRING, BEGIN {}, INIT {}, CHECK {}, UNITCHECK {} and END {} constructs.
See also unshift, push, and pop. shift and unshift do the same thing to the left end of an array that pop and push do to the right end.
I understand outside of subroutines, the array is #ARGV, and inside arguments are passed through #_
I've read countless tutorials on how to use the shift function, but it's always about arrays, and how it removes the first element at the beginning of the array and returns it. But I see sometimes
$eg = shift;
~do more things here~
It seems as if nothing is making sense to me, and I feel like I can't continue reading more until I understand how this works as it's a "basic building block" to the language.
I'm not quite sure if an example of code is needed, as I believe the same principles apply to all programs that use shift. But if wrong I can provide some examples of code.
It depends on your context.
In all cases, shift removes the element at list index 0 and returns it, shifting all remaining elements down one.
Inside a sub, shift without an argument (bare shift) operates on the #_ list of subroutine parameters.
Suppose I call mysub('me', '22')
sub mysub {
my $self = shift; # return $_[0], moves everything else down, $self = 'me', #_ = ( '22' )
my $arg1 = shift; # returns $_[0], moves everything down, $arg1 = '22', #_ = ()
}
Outside a sub, it operates on the #ARGV list of command line parameters.
Given an argument, shift operates on that list.
my #list1 = ( 1,2,3,4,5 );
my $first = shift #list1; # $first = 1, #list1 = (2,3,4,5)
It removes the first element from an array and returns it.
If #_ contains the elements ("foo","bar",123) then the statement
$eg = shift; # same as $eg = shift #_
Assigns the value "foo" to the variable $eg, and leaves #_ containing the elements ("bar",123).
This is very much a basic building block of the language. You will frequently see this construction at the beginning of subroutines as it is one of the ways to copy the arguments to the subroutine (#_) into other variables.
sub func {
my $x = shift; # put first arg into $x
my $y = shift; # put second arg into $y
my #z = #_; # put remaining args into #z
...
}
$r = func(1,2,"foo");
I was wondering what this subroutine does in Perl. I believe i have the general idea but i'm wondering about some of the syntax.
sub _init
{
my $self = shift;
if (#_) {
my %extra = #_;
#$self{keys %extra} = values %extra;
}
}
Here is what i think it does: essentially add any "extra" key-value pairs to the nameless hash refereced by the variable $self. Also i'm not 100% sure about this but i think my $self = shift is actually referring to the variable $self that called the _init() subroutine.
My specific questions are:
Is $self actually referring to the variable that called the subroutine _init() ?
What does the #$ syntax mean when writing #$self{keys %extra} = values %extra;
Your understanding is correct. This allows the user of a class to set any parameters they want into the object.
Yes. If you call, for example, $myobject->_init('color', 'green'), then this code will set $myobject->{'color'} = 'green'.
This is a somewhat confusing hash operation. keys %extra is a list (obviously of keys). We are effectively using a "hash slice" here. Think of it like an array slice, where you can call #$arrayref[1, 3, 4]. We use the # sign here because we're talking about a list - it's the list of values corresponding to the list of keys keys %extra in the hash referenced by $self.
Another way to write this would be:
foreach my $key (keys %extra) {
$self->{$key} = $extra{$key};
}
Is $self actually referring to the variable that called the subroutine _init()?
Variables don't call subroutines.
The invocant (what's on the left of the -> in ->_init()) is passed to the method as its first argument, and you placed this in $self. (shift() is short for shift(#_) in subs.)
What does the #$ syntax mean when writing #$self{keys %extra} = values %extra;
#hash{LIST} is a hash slice.
#{ EXPR }{LIST} is a hash slice where the hash to slice is specified through a reference. The curlies are optional when EXPR is simple scalar lookup, so #{ $hash_ref }{LIST} can be written as #$hash_ref{LIST}.
The method add the arguments to %$self, the hash-based object used as the invocant. It could also have been written as follows:
%$self = ( %$self, #_ );
I was always sure that if I pass a Perl subroutine a simple scalar, it can never change its value outside the subroutine. That is:
my $x = 100;
foo($x);
# without knowing anything about foo(), I'm sure $x still == 100
So if I want foo() to change x, I must pass it a reference to x.
Then I found out this is not the case:
sub foo {
$_[0] = 'CHANGED!';
}
my $x = 100;
foo($x);
print $x, "\n"; # prints 'CHANGED!'
And the same goes for array elements:
my #arr = (1,2,3);
print $arr[0], "\n"; # prints '1'
foo($arr[0]);
print $arr[0], "\n"; # prints 'CHANGED!'
That kinda surprised me. How does this work? Isn't the subroutine only gets the value of the argument? How does it know its address?
In Perl, the subroutine arguments stored in #_ are always aliases to the values at the call site. This aliasing only persists in #_, if you copy values out, that's what you get, values.
so in this sub:
sub example {
# #_ is an alias to the arguments
my ($x, $y, #rest) = #_; # $x $y and #rest contain copies of the values
my $args = \#_; # $args contains a reference to #_ which maintains aliases
}
Note that this aliasing happens after list expansion, so if you passed an array to example, the array expands in list context, and #_ is set to aliases of each element of the array (but the array itself is not available to example). If you wanted the latter, you would pass a reference to the array.
Aliasing of subroutine arguments is a very useful feature, but must be used with care. To prevent unintended modification of external variables, in Perl 6 you must specify that you want writable aliased arguments with is rw.
One of the lesser known but useful tricks is to use this aliasing feature to create array refs of aliases
my ($x, $y) = (1, 2);
my $alias = sub {\#_}->($x, $y);
$$alias[1]++; # $y is now 3
or aliased slices:
my $slice = sub {\#_}->(#somearray[3 .. 10]);
it also turns out that using sub {\#_}->(LIST) to create an array from a list is actually faster than [ LIST ] since Perl does not need to copy every value. Of course the downside (or upside depending on your perspective) is that the values remain aliased, so you can't change them without changing the originals.
As tchrist mentions in a comment to another answer, when you use any of Perl's aliasing constructs on #_, the $_ that they provide you is also an alias to the original subroutine arguments. Such as:
sub trim {s!^\s+!!, s!\s+$!! for #_} # in place trimming of white space
Lastly all of this behavior is nestable, so when using #_ (or a slice of it) in the argument list of another subroutine, it also gets aliases to the first subroutine's arguments:
sub add_1 {$_[0] += 1}
sub add_2 {
add_1(#_) for 1 .. 2;
}
This is all documented in detail in perldoc perlsub. For example:
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.
Perl passes arguments by reference, not by value. See http://www.troubleshooters.com/codecorn/littperl/perlsub.htm
If I pass a hash to a sub:
parse(\%data);
Should I use a variable to $_[0] first or is it okay to keep accessing $_[0] whenever I want to get an element from the hash? clarification:
sub parse
{ $var1 = $_[0]->{'elem1'};
$var2 = $_[0]->{'elem2'};
$var3 = $_[0]->{'elem3'};
$var4 = $_[0]->{'elem4'};
$var5 = $_[0]->{'elem5'};
}
# Versus
sub parse
{ my $hr = $_[0];
$var1 = $hr->{'elem1'};
$var2 = $hr->{'elem2'};
$var3 = $hr->{'elem3'};
$var4 = $hr->{'elem4'};
$var5 = $hr->{'elem5'};
}
Is the second version more correct since it doesn't have to keep accessing the argument array, or does Perl end up interpereting them the same way anyhow?
In this case there is no difference because you are passing reference to hash. But in case of passing scalar there will be difference:
sub rtrim {
## remove tailing spaces from first argument
$_[0] =~ s/\s+$//;
}
rtrim($str); ## value of the variable will be changed
sub rtrim_bugged {
my $str = $_[0]; ## this makes a copy of variable
$str =~ s/\s+$//;
}
rtrim($str); ## value of the variable will stay the same
If you're passing hash reference, then only copy of reference is created. But the hash itself will be the same. So if you care about code readability then I suggest you to create a variable for all your parameters. For example:
sub parse {
## you can easily add new parameters to this function
my ($hr) = #_;
my $var1 = $hr->{'elem1'};
my $var2 = $hr->{'elem2'};
my $var3 = $hr->{'elem3'};
my $var4 = $hr->{'elem4'};
my $var5 = $hr->{'elem5'};
}
Also more descriptive variable names will improve your code too.
For a general discussion of the efficiency of shift vs accessing #_ directly, see:
Is there a difference between Perl's shift versus assignment from #_ for subroutine parameters?
Is 'shift' evil for processing Perl subroutine parameters?
As for your specific code, I'd use shift, but simplify the data extraction with a hash slice:
sub parse
{
my $hr = shift;
my ($var1, $var2, $var3, $var4, $var5) = #{$hr}{qw(elem1 elem2 elem3 elem4 elem5)};
}
I'll assume that this method does something else with these variables that makes it worthwhile to keep them in separate variables (perhaps the hash is read-only, and you need to make some modifications before inserting them into some other data?) -- otherwise why not just leave them in the hashref where they started?
You are micro-optimizing; try to avoid that. Go with whatever is most readable/maintainable. Usually this would be the one where you use a lexical variable, since its name indicates its purpose...but if you use a name like $data or $x this obviously doesn't apply.
In terms of the technical details, for most purposes you can estimate the time taken by counting the number of basic ops perl will use. For your $_[0], an element lookup in a non-lexical array variable takes multiple ops: one to get the glob, one to get the array part of the glob, one or more to get the index (just one for a constant), and one to look up the element. $hr, on the other hand is a single op. To cater to direct users of #_, there's an optimization that reduces the ops for $_[0] to a single combined op (when the index is between 0 and 255 inclusive), but it isn't used in your case because the hash-deref context requires an additional flag on the array element lookup (to support autovivification) and that flag isn't supported by the optimized op.
In summary, using a lexical is going to be both more readable and (if you using it more than once) imperceptibly faster.
My rule is that I try not to use $_[0] in subroutines that are longer than a couple of statements. After that everything gets a user-defined variable.
Why are you copying all of the hash values into variables? Just leave them in the hash where they belong. That's a much better optimization than the one you are thinking about.
Its the same although the second is more clear
Since they work, both are fine, the common practice is to shift off parameters.
sub parse { my $hr = shift; my $var1 = $hr->{'elem1'}; }