I have the following subroutine which i should pass the routine as hashtable and that hashtable should be again called inside another subroutine using perl?
input file(from linux command bdata):
NAME PEND RUN SUSP JLIM JLIMR RATE HAPPY
achandra 0 48 0 2000 50:2000 151217 100%
agutta 1 5 0 100 50:100 16561 83%
My subroutine:
sub g_usrs_data()
{
my($lines) = #_;
my $header_found = 0;
my #headers = ();
my $row_count = 0;
my %table_data = ();
my %row_data = ();
$lines=`bdata`;
#print $lines;
foreach (split("\n",$lines)) {
if (/NAME\s*PEND/) {
$header_found = 1;
#headers =split;
}
elsif (/^\s*$/)
{
$header_found=0;
}
$row_data{$row_count++} = $_;
#print $_;
}
My query:
How can i pass my subroutine as hash into another subroutine?
example:
g_usrs_data() -> this is my subroutine .
the above subroutine should be passed into another subroutine (i.e into usrs_hash as hash table)
example:
create_db(usrs_hash,$sql1m)
Subroutines can be passed around as code references. See perlreftut and perlsub.
An example with an anonymous subroutine
use warnings;
use strict;
my $rc = sub {
my #args = #_;
print "\tIn coderef. Got: |#_|\n";
return 7;
}; # note the semicolon!
sub use_rc {
my ($coderef, #other_args) = #_;
my $ret = $coderef->('arguments', 'to', 'pass');
return $ret;
}
my $res = use_rc($rc);
print "$res\n";
This silly program prints
In coderef. Got: |arguments to pass|
7
Notes on code references
The anonymous subroutine is assigned to a scalar $rc, making that a code reference
With an existing (named) sub, say func, a code reference is made by my $rc = \&func;
This $rc is a normal scalar variable, that can be passed to subroutines like any other
The sub is then called by $rc->(); where in parenthesis we can pass it arguments
Note that the syntax for creating and using them are just like for other data types
As anonymous assign by = sub { }, much like = [ ] (arrayref) and = { } (hashref)
For a named sub use & instead of a sigil, so \& for sub vs. \# (array) and \% (hash)
They are used by ->(), much like ->[] (arrayref) and ->{} (hashref)
For references in general see perlreftut. Subroutines are covered in depth in perlsub.
See for example this post on anonymous subs, with a number of answers.
For far more see this article from Mastering Perl and this article from The Effective Perler.
Related
I have been using perl for some time now.
I want to know how I can run the following operation in perl:
subtract(40)(20)
To get the result:
20
I think I would have to look at custom parsing techniques for Perl.
This is what I am looking at right now:
Devel::Declare
Devel::CallParser
and
http://www.perl.com/pub/2012/10/an-overview-of-lexing-and-parsing.html
Now, I am not sure what to look for or what to do.
Any help on HOW to go about this, WHAT to read would be appreciated. Please be clear.
Thank you.
I recommend trying Parse::Keyword. Parse::Keyword is really great for parsing custom syntax, as it lets you call back various parts of the Perl parser, such as parse_listexpr, parse_block, parse_fullstmt, etc (see perlapi).
It has a drawback in that if you use those to parse expressions that close over variables, these are handled badly, but this can be worked around with PadWalker.
Parse::Keyword (including PadWalker trickery) is what Kavorka uses; and that does some pretty complex stuff! Early versions of p5-mop-redux used it too.
Anyway, here's a demonstration of how your weird function could be parsed...
use v5.14;
use strict;
use warnings;
# This is the package where we define the functions...
BEGIN {
package Math::Weird;
# Set up parsing for the functions
use Parse::Keyword {
add => \&_parser,
subtract => \&_parser,
multiply => \&_parser,
divide => \&_parser,
};
# This package is an exporter of course
use parent 'Exporter::Tiny';
our #EXPORT = qw( add subtract multiply divide );
# We'll need these things from PadWalker
use PadWalker qw( closed_over set_closed_over peek_my );
sub add {
my #numbers = _grab_args(#_);
my $sum = 0;
$sum += $_ for #numbers;
return $sum;
}
sub subtract {
my #numbers = _grab_args(#_);
my $diff = shift #numbers;
$diff -= $_ for #numbers;
return $diff;
}
sub multiply {
my #numbers = _grab_args(#_);
my $product = 1;
$product *= $_ for #numbers;
return $product;
}
sub divide {
my #numbers = _grab_args(#_);
my $quotient = shift #numbers;
$quotient /= $_ for #numbers;
return $quotient;
}
sub _parser {
lex_read_space;
my #args;
while (lex_peek eq '(')
{
# read "("
lex_read(1);
lex_read_space;
# read a term within the parentheses
push #args, parse_termexpr;
lex_read_space;
# read ")"
lex_peek eq ')' or die;
lex_read(1);
lex_read_space;
}
return sub { #args };
}
# In an ideal world _grab_args would be implemented like
# this:
#
# sub _grab_args { map scalar(&$_), #_ }
#
# But because of issues with Parse::Keyword, we need
# something slightly more complex...
#
sub _grab_args {
my $caller_vars = peek_my(2);
map {
my $code = $_;
my $closed_over = closed_over($code);
$closed_over->{$_} = $caller_vars->{$_} for keys %$closed_over;
set_closed_over($code, $closed_over);
scalar $code->();
} #_;
}
# We've defined a package inline. Mark it as loaded, so
# that we can `use` it below.
$INC{'Math/Weird.pm'} = __FILE__;
};
use Math::Weird qw( add subtract multiply );
say add(2)(3); # says 5
say subtract(40)(20); # says 20
say multiply( add(2)(3) )( subtract(40)(20) ); # says 100
If you can live with the additions of a sigil and an arrow, you could curry subtract as in
my $subtract = sub {
my($x) = #_;
sub { my($y) = #_; $x - $y };
};
Call it as in
my $result = $subtract->(40)(20);
If the arrow is acceptable but not the sigil, recast subtract as
sub subtract {
my($x) = #_;
sub { my($y) = #_; $x - $y };
};
Invocation in this case looks like
my $result = subtract(40)->(20);
Please don't tack on broken syntax extensions on your program to solve a solved problem.
What you want are closures, and a technique sometimes called currying.
Currying is the job of transforming a function that takes multiple arguments into a function that is invoked multiple times with one argument each. For example, consider
sub subtract {
my ($x, $y) = #_;
return $x - $y;
}
Now we can create a subroutine that already provides the first argument:
sub subtract1 { subtract(40, #_) }
Invoking subtract1(20) now evaluates to 20.
We can use anonymous subroutines instead, which makes this more flexible:
my $subtract = sub { subtract(40, #_) };
$subtract->(20);
We don't need that variable:
sub { subtract(40, #_) }->(20); # equivalent to subtract(40, 20)
We can write subtract in a way that does this directly:
sub subtract_curried {
my $x = shift;
# don't return the result, but a subroutine that calculates the result
return sub {
my $y = shift;
return $x - $y;
};
}
Now: subtract_curried(40)->(20) – notice the arrow in between, as we are dealing with a code reference (another name for anonymous subroutine, or closures).
This style of writing functions is much more common in functional languages like Haskell or OCaml where the syntax for this is prettier. It allows very flexible combinations of functions. If you are interested in this kind of programming in Perl, you might want to read Higher-Order Perl.
#Heartache: Please forget this challenge as it makes no sense for the parser and the user.
You can think of using fn[x][y] or fn{x}{y} which are valid syntax variants - i.e. you can stack [] and {} but not lists,
or fn(x,y) or fn(x)->(y) which do look nice, are also valid and meaningful syntax variants.
But fn(x)(y) will not know in which context the second list should be used.
For fn(x)(y) the common interpretation would be fn(x); (y) => (y). It returns the 2nd list after evaluating the first call.
You may create the source code filter:
package BracketFilter;
use Filter::Util::Call;
sub import {
filter_add(sub {
my $status;
s/\)\(/, /g if ($status = filter_read()) > 0;
return $status ;
});
}
1;
And use it:
#!/usr/bin/perl
use BracketFilter;
subtract(40)(20);
sub subtract {
return $_[0] - $_[1];
}
I just looked in disbelief at this sequence:
my $line;
$rc = getline($line); # read next line and store in $line
I had understood all along that Perl arguments were passed by value, so whenever I've needed to pass in a large structure, or pass in a variable to be updated, I've passed a ref.
Reading the fine print in perldoc, however, I've learned that #_ is composed of aliases to the variables mentioned in the argument list. After reading the next bit of data, getline() returns it with $_[0] = $data;, which stores $data directly into $line.
I do like this - it's like passing by reference in C++. However, I haven't found a way to assign a more meaningful name to $_[0]. Is there any?
You can, its not very pretty:
use strict;
use warnings;
sub inc {
# manipulate the local symbol table
# to refer to the alias by $name
our $name; local *name = \$_[0];
# $name is an alias to first argument
$name++;
}
my $x = 1;
inc($x);
print $x; # 2
The easiest way is probably just to use a loop, since loops alias their arguments to a name; i.e.
sub my_sub {
for my $arg ( $_[0] ) {
code here sees $arg as an alias for $_[0]
}
}
A version of #Steve's code that allows for multiple distinct arguments:
sub my_sub {
SUB:
for my $thisarg ( $_[0] ) {
for my $thatarg ($_[1]) {
code here sees $thisarg and $thatarg as aliases
last SUB;
}
}
}
Of course this brings multilevel nestings and its own code readability issues, so use it only when absolutely neccessary.
(sub {
print 1;
})();
sub {
print 1;
}();
I tried various ways, all are wrong...
(sub { ... }) will give you the pointer to the function so you must call by reference.
(sub { print "Hello world\n" })->();
The other easy method, as pointed out by Blagovest Buyukliev would be to dereference the function pointer and call that using the { } operators
&{ sub { print "Hello World" }}();
Yay, I didn't expect you folks to come up with that much possibilities. But you're right, this is perl and TIMTOWTDI: +1 for creativitiy!
But to be honest, I use hardly another form than the following:
The Basic Syntax
my $greet = sub {
my ( $name ) = #_;
print "Hello $name\n";
};
# ...
$greet->( 'asker' )
It's pretty straight forward: sub {} returns a reference to a sub routine, which you can store and pass around like any other scalar. You can than call it by dereferencing. There is also a second syntax to dereference: &{ $sub }( 'asker' ), but I personally prefer the arrow syntax, because I find it more readable and it pretty much aligns with dereferencing hashes $hash->{ $key } and arrays $array->[ $index ]. More information on references can be found in perldoc perlref.
I think the other given examples are a bit advanced, but why not have a look at them:
Goto
sub bar {goto $foo};
bar;
Rarely seen and much feared these days. But at least it's a goto &function, which is considered less harmful than it's crooked friends: goto LABEL or goto EXPRESSION ( they are deprecated since 5.12 and raise a warning ). There are actually some circumstances, when you want to use that form, because this is not a usual function call. The calling function ( bar in the given example ) will not appear in the callling stack. And you don't pass your parameters, but the current #_ will be used. Have a look at this:
use Carp qw( cluck );
my $cluck = sub {
my ( $message ) = #_;
cluck $message . "\n";
};
sub invisible {
#_ = ( 'fake' );
goto $cluck;
}
invisible( 'real' );
Output:
fake at bar.pl line 5
main::__ANON__('fake') called at bar.pl line 14
And there is no hint of an invisible function in the stack trace. More info on goto in perldoc -f goto.
Method Calls
''->$foo;
# or
undef->$foo;
If you call a method on an object, the first parameter passed to that method will be the invocant ( usually an instance or the class name ). Did i already say that TIMTOWTCallAFunction?
# this is just a normal named sub
sub ask {
my ( $name, $question ) = #_;
print "$question, $name?\n";
};
my $ask = \&ask; # lets take a reference to that sub
my $question = "What's up";
'asker'->ask( $question ); # 1: doesn't work
my $meth_name = 'ask';
'asker'->$meth_name( $question ); # 2: doesn't work either
'asker'->$ask( $question ); # 1: this works
In the snippet above are two calls, which won't work, because perl will try to find a method called ask in package asker ( actually it would work if that code was in the said package ). But the third one succeeds, because you already give perl the right method and it doesn't need to search for it. As always: more info in the perldoc I can't find any reason right now, to excuse this in production code.
Conclusion
Originally I didn't intend to write that much, but I think it's important to have the common solution at the beginning of an answer and some explanations to the unusual constructs. I admit to be kind of selfish here: Every one of us could end up maintaining someones code, who found this question and just copied the topmost example.
There is not much need in Perl to call an anonymous subroutine where it is defined. In general you can achieve any type of scoping you need with bare blocks. The one use case that comes to mind is to create an aliased array:
my $alias = sub {\#_}->(my ($x, $y, $z));
$x = $z = 0;
$y = 1;
print "#$alias"; # '0 1 0'
Otherwise, you would usually store an anonymous subroutine in a variable or data structure. The following calling styles work with both a variable and a sub {...} declaration:
dereference arrow: sub {...}->(args) or $code->(args)
dereference sigil: &{sub {...}}(args) or &$code(args)
if you have the coderef in a scalar, you can also use it as a method on regular and blessed values.
my $method = sub {...};
$obj->$method # same as $method->($obj)
$obj->$method(...) # $method->($obj, ...)
[1, 2, 3]->$method # $method->([1, 2, 3])
[1, 2, 3]->$method(...) # $method->([1, 2, 3], ...)
I'm endlessly amused by finding ways to call anonymous functions:
$foo = sub {say 1};
sub bar {goto $foo};
bar;
''->$foo; # technically a method, along with the lovely:
undef->$foo;
() = sort $foo 1,1; # if you have only two arguments
and, of course, the obvious:
&$foo();
$foo->();
You need arrow operator:
(sub { print 1;})->();
You might not even need an anonymous function if you want to run a block of code and there is zero or one input. You can use map instead.
Just for the side effect:
map { print 1 } 1;
Transform data, take care to assign to a list:
my ($data) = map { $_ * $_ } 2;
# ------------------------------------------------------
# perl: filter array using given function
# ------------------------------------------------------
sub filter {
my ($arr1, $func) = #_;
my #arr2=();
foreach ( #{$arr1} ) {
push ( #arr2, $_ ) if $func->( $_ );
};
return #arr2;
}
# ------------------------------------------------------
# get files from dir
# ------------------------------------------------------
sub getFiles{
my ($p) = #_;
opendir my $dir, $p or die "Cannot open directory: $!";
my #files=readdir $dir;
closedir $dir;
#return files and directories that not ignored but not links
return filter \#files, (sub { my $f = $p.(shift);return ((-f $f) || (-d $f)) && (! -l $f) } );
}
I've always been somewhat confused about the purpose and usage of anonymous subs in perl. I understand the concept, but looking for examples and explanations on the value of this practice.
To be clear:
sub foo { ... } # <--- named sub
sub { ... } # <--- anonymous sub
For example:
$ perl -e 'print sub { 1 }'
CODE(0xa4ab6c)
Tells me that sub returns a scalar value. So, I can do:
$ perl -e '$a = sub { 1 }; print $a'
For the same output as above. This of course holds true for all scalar values, so you can load arrays or hashes with anonymous subs.
The question is, how do I use these subs? Why would I want to use them?
And for a gold star, is there any problem which can only be resolved with an anonymous sub?
Anonymous subroutines can be used for all sorts of things.
Callbacks for event handling systems:
my $obj = Some::Obj->new;
$obj->on_event(sub {...});
Iterators:
sub stream {my $args = \#_; sub {shift #$args}}
my $s = stream 1, 2, 3;
say $s->(); # 1
say $s->(); # 2
Higher Order Functions:
sub apply (&#) {
my $code = shift;
$code->() for my #ret = #_;
#ret
}
my #clean = apply {s/\W+/_/g} 'some string', 'another string.';
say $clean[0]; # 'some_string'
Creating aliased arrays:
my $alias = sub {\#_}->(my $x, my $y);
$alias[0]++;
$alias[1] = 5;
say "$x $y"; # '1 5''
Dynamic programming with closures (such as creating a bunch of subroutines that only differ by a small amount):
for my $name (qw(list of names)) {
no strict 'refs';
*$name = sub {... something_with($name) ...};
}
There is no situation where an anonymous subroutine can do anything that a named subroutine can not. The my $ref = sub {...} constructor is equivalent to the following:
sub throw_away_name {...}
my $ref = \&throw_away_name;
without having to bother with deciding on a unique 'throw_away_name' for each sub.
The equivalence also goes the other way, with sub name {...} being equivalent to:
BEGIN {*name = sub {...}}
So other than the name, the code reference created by either method is the same.
To call a subroutine reference, you can use any of the following:
$code->(); # calls with no args
$code->(1, 2, 3); # calls with args (1, 2, 3)
&$code(); # calls with no args
&$code; # calls with whatever #_ currently is
You can even use code references as methods on blessed or unblessed scalars:
my $list = sub {#{ $_[0] }};
say for [1 .. 10]->$list # which prints 1 .. 10
You can use it to create iterators.
use strict;
use warnings;
use 5.012;
sub fib_it {
my ($m, $n) = (0, 0);
return sub {
my $val = ( $m + $n );
$val = 1 unless $val;
($m, $n) = ($n, $val);
return $val;
}
}
my $fibber = fib_it;
say $fibber->() for (1..3); ### 1 1 2
my $fibber2 = fib_it;
say $fibber2->() for (1..5); ### 1 1 2 3 5
say $fibber->() for (1..3); #### 3 5 8
Anonymous subroutines can be used to create closures.
Closure is a notion out of the Lisp world that says if you define an anonymous function in a particular lexical context, it pretends to run in that context even when it's called outside the context.
perlref
What's a closure?
Here's something similar you might have seen before:
#new_list = map { $_ + 1 } #old_list;
And also:
#sorted = sort { $a <=> $b } #unsorted;
Neither of those are anonymous subs, but their behavior can be imitated in your functions with anonymous subs. They don't need the sub keyword because the functions are (essentially) prototyped to have their first argument be a subroutine, and Perl recognizes that as a special case where sub can be left off. (The functions also set the requisite variables to meaningful values before calling the subroutines you provided in order to simplify argument passing, but that's not related.)
You can write your own map-like function:
sub mapgrep (&#) { # make changes and also filter based on defined-ness
my ($func, #list) = #_;
my #new;
for my $i (#list) {
my $j = $func->($i);
push #new, $j if defined $j;
}
}
The magic to make it work with $_ is a bit much to write here - the above version only works for subs that take arguments.
Well I wrote a SAX parser for perl that is event driven. You can pass anonymous subs to the begin/end events on an element.
my $str = "<xml><row><data></data></row></xml>":
my $parser = SAXParser->new();
$parser->when('row')->begin(sub {
my ($element) = #_;
push(#rows, $row);
});
$parser->when('row')->end(sub {
## do something like serialize it or whatever
});
$parser->parse($str);
They are generally used when you want to pass a sub to another bit of code. Often this is a case of "When X happens (in third party code) do Y".
For example. When defining an attribute in Moose, you can specify the default value of that attribute using a sub. Given a class which has, as part of its definition:
has 'size' => (
is => 'ro',
default =>
sub { ( 'small', 'medium', 'large' )[ int( rand 3 ) ] },
predicate => 'has_size',
);
Whenever an instance of that class is created without an explicit size being passed, the sub will be called and the return value will be the size for that object.
If we switch to another language to give a different example, you'll find a similar concept in JavaScript.
var b = document.getElementById('my_button').
b.addEventListener('click', function (e) { alert('clicked!'); });
In your example, you haven't actually called created subroutine. Call is performed with either &$a or $a->() syntax. What you've done is that you stored a reference to subroutine in $a, then stringifyed it and printed result. Compare:
my $a = sub {1};
my $b = sub {1};
print join("\n", $a, $a->(), $b, $b->());
These are subs for the lazy programmer. You can use them for local throw-away functions and can save some typing. Instead of
sub x { ... }
my $function_ptr = \&x;
you can now use
my $function_ptr = sub { ... };
The anonymous functions are also private, and can only be accessed through the $function_ptr, so they don't have an entry in the symbol table.
I do not get to understand how the Perl read($buf) function is able to modify the content of the $buf variable. $buf is not a reference, so the parameter is given by copy (from my c/c++ knowledge). So how come the $buf variable is modified in the caller ?
Is it a tie variable or something ? The C documentation about setbuf is also quite elusive and unclear to me
# Example 1
$buf=''; # It is a scalar, not a ref
$bytes = $fh->read($buf);
print $buf; # $buf was modified, what is the magic ?
# Example 2
sub read_it {
my $buf = shift;
return $fh->read($buf);
}
my $buf;
$bytes = read_it($buf);
print $buf; # As expected, this scope $buf was not modified
No magic is needed -- all perl subroutines are call-by-alias, if you will. Quoth perlsub:
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).
For example:
sub increment {
$_[0] += 1;
}
my $i = 0;
increment($i); # now $i == 1
In your "Example 2", your read_it sub copies the first element of #_ to the lexical $buf, which copy is then modified "in place" by the call to read(). Pass in $_[0] instead of copying, and see what happens:
sub read_this {
$fh->read($_[0]); # will modify caller's variable
}
sub read_that {
$fh->read(shift); # so will this...
}
read() is a built-in function, and so can do magic. You can accomplish something similar with your own functions, though, by declaring a function prototype:
sub myread(\$) { ... }
The argument declaration \$ means that the argument is implicitly passed as a reference.
The only magic in the built-in read is that it works even when called indirectly or as a filehandle method, which doesn't work for regular functions.