I am new to Perl. I am still trying to learn the syntax of it. I have seen someone using // and //= in Perl but I couldn't find any resources on the web that explain this.
Can someone explain to me what exactly does it mean in layman terms? And what it actually does?
As kjprice mentioned, // is the logical 'defined or' operator and is documented here on the perlop page and the relevant excerpt is
it's exactly the same as ||, except that it tests the left hand side's definedness instead of its truth. Thus, EXPR1 // EXPR2 returns the value of EXPR1 if it's defined, otherwise, the value of EXPR2 is returned.
You can think of
my $var = EXPR1 // EXPR2;
as a short hand way of writing:
my $var;
if ( defined EXPR1 ) {
$var = EXPR1;
} else {
$var = EXPR2;
}
I often use this to either assign a default value to a variable unless a supplied by command line or config file value is supplied. Something like:
my $var = $config_version // 'foo';
The //= is a variation of this with an assignemnt mixed in. That same perlop page says this:
Modifying an assignment is equivalent to doing the assignment and then
modifying the variable that was assigned to.
For //= that means instead of writing something like
my $var = EXPR1 // EXPR2;
You could write
my $var = EXPR1;
$var //= EXPR2;
and get equivalent values.
From the perldoc perlop:
Logical Defined-Or
Although it has no direct equivalent in C, Perl's // operator is related to its C-style or. In fact, it's exactly the same as ||, except that it tests the left hand side's definedness instead of its truth. Thus, EXPR1 // EXPR2 returns the value of EXPR1 if it's defined, otherwise, the value of EXPR2 is returned. (EXPR1 is evaluated in scalar context, EXPR2 in the context of // itself). Usually, this is the same result as defined(EXPR1) ? EXPR1 : EXPR2 (except that the ternary-operator form can be used as a lvalue, while EXPR1 // EXPR2cannot). This is very useful for providing default values for variables. If you actually want to test if at least one of $a and $b is defined, use defined($a // $b).
So:
$NODEFINED // $DEFINED # will return the value of defined
$DEFINED1 // $DEFINED2 # will return the value of $DEFINED1
$a //= $b;
is shorthand for:
$a = $a // $b;
So $a will be set to the value $b ONLY if $a is undefined.
The $a //= 42; form is useful for setting a default for a variable that may not yet be defined.
Related
I'm trying to write like:
my $q='select name as N from names';
my $sth=$dbh->prepare( $q ) ;
$sth->execute;
$test->{ $_->{N} } = 1 while $sth->fetchrow_hashref();
but $_ is undef. Does the return value get stored somewhere accessible without explicitly assigning it? Tim, it sure would be handy if it got stored somewhere!
The while loop does not always implicitly assign a value to the default operator $_. As described in perldoc perlsyn:
If the condition expression of a while statement is based on any of a
group of iterative expression types then it gets some magic treatment.
The affected iterative expression types are readline, the
input operator, readdir, glob, the globbing operator, and
each. If the condition expression is one of these expression types,
then the value yielded by the iterative operator will be implicitly
assigned to $_.
(...and otherwise it is not, is implied here)
To do what you want, you need to do
... while $_ = $sth->fetchrow_hashref();
Or better yet, use the idiomatic syntax:
while (my $row = $sth->fetchrow_hashref()) {
$test->{ $row->{N} } = 1;
}
I'm new to Perl and came across this piece of code at work, I search for a while but did not find the answer. Can anyone help to explain its function in plain english? thanks.
my $abc = delete $args{ 'abc' } // croak 'some information!';
From this page here: http://perldoc.perl.org/perlop.html#Logical-Defined-Or
Although it has no direct equivalent in C, Perl's // operator is related to its C-style or. In fact, it's exactly the same as ||, except that it tests the left hand side's definedness instead of its truth. Thus, EXPR1 // EXPR2 returns the value of EXPR1 if it's defined, otherwise, the value of EXPR2 is returned. (EXPR1 is evaluated in scalar context, EXPR2 in the context of // itself). Usually, this is the same result as defined(EXPR1) ? EXPR1 : EXPR2 (except that the ternary-operator form can be used as a lvalue, while EXPR1 // EXPR2 cannot, and EXPR1 will only be evaluated once). This is very useful for providing default values for variables. If you actually want to test if at least one of $a and $b is defined, use defined($a // $b).
Check for Logical Defined-Or in perlop, it is similar to || but it checks for undef value (not false one).
Although it has no direct equivalent in C, Perl's // operator is related to its C-style or. In fact, it's exactly the same as ||, except that it tests the left hand side's definedness instead of its truth.
So in short,
my $abc = delete $args{ 'abc' } // croak 'some information!';
will croak when $args{ 'abc' } returns undef value.
I'm new to Perl and came across this piece of code at work, I search for a while but did not find the answer. Can anyone help to explain its function in plain english? thanks.
my $abc = delete $args{ 'abc' } // croak 'some information!';
From this page here: http://perldoc.perl.org/perlop.html#Logical-Defined-Or
Although it has no direct equivalent in C, Perl's // operator is related to its C-style or. In fact, it's exactly the same as ||, except that it tests the left hand side's definedness instead of its truth. Thus, EXPR1 // EXPR2 returns the value of EXPR1 if it's defined, otherwise, the value of EXPR2 is returned. (EXPR1 is evaluated in scalar context, EXPR2 in the context of // itself). Usually, this is the same result as defined(EXPR1) ? EXPR1 : EXPR2 (except that the ternary-operator form can be used as a lvalue, while EXPR1 // EXPR2 cannot, and EXPR1 will only be evaluated once). This is very useful for providing default values for variables. If you actually want to test if at least one of $a and $b is defined, use defined($a // $b).
Check for Logical Defined-Or in perlop, it is similar to || but it checks for undef value (not false one).
Although it has no direct equivalent in C, Perl's // operator is related to its C-style or. In fact, it's exactly the same as ||, except that it tests the left hand side's definedness instead of its truth.
So in short,
my $abc = delete $args{ 'abc' } // croak 'some information!';
will croak when $args{ 'abc' } returns undef value.
I am new to perl. Can anyone please explain the meaning of // operator in perl.
It's the definedness operator. The expression:
A // B
will return A if it's defined, otherwise B.
It's very useful for getting default values if the source of the information is not defined, with things like:
$actualBalance = $balanceFromBank // 0;
or:
$confirmation = $userInput // "N";
See the relevant part of the perlop page for more detail and make a link of perlop for future reference, since Google searches and the punctuational Perl code don't mix that well :-)
The // operator is a logical defined-or. Perlop says:
Although it has no direct equivalent in C, Perl's // operator is
related to its C-style or. In fact, it's exactly the same as ||,
except that it tests the left hand side's definedness instead of its
truth. Thus, EXPR1 // EXPR2 returns the value of EXPR1 if it's
defined, otherwise, the value of EXPR2 is returned. (EXPR1 is
evaluated in scalar context, EXPR2 in the context of // itself).
Usually, this is the same result as defined(EXPR1) ? EXPR1 : EXPR2
(except that the ternary-operator form can be used as a lvalue, while
EXPR1 // EXPR2 cannot). This is very useful for providing default
values for variables. If you actually want to test if at least one of
$a and $b is defined, use defined($a // $b) .
In short: It returns the left side if that expression is defined (as in not undef), or the right side.
my $foo = undef;
say $foo // 42;
# 42
my $bar = 'bar';
say $bar // 42;
# bar
It's so called defined-or operator, which has been implemented in Perl 5.10. Example from the doc:
The following expression:
$a // $b
... is merely equivalent to
defined $a ? $a : $b
And the statement:
$c //= $d;
... can now be used instead of
$c = $d unless defined $c;
Here's how || and // are different:
use 5.010;
my $rabbits = 0;
say $rabbits || 1; # 1, as 0 || 1 evaluates to 1
say $rabbits // 1; # 0, as 0 is not `undef`
That is "defined-or". $abc // "default" is equivalent to defined($abc) ? $abc : "default". Meaning if the left side of // has a defined value then that value is used, otherwise the right side of it.
See "Logical defined-or" in the perlop man page.
is defined or
like,
my $a //= 3;
will assign 3 to $a
it is different from ||, which is simply, or in that:
my $a = "";
$a //= 3;
print "|$a|\n";
$a = "";
$a ||=5;
print "|$a|\n";
will print only |5|, because in the first case $a is defined (with a false value), while in the second it matters if $a evaluates to true or not.
Why does // have lower precedence than == in (at least) perl 5.010?
For example, this
use 5.010;
my $may_be_undefined = 1;
my $is_equal_to_two = ($may_be_undefined//0 == 2);
say $is_equal_to_two;
prints (for me) very unexpected result.
It's because of the category of operators which // falls under, aswell as ==.
== is an "equality operator" though // falls under the category of "C-style logical operators".
As an example; && is in the same "category" as //, with that said both of the statements below are equivalent when it comes to operator precedence. That might make it easier to understand?
print "hello world" if $may_be_undefined && 0 == 2;
print "hello world" if $may_be_undefined // 0 == 2;
Documentation of C-style Logical Defined-Or ( // )
Although it has no direct equivalent in C, Perl's // operator is related to its C-style or. In fact, it's exactly the same as ||, except that it tests the left hand side's definedness instead of its truth.
Thus, $a // $b is similar to defined($a) || $b (except that it returns the value of $a rather than the value of defined($a)) and yields the same result as defined($a) ? $a : $b (except that the ternary-operator form can be used as a lvalue, while $a // $b cannot).
This is very useful for providing default values for variables. If you actually want to test if at least one of $a and $b is defined, use defined($a // $b) .
The ||, // and && operators return the last value evaluated (unlike C's || and &&, which return 0 or 1).
Documentation of Operator Precedence and Associativity