daily notes[7]

perl note

1. A hash in perl can be initialized with array,for example:

my %numbers = ("one", 1, "two", 2);
print $fruit_color{"one"};  

2. it is wonderful that the hash can be sliced to result in an array as similar as array.for example:

my %numbers = ("one", 1, "two", 2,"three",3);
my @nums= @numbers{"one","three"};
print @nums;

13

3. keys and values get all keys and all values of a hash .by the way,the slicing in hash also is a hash.for example:

my %numbers = ("one", 1, "two", 2,"three",3,"uno",1);
my %nums= %numbers{"one","three","uno"};
print keys %nums;
print values %nums;

unoonethree113

4. the value in hash of perl can belong to a hash even any other Perl data type.

my $numbers = {"two"=> {"due"=>2,"二"=>2},"three"=>3};
print $numbers->{"two"}->{"二"};
print $numbers->{"three"};

23

5. unless you want to create a global variable ,you ought to use my to identify the variable scoping.
6. the loop structure of perl is similar as most programming language.

while ( condition ) {...
}

until ( condition ) {...
}

for ($k = 0; $k <= $max; $k++) {...
}

foreach my $key (keys %hash) {print "the $key corresponding to  $hash{$key}\n";
}

7. regular expression

.                   a single character
\s                  a whitespace character (space, tab, newline, ...)
\S                  non-whitespace character
\d                  a digit (0-9)
\D                  a non-digit
\w                  a word character (a-z, A-Z, 0-9, _)
\W                  a non-word character
[aeiou]             matches a single character in the given set
[^aeiou]            matches a single character outside the givenset
(foo|bar|baz)       matches any of the alternatives specified^                   start of string
$                   end of string

*                   zero or more of the previous thing
+                   one or more of the previous thing
?                   zero or one of the previous thing
{k}                 matches exactly k of the previous thing
{k1,k2}               matches between k1 and k2 of the previous thing
{k1,}                matches k1 or more of the previous thing

references

1. https://learn.perl.org/