Workshop in Computational Bioskills - Spring 2009
Back
to Lesson 2
Part 1 - Hash tables/Associative Arrays (%)
Part 2 - Control flow
Hashes (Associative Arrays):
a Hash or
an Associative Array is a different
kind of a list.
Here, a cell is labeled by any scalar, instead of the natural
numbers in lists.
As always, the cell content can be any type of scalar.
$h{"sunday"}
= "blue";
$h{"monday"}
= "red";
$h{"tuesday"}
= "black";
$h{"wednesday"}=
"yellow";
$h{"thursday"}
= "orange";
$h{"friday"}
= "green";
$h{"saturday"}
= "purple";
This can
be written nicer in the following format:
%h = (
sunday => blue,
monday
=> red,
tuesday
=> black,
...
saturday
=> purple,
);
Keys can be just as well numbers:
$h{13.2} = 12.3;
$h{1e-5} = 42;
$h{0.00001} = 13; # This will
overide 42, since 1e-5 = 0.00001 !
Hash elements has no order:
Due to the revolutionary naming scheme, there is no inherent order on the hash elements.
Hash commands:
- keys
The "keys(%h)" command returns a list (in some
arbitrary order) of the cells' names in %h.
- values
Similarly, "values(%h)" returns a list of the values stored in
the hash %h.
Luckily enough, the two commands: keys() & values(), should
work in the same order.
- each
There's another way of iterating a hash, though.
"each(%h)" returns the next pair of (key, value).
while (($k,
$v) = each (%h)) {
print
"$k --> $v\n";
}
This code will iterate ovel all elements in the hash %h, and
print them.
- delete
So far, we've seen ways of adding elements to a hash. But how can
we delete them ?
$k =
"tuesday";
delete $h{$k}; # will delete the element named $k.
- exists
It is possible to check if a cell exists before reading from it.
(instead of getting an "undef" value)
delete
$h{"friday"} if (exists $h{"friday"});
Slicing a hash:
Just like lists, hashes can be
sliced.
@colors =
@h{"sunday","monday","wednesday"};
@h{"sunday","monday","wednesday"}
= ("red", "cyan", "magenta");
This summarizes Hashes. Now Let's continue to Lesson 3 - Perl II
[Based upon "Learning Perl, 2nd
Ed.", by Randel L. Schwartz & Tom Christiansen, Chap. 5]