There was some comparison with Python yesterday.
Though perl is not as good as python in many ways for some reason perl
has a very strong developer community around it. It has
got a certain way to obtain copious documentation in UNIX man page format.
Perl has CPAN(www.cpan.org) which is integrated into the standard distribution.
The sheer energy and force behind perl seems enormous. No other
language has enjoyed
such sheer patronage. Not C, not java,not even php.
And perl's UNIX focus lies in its manpage format for documentation
unlike python's help(<module>)
style, though every language has OS bindings perl is a direct copy
when it comes to regex, its
IPC and so on. It is nothing but UNIX in a new flavor.
Today we will see some more examples.
$ cat 0.pl
-------------------------------------------------------
#!/usr/bin/perl
$scalar1 = "This is a string scalar";
@arr = (1,2,3,5, "string1", "one more", $scalar1);
%h = ("key1" => 6, "key2" => 10);
print "The array is @arr\n";
for(keys %h) {
print "key is $_, value is $h{$_}\n";
}
-------------------------------------------------------
I will explain the above code.
We have seen scalars yesterday. All scalars start with $ and hold
singular values.
If you want to store a list of values you require arrays that start
with the @ character.
Then we see the hash which starts with the % and it uses the same
assignment style.
Meaning that we use (...) to assign values to the hash.
This is because hash is also an array. Instead of numeric prefix it
uses named keys. That is
why it is called an associative array.
So it has exactly 1/2 of the array length.
This is because half of the assignment items go towards making the key
and half the values.
Hashes have no ordering. It is stored and retrieved in a different
order. So in order to use
ordering we need to resort to the sort() function in perl.
We saw that the array can be printed just by enclosing the array in
double quotes(").
If you print it like this:
print @arr;
you will get the same result without the space between array elements.
This is a key feature of perl. Perl is context aware.
Depending on whether the lvalue is singular or plural(scalar or
array), the RHS will be assigned
as the length of the RHS or the list in the RHS itself.
For instance, if you write,
$sca = (1,3,4,5);
You will see that $sca contains 4.
$ perl -e '$sca = (1,2,3,4); print $sca;'
4$
But if you do this:
@a = (1,2,3,4);
you will be able to assign the whole array.
This can trip many people, even experienced programmers. One can even
argue it is a bug.
For instance in many situations it will be obvious we want the entire
list but you won't get it
without resorting to () on the LHS.
Anyway we will get to that on a future date.
For now, the hash statement and the print in the above code is confusing.
%hashsample = (1, 100, 2, 200);
will get assigned as :
1 -> 100
2 -> 200
So,
$hashsample{1} = 100;
$hashsample{2} = 200;
Note the syntax. This will drive you mad initially. Perl uses a lot of
funny characters.
One more thing. Another confusing thing but you can say it is a feature.
You assign the hash like this also and get the same result.
%foo = (1 => 100, 2 => 200);
This is because => is an alias for ,.
Howzzat?
Anyway array values are accessed with [<index>] and hash values are
accessed with {$key}.
For example,
@a = (1,3, 10);
print "first value is $a[0]\n";
%foo = (1 => 100, 2 => 200);
print "hash content with key 1 is $foo{1}\n";
Since hash always contains scalars as keys and could contain anything
as value it is always one to one.
For instance you can hold an array or function pointer as value but it
has to be an array pointer.
So every time you access a hash value, it will be $hashname{$key};
The usage,
for(keys %h) {
print "key is $_, value is $h{$_}\n";
}
will be explained tomorrow in iterators. This is the foreach() iterator.
Perl makes it very convenient to use array elements in a for loop.
More tomorrow.
-Girish
--
G3 Tech
Networking appliance company
web: http://g3tech.in ?mail: girish at g3tech.in
