some rewriting

Author: szabgab

perl-hashes.tt

@@ -1,6 +1,6 @@
-=title Hashes
-=timestamp 2013-03-17T11:12:06
-=indexes hash, key, value, associative
+=title Hashes in Perl
+=timestamp 2013-03-19T11:12:06
+=indexes hash, key, value, associative, %, =>, fat arrow, fat comma
 =status show
 =books beginner_book
 =author leprevost
@@ -12,110 +12,163 @@
 
 =abstract start
 
-In this chapter we are going to learn about one of the powerful tools in Perl programming: hashes.
-Some times called associative arrays, dictionaries or maps, hashes are one of the basic data structures available in Perl.
+In this article of the <a href="/perl-tutorial">Perl Tutorial</a>
+we are going to learn about one of the powerful tools in Perl programming: <b>hashes</b>.
+Some times called associative arrays, dictionaries or maps, hashes are one of the data structures available in Perl.
 
 =abstract end
 
-Hashes, like other Perl variables, are declared using the <hl>my</hl> keyword. The variable name is preceded by the <hl>%</hl> symbol.
+A hash is an un-ordered group of key-value pairs. The keys are uniques strings. The values are scalar values.
+Each value can be either a number, a string or a reference. (We'll learn about references later, but those allow
+us to build multi-dimensional data strctures.
 
-It's a little mnemonic trick to help you remind about the key-value structure. Don't worry you will get it in a moment.
+Hashes, like other Perl variables, are declared using the <hl>my</hl> keyword. The variable name is preceded by the
+percentage (<hl>%</hl>) symbol.
 
-Similarly to arrays, hashes can hold a series of elements that can also be retrieved at will, the major difference is the way how this happens. When retrieving an element from an array you should use its <hl>index</hl>. When retrieving elements from hashes, you should use names , or its <hl>keys</hl>.
+It's a little mnemonic trick to help you remind about the key-value structure.
 
-Each hash key is associated to a <hl>value</hl> and they are all unique inside the hash structure. That means no repetitive keys are allowed.
+Some people think that hashed are like arrays (the old name 'associative array' also indicates this, and in some other
+languages like PHP, there is no difference between the two), but there are two major differences between arrays and
+hashes. Arrays are ordered, and you access an element of an array using its numerical index.
+Hashes are un-ordered and you access a value using a key which is a string.
 
-In practice, when you place inside a hash a pair of key and value, you are placing inside its structure two scalars that are converted to text.
+Each hash key is associated with a single <b>value</b> and the keys are all unique inside a single hash structure.
+That means no repetitive keys are allowed. (If you want to have more than one values for a key, you'll will need to wait
+a bit till we reach the references.)
 
-<h2> Hash basics </h2>
+Let's see some code now:
 
-Lets create a hash and see how they work:
+<h2>Create an empty hash</h2>
 
 <code lang="perl">
-my %hash;
+my %color_of;
 </code>
 
-In order to access a hash element you should do the following:
+<h2>Insert element in a hash</h2>
+
+Insert a key-value pair: In this case 'apple' is the key and 'red' is the associated value.
 
 <code lang="perl">
-$hash{$<key_name>};
+$color_of{'apple'} = 'red';
 </code>
 
-When accessing a specific key-value pair, the hash is called using the <hl>$</hl> symbol because pairs are stored as scalars.
+You can also use a variable instead of the key and then you don't need to put the
+variable in quotes:
+
+<code lang="perl">
+my $fruit = 'apple';
+$color_of{$fruit} = 'red';
+</code>
 
-Since our hash was create and not populated the code written above will return nothing, so lets place inside the hash a few elements:
+Actually, if the key is a simple string, you could leave out the quotes even when you use the string directly:
 
 <code lang="perl">
-$hash{“apple”} = “red”;
-$hash{“orange”} = “orange”;
-$hash{“grape”} = “purple”;
+$color_of{apple} = 'red';
 </code>
 
-We could also have instantiated the variable with the key-value pairs simultaneous passing to the hash a list of key-value pairs:
+As you can see above, when accessing a specific key-value pair, we used the <hl>$</hl> symbol (and not the % symbol)
+because we are accessing a single value which is a <b>scalar</b>.
+
+<h2>Fetch and element of a hash</h2>
+
+Quite similar to the way we insterted an element we can also fetch the value of an element.
 
 <code lang="perl">
-my %hash = (
-“apple” => “red”,
-“orange” => “orange”,
-“grape” => “purple”,
-);
+print $color_of{apple};
 </code>
 
-This is called the 'big arrow notation', and its used to indicate pairs of elements.
+If the key does not exist, the hash will return an <a href="http://perl5maven.com/undef-and-defined-in-perl">undef</a>,
+and if <hl>warnings</hl> are enabled, as they should be, then we'll get a
+<a href="http://perl5maven.com/use-of-uninitialized-value">warning about uninitialized value</a>.
+
+<code lang="perl">
+print $color_of{orange};
+</code>
 
-Now, we can do the following in order to access the value of the <i>apple</i> key:
+Let's a few more key-value pairs to the hash:
 
 <code lang="perl">
-print $hash{“apple”};
+$color_of{orange} = “orange”;
+$color_of{grape} = “purple”;
 </code>
 
-The returned value is :
+<h2>Initialize a hash with values</h2>
+
+We could have instantiated the variable with the key-value pairs simultaneous passing to the hash a list of key-value pairs:
 
 <code lang="perl">
-> red.
+my %color_of = (
+    “apple”  => “red”,
+    “orange” => “orange”,
+    “grape”  => “purple”,
+);
 </code>
 
-Now lets see what happens when we include another key-value pair using a already stored key:
+This is called the <b>fat arrow</b> or <b>fat comma</b>, and it is used to indicate pairs of elements.
+The first name fat arrow will be clear once we see the the other arrow (->) used in Perl.
+The name fat comma is used, because these arrows are basically the same as commas. So we could have written this too:
 
 <code lang="perl">
-$hash{“apple”} = “green”;
-print $hash{“apple”};
+my %color_of = (
+    “apple”,  “red”,
+    “orange”, “orange”,
+    “grape”,  “purple”,
+);
 </code>
 
-The result will be:
+Actually the fat comma allows you to leave out the quotes on the left-hand side writing like this:
+
 <code lang="perl">
-> “green”;
+my %color_of = (
+    apple  => “red”,
+    orange => “orange”,
+    grape  => “purple”,
+);
 </code>
 
-As you can see the value associated to the <hl>apple</hl> key was overwritten, so remember, keys should be unique.
 
-Now you can understand why the <hl>%</hl> symbol, the two little spheres are like pairs of things, or in our case, keys-values.
+<h2>Assignment to a hash element</h2>
+
+Let's see what happens when we assign another value to an existing key:
+
+<code lang="perl">
+$color_of{apple} = “green”;
+print $color_of{“apple”};     # green
+</code>
+
+The assignment changed the value associated with the <hl>apple</hl> key. Remember, keys are unique and each key has a
+single value.
 
-<h2> Hash size </h2>
+<h2>Iterating over hashes</h2>
 
-There is a quick way to know the size of a hash, and for that you have to use the <hl>keys</hl> keyword:
+In order to access a value in a hash you need to know the key.
+When the keys of a hash are not pre-defined values you can use the <hl>keys</hl> function
+to get the list of keys. Then you can iterate over those keys:
 
 <code lang="perl">
-my $hash_size = keys %hash;
-print $hash_size;
+my @fruits = keys %color_of;
+for my $fruit (@fruits) {
+    print “The color of '$fruit' is $color_of{$fruit}\n”;
+}
 </code>
 
-This will print the number of key-value pairs:
+You don't even need to use the temporary varible <hl>@fruits</hl>, you can iterate
+over directly the return values of the <hl>keys</hl> function:
 
 <code lang="perl">
-> 3
+for my $fruit (keys %color_of) {
+    print “The color of '$fruit' is $color_of{$fruit}\n”;
+}
 </code>
 
-<h2> Iterating over hashes </h2>
 
-This is how a for loop looks like when iterating over a hash:
+<h2>The size of a hash</h2>
+
+When we say the size of a hash, usually we mean the number of key-value pairs.
+You can get it by placing the <hl>keys</hl> function in scalar context.
 
 <code lang="perl">
-for my $key (keys %hash) {
-    print “the key is: $key and the value is $hash{$key}”;
-}
+print scalar keys %hash;
 </code>
 
-<h2> Conclusion </h2>
-You have learned the basics about one of the most important and powerful data structures in Perl, a little step closer to become a Perl Maven.