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add perl-arrays article
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‎for-loop-in-perl.tt‎

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=title The for loop in Perl
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=timestamp 2013-01-17T14:45:56
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=tags Perl, for, foreach, loop, infinite loop
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=indexes for, foreach, loop
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=status draft ready for proofreading
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=timestamp 2013-03-23T20:45:13
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=indexes for, foreach, loop, infinite loop
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=status show
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=author szabgab
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=index 1
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=archive 1
@@ -23,11 +22,10 @@ but this construct is actually available in many programming languages.
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The <b>for</b> keyword in Perl can work in two different ways.
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It can work just as a <b>foreach</b> loop works and it can act
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as a 3-part C-style for loop. It is called C-style though
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this is employed in many languages.
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it is available in many languages.
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I'll describe how this works though in Perl people usually
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prefer to write the <hl>foreach</hl> style loop as described
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in the section about <a href="http://szabgab.com/perl-arrays.html">perl arrays</a>.
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I'll describe how this works although I prefer to write the <hl>foreach</hl>
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style loop as described in the section about <a href="http://szabgab.com/perl-arrays.html">perl arrays</a>.
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The two keywords <hl>for</hl> and <hl>foreach</hl> can be used as synonyms.
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Perl will work out which meaning you had in mind.

‎perl-arrays.tt‎

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=title Perl Arrays
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=timestamp 2013-03-23T20:45:02
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=indexes @, array, arrays, length, size, foreach, Data::Dumper, scalar, push, pop, shift
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=status show
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=author szabgab
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=index 1
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=archive 1
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=feed 1
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=comments 1
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=social 1
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=abstract start
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In this episode of the <a href="/perl-tutorial">Perl Tutorial</a> we are going to learn about <b>arrays in Perl</b>.
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This is an overview of how arrays work in Perl. We'll see more detailed explanations later.
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Variable names of arrays in Perl start with the at mark: <hl>@</hl>.
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Due to our insistance on using <hl>strict</hl> you have to declare these variables using the <hl>my</hl> keyword
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before the first usage.
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=abstract end
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Remember, all the examples below assume your file starts with
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<code lang="perl">
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use strict;
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use warnings;
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use 5.010;
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</code>
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Declare an array:
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<code lang="perl">
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my @names;
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</code>
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Declare and assign values:
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<code lang="perl">
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my @names = ("Foo", "Bar", "Baz");
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</code>
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<h2>Debugging of an array</h2>
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<code lang="perl">
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use Data::Dumper qw(Dumper);
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my @names = ("Foo", "Bar", "Baz");
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say Dumper \@names;
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</code>
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The output is:
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<code>
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$VAR1 = [
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'Foo',
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'Bar',
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'Baz'
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];
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</code>
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<h2>foreach loop and perl arrays</h2>
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<code lang="perl">
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my @names = ("Foo", "Bar", "Baz");
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foreach my $n (@names) {
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say $n;
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}
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</code>
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will print:
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<code>
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Foo
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Bar
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Baz
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</code>
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<h2>Accessing an element of an array</h2>
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<code lang="perl">
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my @names = ("Foo", "Bar", "Baz");
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say $names[0];
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</code>
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Note, when accessing a single element of an array the leading sigil changes from <hl>@</hl> to <hl>$</hl>.
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This might cause confustion to some people, but if you think about it, it is quite obvious why.
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<hl>@</hl> marks plural and <hl>$</hl> marks singular. When accessing a single element
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of an array it behaves just as a regular scalar variable.
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<h2>Indexing array</h2>
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The indexes of an array start from 0. The largest index is always in the variable called
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<hl>$#name_of_the_array</hl>. So
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<code lang="perl">
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my @names = ("Foo", "Bar", "Baz");
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say $#names;
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</code>
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Will print 2 because the indexes are 0,1 and 2.
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<h2>Length or size of an array</h2>
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In Perl there is no special function to fetch the size of an array, but there
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are several ways to obtain that value. For one, the size of the array is one more
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than the largest index. In the above case <hl>$#names+1</hl> is the <b>size</b> or
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<b>length</b> of the array.
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In addition the <hl>scalar</hl> function can be used to to obtain the size of an array:
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<code lang="perl">
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my @names = ("Foo", "Bar", "Baz");
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say scalar @names;
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</code>
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Will print 3.
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The scalar function is sort of a casting function that - among other things - converts an
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array to a scalar. Due to an arbitrary, but clever decision this conversion yields the size
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of the array.
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<h2>Loop on the indexes of an array</h2>
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There are cases when looping over the values of an array is not enough.
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We might need both the value and the index of that value.
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In that case we need to loop over the indexes, and obtain the values using the
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indexes:
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<code lang="perl">
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my @names = ("Foo", "Bar", "Baz");
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foreach my $i (0 .. $#names) {
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say "$i - $names[$i]";
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}
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</code>
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prints:
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<code>
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0 - Foo
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1 - Bar
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2 - Baz
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</code>
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<h2>Push on Perl array</h2>
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<hl>push</hl> appends a new value to the end of the array, extending it:
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<code lang="perl">
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my @names = ("Foo", "Bar", "Baz");
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push @names, 'Moo';
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say Dumper \@names;
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</code>
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The result is:
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<code>
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$VAR1 = [
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'Foo',
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'Bar',
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'Baz',
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'Moo'
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];
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</code>
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<h2>Pop from Perl array</h2>
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<hl>pop</hl> fetches the last element from the array:
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<code lang="perl">
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my @names = ("Foo", "Bar", "Baz");
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my $last_value = pop @names;
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say "Last: $last_value";
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say Dumper \@names;
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</code>
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The result is:
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<code>
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Last: Baz
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$VAR1 = [
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'Foo',
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'Bar',
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];
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</code>
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<h2>shift the Perl array</h2>
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<hl>shift</hl> will return the left most element
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of an array and move all the other elements to the left.
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<code lang="perl">
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my @names = ("Foo", "Bar", "Baz");
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my $first_value = shift @names;
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say "First: $first_value";
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say Dumper \@names;
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</code>
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The result is:
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<code>
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First: Foo
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$VAR1 = [
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'Bar',
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'Baz',
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];
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</code>
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‎perl-tutorial.tt‎

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It will be a good foundation for you to build on.
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The free on-line version of the tutorial is currently in development. Many parts are ready. Additional
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parts are being published every few days. The latest one was published on March 21, 2013.
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parts are being published every few days. The latest one was published on March 23, 2013.
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If you are interested in getting updated when new parts are published,
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please <a href="/register">subscribe to the newsletter</a>.
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<li><a href="http://szabgab.com/for-loop-in-perl.html">The for loop in Perl</a></li>
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<li>Lists in Perl</li>
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<li>Using Modules</li>
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<li><a href="http://szabgab.com/perl-arrays.html">Arrays in Perl</a></li>
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<li><a href="/perl-arrays">Arrays in Perl</a></li>
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<li>Process command line parameters @ARGV, Getopt::Long</li>
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<li><a href="/how-to-read-a-csv-file-using-perl">How to read and process a CSV file? (split, Text::CSV_XS)</a></li>
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<li><a href="/join">join</a></li>

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