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Beginners/Idiomatic Perl

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Perl allows a great many short cuts that are used regularly by its programmers. Some of these are detailed below.

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Short-circuiting operators

The conditional "or" operators (|| and or) can be used to short-circuit an expression. If the left-hand side of the expression returns true, the right-hand side will not be evaluated. Thus we can write:

    open(INPUT, "<", "myfile.txt") or die "File could not be opened";
    open(INPUT, "<", "myfile.txt") || die "File could not be opened";

If the file cannot be opened for whatever reason the expression on the right-hand side will be evalated and Perl will die with an error.

The conditional "and" operators can also be used similarly as follows:

    ($a == 5) && print "a is 5";
    $a == 5 and warn "a is 5"

however these are generally better written with if statements:

    print "a is 5" if $a == 5;
    if($a == 5) {
         print "a is 5";
    }

Default values

The same short-circuiting properties described above can be used to assign default values:

    my $b = $a || 5;
    my $param = shift || "default_value";

In this case, the left-hand side of the expression is tested for truth, and if it's false, the value on the right-hand side is assigned (even if it is also false). You'll also see this used to ensure that a variable has a defined value:

    $a = $a || 0;  
    # also written as:
    $a ||= 0;

Chaining comparisons

The || operator can be used to chain comparison operators such as cmp and <=>. In these cases if the current comparison returns equality, the later comparisons will be compared.

    my @sorted_persons =
        sort
             {
                    ($a->{last_name} cmp $b->{last_name}) ||
                    ($a->{first_name} cmp $b->{first_name})
             }
             @persons;

map

The map function can be used to map one array into another. Each element can be converted to multiple elements if necessary.

     my @mapped_filenames = map { $_.".old" } @filenames;

grep

grep in scalar context can be used to determine if an array contains a value, or has a value that matches a certain criterion:

    if (grep { $_ eq $value } @array)
    {
        # Do something
    }
    # Do something if one of the elements of the array is active.
    if (grep { is_active($_) } @array)
    {
         .
         .
         .
    }

local

local can be used to temporarily set the value of a variable until the end of the declared scope. It can also be used with most built-in variables, with the subscripts of arrays or hashes, and with typeglobs.

    my $content;
    {
         # Temporarily undef $/
         local $/;
         # Read the entire file into $content
         open my $fh, "<", "hello";
         $content = <$fh>;
         close($fh);
    }
    # Now $/'s value is restored again.
    {
         local $hash{"hello"} = "yellow";
         myfunc(\%hash); # Now myfunc will think that $hash{"hello"} is "yellow".
    }

Slicing

A hash or an array can be subscripted (sliced) using another array or list.

    my @a = @array[3,4,5];       # using a list
    my @a = @array[@b];          # using an array
    my @a = @hash{@b};
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