Conditionals

Conditionals: if and unless

We’re on the home-straight in understanding the code from the last post now:

#!/usr/bin/perl

use strict;
use warnings;

my @peas = qw/chick mushy split/;
while ( my $type = pop @peas ) {
    print "$type peas are ", flavour( $type ), ".\n";
}

sub flavour {
    my $query = shift @_;
    my @peas = qw/chick garbanzo/;
    foreach ( @peas ) {
        if ( $query eq $_ ) {
            return "delicious";
        }
    }
    return "disgusting";
}

The very last bit is the conditional statement in the flavour() subroutine. This part compares the type of pea the subroutine was passed with all the peas in its own @peas, and if it matches any of them, the subroutine returns ‘delicious’. The if conditional has the general form:

if     ( THIS_IS_TRUE ) { DO_SOMETHING; }

which is analogous to:

while  ( THIS_IS_TRUE ) { DO_SOMETHING; }

The equivalent of:

until  ( THIS_IS_TRUE ) { DO_SOMETHING; }

is:

unless ( THIS_IS_TRUE ) { DO_SOMETHING; }

Comparison operators: eq and ==

The actual comparison the if statement makes is:

$query eq $_

The eq tests to see if two strings are identical. Perl has two sets of comparisons: numerical and string. The ’equal to’ test is eq for strings, and == for numbers (that’s two = signs). Perl goof number one is getting == comparison and = assignment mixed up.

In addition to ’equal to’ comparisons, Perl also has greater than, less than, greater than or equal to, less than or equal to, and not equal to comparisons. For numbers these are >, <, <=, >=, and != respectively. The equivalents for strings are gt, lt, ge, le, and ne.

The reason Perl makes a distinction between numerical and string comparisons is because “2” and “2.0” are numerically equal, but not stringily equal : "2" == "2.0" is TRUE because 2 and 2.0 are the same numerically (mathematicians: shush). However, "2" eq "2.0" is FALSE, because they are clearly not the same string of characters. You want the maths symbols to compare things as numbers, and the language symbols to compare them as strings.

if statements can be optionally followed by any number of elsif statements, and an optional else statement, so:

if    ( THIS_IS_TRUE ) {
    DO_THIS_THING;
}
elsif ( THIS_OTHER_THING_IS_TRUE ) {
    DO_THIS_OTHER_THING;
}
else {
    DO_THE_DEFAULT_THING;
}

Which is all very simple and obvious. You can also nest if’s inside other if’s to a gazillion degrees, which is a perfect way of making code unreadable, but will be necessary from time to time.

Switch statements: given and when

If you come from a C background, you may be wondering if Perl has a switch statement, which, if you don’t, is basically a shorthand for a very long if...elsif...elsif...elsif...else statement. As of Perl 5.10.0, it does, but you need to explicitly enable it:

use 5.14.1;
given ( $arg ) {
    when ( $_ eq 'quit' )  { exit;               }
    when ( $_ eq 'squit' ) { say "Norovirus";    }
    default                { system "perldoc Math::Complex" }
}

given sets $_ to the argument you give it ($arg), and you can then test  $_ against various values using  when. If one of the  when cases succeeds, control leaves the given switch structure. If none of the cases succeed, an optional  default block can be called.  The new keyword exit simply causes a Perl program to stop. system we’ll come across later, but for the moment, just see what happens if you enter neither ‘quit’ nor ‘squit’ when you run this script.

Next up…More hashes and sorting

Posted in Perl

• Perl