Hi All,
Here is the next chapter. Once again all feedback greatly appreciated.
Arrays
Defined using the @ symbol in front of the array name, arrays store a
list variables with numbered locations starting at 0. e.g
my @array = (‘one’, ‘two’);
stores ‘one’ at location 0 and ‘two’ at location 1. This is an important
thing to note with a lot of computer arithmetic, numbering generally
starts at 0.
To add a variable to the end of any array you can use the push command.
push( @array, ’three’ );
or simply use:
@array = ( @array, ’three’ );
To join an array
my @join = ( @array1, @array2 );
To take an item off the end of an array, use pop.
my $item = pop( @array );
So push and pop are a pair of functions that deal with adding/removing
from the end of an array. Their counterparts are unshift and shift, that
deal with adding/removing from the start of an array.
shift takes the first item off of an array.
my $item = shift( @array );
and unshift puts it back on
unshift( @array, 'zero' );
To access a value in an array you need to reference its location e.g
my @array = (‘one’, ‘two’,’three’);
my $item = $array[1];
$item will contain 'two', if no value is returned you get undef. Note
that when obtaining an arrays value you use $ rather than @ and square
brackets []. This is because you are accessing a single Scalar value
form the array, not the whole array.
To sort an array
@array = sort @array;
The default sort is an ASCII one, so A-Z goes before a-z. We'll cover
more on sorting later.
Getting input from the user
There are different ways of getting input from a user. From command line
arguments to sockets. Here are a couple of common methods.
Command line arguments
Perl has a special variable named @ARGV. It's an array containing all of
the parameters passed into the script when it's been called from the
command line. For instance the script:-
#!c:/perl/bin/perl.exe
print $ARGV[0];
Would print the first parameter passed in. So if called with “perl
script.pl Hello” it would display “Hello”. If called with “perl
script.pl Hello Lyle” it would still only display “Hello”. This is
because it's only printing the first parameter passed. You could use
parenthesis to make the first parameter contain both Hello and Lyle,
such as 'perl script.pl “Hello Lyle”' would print “Hello Lyle”.
STDIN file handle
STDIN stands for 'Standard Input”. Generally speaking, all Unix derived
languages will have a STDIN, STDOUT 'Standard Output' and STDERR
'Standard Error'. They are treated as file handles that you can either
read from, or write to. You've already used STDOUT, by default print
goes to STDOUT, so the following lines are the same:-
print “Hello”;
print STDOUT “Hello”; # Same
You can read from STDIN using the readline command, although Perl
provides shorthand in the form of <FILEHANDLE>, such that:-
my $name = readline(*STDIN);
my $name = <STDIN>; # Same
It's uncommon to use readline, so stick with <STDIN>. You'll notice that
in the readline example, STDIN has a * sigil, this is because to pass
file handles into functions you must pass the whole GLOB. I'm not
covering GLOBs here, for now think of them as another kind of variable.
The example code above will pause the program and wait for user input,
once the user hits enter the program will continue and $name will
contain whatever they typed.
Conditionals
Things would be pretty boring if programs just went in a straight line
all the time. Conditionals allow the software to make decisions based on
variables, often ones that have been input. Perl offers if, elsif, else
and for convenience unless.
if
The if statement allows you to run a block of code if a condition is
true. They are in the format:-
if ( CONDITION ) {
CODE
}#if
You'll notice the use of curly brackets again. This time they surround
the block of code that is to be run if the condition is true. The small
comment on the end #if is a personal touch and not required. In larger
programs you can have many code blocks within code blocks, a simple
comment on the end bracket can help you keep track of where they started.
In singular context, Perl sees anything that isn't undef, 0, 0.0 or ''
(empty string), '0' (string containing 0) or an empty list as true.
Examples:-
if ( 0 ) { } # False
if ( 1 ) { } # True
if ( 0.0 ) { } # False
if ( '' ) { } # False
if ( 'text' ) { } # True
if ( undef ) { } # False
You can also reverse this with not or !, such as:-
if ( ! 0 ) { } # True
if ( not 1 ) { } # False
You can also make comparisons or check if an operation or function
returns a true value:-
if ( 0 == 0 ) { } # True
if ( 1 == 0 ) { } # False
Note that for comparisons you need to use == rather than just =. This is
because = is used for an assignment. Example:-
# Correct way
my $value = 1;
if ( $value == 1 ) { } # True
# Wrong way
if ( $value = 0 ) { } # True
Both examples return true. This is because the second one assigns 0 to
the scalar $value which is a successful operation returning true. !, >,
< can also be used:-
if ( 0 != 0 ) { } # False
if ( 1 > 0 ) { } # True
if ( 0 <= 0 ) { } # True
When comparing strings you use the eq and ne syntax instead (equals, not
equals respectively):-
if ( 'a' eq 'b' ) { } # False
if ( 'a' ne 'b' ) { } # True
You also have OR, ||, AND, &&. OR and || do basically the same thing, as
do AND and &&. There are subtle differences in preference, but I won't
be covering them as the vast majority of the time they are interchangeable.
if ( 'a' eq 'b' || 1 > 0 ) { } # True
if ( 'a' ne 'b' && 1 < 0 ) { } # False
else
You may also use the else statement with if, allowing to to do one thing
or the other. Such as:-
if ( $name eq 'Lyle' ) {
print 'Thanks for teaching us Lyle';
}#if
else {
print “Hello $name”;
}#else
elsif
Often 'else if' or 'elseif' in other languages, shortened to elsif in
Perl. It allows you to run another condition only if the original if is
false. You can have many elsif statements as you want, often you end
with a else. Such as:-
if ( $name eq 'Lyle' ) {
print 'Thanks for teaching us Lyle';
}#if
elsif ( $name eq '' ) {
print “Please input your name”;
}#elsif
else {
print “Hello $name”;
}#else
unless
For general convenience and to make things read easier, unless is
provided as an alternative to if not.
if ( not( 9 > 10 ) ) { } # True
unless ( 9 > 10 ) { } # Same
Lyle
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