Representing Data in Perl

Scalars
Arrays
Hashes

Scalars

A scalar is an individual piece of data:
- a single number
  123.45
- a single string
  "CGI Programming with Perl"

Scalar Variables

Scalar variables always begin with a dollar sign ($)

The "$" should remind you of the "S" in scalar

$firstName = "Michael";
$studentNumber = 123456;

Strings

Strings should be enclosed in single or double quotes

Unlike other languages, in Perl there is a big difference

$prize = 100.00;
print 'You won a $prize dollar cheque!;
print "You won a $prize dollar cheque!";

See the output

Arithmetic Operators

Perl supports the usual operators
```
+     -     *     /     **     %
```
Rules of precedence:
```
**
*     /     %
+     -
```

Strings in Arithmetic Expressions

Strings in an arithmetic expression are converted to a number
```
$result = '17' + 3;
$value  = "17" + "3";
```
All non-numerical characters are stripped out in that conversion
```
$sum    = "1s7" + "3w";
```

String Concatenation

The string concatenation operator is the period (.)

$string = "up" . "side" . "down";
$result = "up" + "side" + "down";
$value  = "1potato" + "2potato";

See the output

Shorthand Assignment Operators

For each arithmetic operator there is a shorthand assignment operator
```
+=     *=     /=   -=   %=   **=   .=
```

Example:

$result = 10;
$result = $result + 5;
$result += 5;

Shorthand Increment (Decrement) Operators

Perl has a shorthand for adding (subtracting) 1 from a variable

$index = 3;	
$result = ++$index;         # $result will be 4
$nextResult = $index--;     # $result will still be 4

++index
Value of index is incremented before it is used
index--
Value of index is used first and then decremented

Arrays

An array (often called a list), is a collection of scalars
Array variables always begin with an at sign (@)
The "a" in "at" is a mnemonic for array

A list literal is an array comprised entirely of constants

@days = ("Monday", "Tuesday", "Wednesday", "Thursday", "Friday");
@languages = qw(Java Perl C++);
@weights = (123.45, 678.9, 1020.3);

Accessing Array Elements

Array elements are numbered starting with 0
When accessing an array element you must use a "$" in front of the variable name (this indicates the type of the element - a scalar)
```
@days = ("Monday", "Tuesday", "Wednesday", "Thursday", "Friday");
print "$days[2]\n";
```
See the output

List Assignment

To assign multiple consecutive array elements to scalars, place the scalars into a list
```
($d1) = @days;
($d1, $d2) = @days;
```
Negative index numbers (starting with -1) access elements from the end of the array
```
$lastDay = $days[-1];
```

Modifying All Array Elements

If you operate on each element of an array, for example, in a foreach loop, the elements are permanently changed
```
@numbers = (1 4 9 16 25);
foreach $number (@numbers){
  $number = sqrt($number);
}
print "@numbers";
```
See the output

Length of an Array

Assigning an array to a scalar assigns it the length of the array (you can also use the scalar operator)
```
$length = @days;
$length = scalar @days;
```
$#array returns the index of the last element in the array
```
$length = $#days + 1;
```

Taking a Slice of an Array

Extracting more than one element at a time from an array is taking a slice
Specify the indices of the desired elements between brackets:
```
@array[n,m,...]
```

Example:

@select = (0, 2, 4);
@fruits = qw(lemons, peaches, oranges, apples, limes, bananas);
@citrus = @fruits[0,2,4];     # these are equivalent ways ...
@citrus = @fruits[@select];   # ... of specifying the slice
print "@citrus";

See the output

Removing Elements from an Array

Use shift to remove elements from the beginning of an array
```
$first = shift(@days);
```
Use pop to remove elements from the end of an array
```
$last = pop(@days);
```

Adding Elements to an Array

Use unshift to add elements to the beginning of an array
```
unshift(@days, "Sunday");
print "@days\n";
```
See the output
Use push to add elements to the end of an array
```
push(@days, "Saturday");
print "@days\n";
```
See the output

Combining Arrays

Syntax for adding the elements from one array to another array:
```
(@first, @second)
```
Similarly, you can concatenate more than two arrays:
```
(@first, @second, @thirds, ...)
```
Since arrays can only contain scalars, this simply inserts the elements of each array into a new array

Replacing Elements in an Array

To replace an element in an array, assign a new value to the element
```
$fruits[4] = "pear";
```
You can replace several elements simultaneously by assigning an array with the new elements to a slice of the array
```
@fruits[0,2] = ("persimmon", "pommegranade");
```

Sorting Arrays

Use the sort operator to sort an array

sort @array;               # ASCII order

To sort in any other order, you need to provide a sort block (i.e. code that compares two elements)

sort {$b cmp $a} @array;   # reverse ASCII order
sort {$a <=> $b} @array;   # ascending numerical order
sort {$b <=> $a} @array;   # descending numerical order

Hashes

A hash (sometimes called an associative array), is a collection of scalars in key-value pairs
Hash variables begin with a percent sign (%)
For example, the environoment variables of CGI scripts are stored in a hash called %ENV

Accessing Hash Elements

Syntax for accessing individual hash elements:
```
$query = $ENV{'QUERY_STRING'};
```
Syntax for accessing multiple hash elements at the same time:
```
@numbers = @phone{'Bob', 'Alice', 'Mary'};
```

Constructing a Hash

You can set hash elements one at a time:

$phone{'Bob'} = '1234';
$phone{'Alice'} = '2345';
$phone{'Mary'} = '3456';

You can specify all elements in an array:

%phone = ('Bob', '1234', 'Alice', '2345', 'Mary', '3456');

Or you can use a more expressive notation:

%phone = (
    Bob => '1234',
    Alice => '2345',
    Mary => '3456'
);

Note: In this notation, the keys are assumed to be strings and don't need to be quoted

Getting All Keys of a Hash

Use the keys operator to get all keys of a hash
```
@listings = keys %phone;
```

You can now print or process the keys:

foreach $key (keys %phone) {
  print $key;
}

See the output

Getting all Values of a Hash

Use the value operator to get all values of a hash
```
@numbers = values %phone;
```

Accessing Keys and Values Simultaneously

There are two ways to access keys and values of hash at the same time:
- Use the each operator
- Use the keys operator combined with a foreach loop

Each time each is invoked it returns the next key-value pair

while (($key, $value) = each %phone) {
   print "<p>The phone number for $key is $value.\n";
}

See the output

Looping through the keys

foreach $key (keys %phone) {
   print "<p>The phone number for $key is $phone{value}.\n";
}

Checking if a Hash Contains a Key

To check if a key is pars of a hash, use the exits operator

$name = 'Joe';
if(exists $phone{$name))  {
   print "The directory contains a listing for $name.";
} else {
   print "No listing for $name."
}

Removing Key-Value Pairs

To remove a key-value pair permanently from the hash, us the delete operator
```
$value = delete $phone{'Mary'};
```
The delete operator returns the deleted value

Reading for Next Class

Read Chapters 8 to 11 in Castro, Perl and CGI