Coding School

Perl References

I’m happiest writing Perl code that does not use references because they always give me a mild headache. Here’s the short version of how they work. The backslash operator () computes a reference to something. The reference is a scalar that points to the original thing. The ‘$’ dereferences to access the original thing. Suppose there is a string…

$str = “hello”; ## original string

And there is a reference that points to that string…

$ref = $str; ## compute $ref that points to $str

The expression to access $str is $$ref. Essentially, the alphabetic part of the variable, ‘str’, is replaced with the dereference expression ‘$ref’…

print “$$refn”; ## prints “hello” — identical to “$strn”;

Here’s an example of the same principle with a reference to an array…

@a = (1, 2, 3); ## original array

$aRef = @a; ## reference to the array

print “a: @an”; ## prints “a: 1 2 3”

print “a: @$aRefn”; ## exactly the same

Curly braces { } can be added in code and in strings to help clarify the stack of @, $, …

print “a: @{$aRef}n”; ## use { } for clarity

Here’s how you put references to arrays in another array to make it look two dimensional…

@a = (1, 2, 3); @b = (4, 5, 6);

@root = (@a, @b);

print “a: @an”; ## a: (1 2 3)

print “a: @{$root[0]}n”; ## a: (1 2 3)

print “b: @{$root[1]}n”; ## b: (4 5 6)

scalar(@root) ## root len == 2

scalar(@{$root[0]}) ## a len: == 3

For arrays of arrays, the [ ] operations can stack together so the syntax is more C like…

$root[1][0] ## this is 4

The if…else Statement

This statement uses a relational expression to check the validity of a condition and execute a set of statements enclosed in braces. It returns a Boolean value, true or false, according to the validity of the condition. The syntax of the if…else statement is:

  if(condition)  {  	block of statement(s);  }  else  {  	block of statement(s);  }  

In this syntax, condition is a relational expression. If the result of this expression is true, then the block of statements following the if statement is executed. Otherwise, the block of statements following the else statement is executed.

In Perl, unlike other languages, all loops and conditional constructs require statements to be enclosed in braces, even for single statements.

  #! /usr/bin/perl  print "Enter a value for a: ";  $a = ;  print "Enter a value for b: ";  $b  = ;  if ($a>$b)  {     print "a is greater than bn";  }  else  {     print "b is greater than an";  }  

In this example, the if clause checks whether $a is greater than $b. If the value of $a is greater than $b, then the result is: a is greater than b. Otherwise, the control transfers to the code following the else clause and the statement associated with the else clause is printed as a result.

The if…elsif…else Statement

This statement is used when there is more than one condition to be checked. The syntax of the if…elsif…else statement is:

  if (condition)  {  	block of statement(s);  }  elsif (condition)  {  	block of statement(s);  }  else  {  	block of statement(s);  }  

In this syntax, if the condition associated with the if clause is false, the control transfers to elsif clause that checks for the next condition. The code associated with elsif clause is executed only if the condition is true or the code associated with the else clause is executed.

  #! /usr/bin/perl  print "Enter the score of a student: ";  $score = ;  if($score>=90)  {    print "Excellent Performancen";  }  elsif($score>=70 && $score<90)  {    print "Good Performancen";  }  else  {    print "Try hardn";  }  

The Loop Statements

Different loop statements in perl…

• The for Loop
• The foreach Loop
• The while Loop
• The do-while Loop
• The until Loop

The process of executing a code block repetitively is known as

iteration. To perform iteration in applications, use the loop

statements, such as for and while.

The loop statements check a condition and repetitively execute the

enclosed statements until the condition is true. The loop

terminates only when the condition becomes invalid.

The for Loop

This loop is used to execute a given set of statements for a fixed

number of times. The syntax of the for loop is:

  for(initialization;testing;updation)  {  	block of statement(s);  }  

In these statements:

initialization: Is the code to declare and initialize the

loop counter. Loop counter is a variable to keep a check on the

number of iterations. Initialization happens only once before the

beginning of the loop.

testing: Is the code, which specifies the condition to

control the number of iterations. The loop executes until the

result of testing is true. When the condition becomes false, the

control passes to the statement following the loop.

updation: Is the code to modify the loop counter after each

iteration. It can increment or decrement the loop counter,

according to the program requirements. Updation occurs at the end

of the loop.

block of statement(s): Is the code to be executed

iteratively. This code must be enclosed within the curly braces.

the for loop will be an infinite loop.

  #! /usr/bin/perl  print "Enter a digit to create its table: ";  $a = ;  chomp($a);  for($b=1;$b<=10;$b++)  {  	print $a.' x '.$b.' = '. $a*$b."n";  }  

  #! /usr/bin/perl  for($a=0;$a<=9;$a++){          for($b=0;$b<=$a;$b++){                  print "*";          }          print "n";  }   

  foreach $var_name (@array_name)  {  	block of statement(s);  }  

  #! /usr/bin/perl  @names = ("George", "Jack", "Davis");  foreach $word(@names)  {  print "$wordn";  }  

  while (condition)  {  	block of statement(s);  }  

  #! /usr/bin/perl  $a = 1;  while ($a <= 10)  {  	print "$an";  	$a++;  }  

  do   {  	block of statement(s);  }  while (condition);  

  #! /usr/bin/perl  $a = 2;  do  {  	print "$an";  	$a+=2;  } while ($a<=20);  

  until(condition)  {  	block of statement(s);  }  

  #! /usr/bin/perl  $a = 1;  until(a == 11)  {  	print $a."n";  $a++;  }  

@ARGV and %ENV

The built-in array @ARGV contains the command line arguments for a Perl program. The following run of the Perl program critic.pl will have the ARGV array (“-poetry”, “poem.txt”).

unix% perl critic.pl -poetry poem.txt

%ENV contains the environment variables of the context that launched the Perl program.

@ARGV and %ENV make the most sense in a Unix environment.

So for example you have a script like test.pl

  #!/usr/bin/perl  use strict;  my $column=$ARGV[0];  my $database=$ARGV[1];    ------run the program now---  $./test.pl ssn employees  

in above example we are passing two values as commandline arguments. so $column will have “ssn” and $database will have “employees”.
Note: Pass values in quoutes if values have spaces.

Perl Hash Maps/associative arrays

ASSIGNING HASH VALUES

  %states=  ("California","Sacramento","Wisconsin","Madison","New York","Albany");  

  %states=  (California=>"Sacramento",Wisconsin=>"Madison","New York"=>"Albany");  

  print "Capital of California is " . $states{"California"} . "nn";  

  #!/usr/bin/perl  %states=  (California=>"Sacramento",Wisconsin=>"Madison","New York"=>"Albany");  foreach my $keys(keys %states)  {    print "KEY:$keys VALUE:$states{$keys}n";    }    output  KEY:Wisconsin VALUE:Madison  KEY:New York VALUE:Albany  KEY:California VALUE:Sacramento  

Arrays @

Array constants are specified using parenthesis ( ) and the elements are separated with

commas. Perl arrays are like lists or collections in other languages since they can grow

and shrink, but in Perl they are just called “arrays”. Array variable names begin with the

at-sign (@). Unlike C, the assignment operator (=) works for arrays — an independent copy

of the array and its elements is made. Arrays may not contain other arrays as elements.

Perl has sort of a “1-deep” mentality. Actually, it’s possible to get around the 1-deep

constraint using “references”, but it’s no fun. Arrays work best if they just contain

scalars (strings and numbers). The elements in an array do not all need to be the same

type.

  @array = (1, 2, "hello");  ## a 3 element array 
  @empty = ();               ## the array with 0 elements 
  $x = 1;   $y = 2;   @nums = ($x + $y, $x - $y);    ## @nums is now (3, -1)   

Just as in C, square brackets [ ] are used to refer to elements, so $a[6] is the element

at index 6 in the array @a. As in C, array indexes start at 0. Notice that the syntax to

access an element begins with ‘$’ not ‘@’ — use ‘@’ only when referring to the whole

array (remember: all scalar expressions begin with $).

  @array = (1, 2, "hello", "there");   $array[0] = $array[0] + $array[1];## $array[0] is now 3   

Perl arrays are not bounds checked. If code attempts to read an element outside the array

size, undef is returned. If code writes outside the array size, the array grows

automatically to be big enough. Well written code probably should not rely on either of

those features.

  @array = (1, 2, "hello", "there");   $sum = $array[0] + $array[27];    ## $sum is now 1, since $array[27] returned undef   $array[99] = "the end";  ## array grows to be size 100   

When used in a scalar context, an array evaluates to its length. The “scalar” operator

will force the evaluation of something in a scalar context, so you can use scalar() to get

the length of an array. As an alternative to using scalar, the expression $#array is the

index of the last element of the array which is always one less than the length.

  @array = (1, 2, "hello", "there");   $len = @array;                  ## $len is now 4 (the length of @array)   $len = scalar(@array);  ## same as above, since $len represented a scalar   ## context anyway, but this is more explicit   

  @letters = ("a", "b", "c");   $i = $#letters;## $i is now 2   

That scalar(@array) is the way to refer to the length of an array is not a great moment in

the history of readable code. At least I haven’t showed you the even more vulgar forms

such as (0 + @a).

The sort operator (sort @a) returns a copy of the array sorted in ascending alphabetic

order. Note that sort does not change the original array. Here are some common ways to sort…

  (sort @array)  ## sort alphabetically, with uppercase first   (sort {$a  $b} @array)              ## sort numerically   (sort {$b cmp $a} @array)              ## sort reverse alphabetically   (sort {lc($a) cmp lc($b)} @array)      ## sort alphabetically, ignoring case (somewhat inefficient)   

The sort expression above pass a comparator function {…} to the sort operator, where the

special variables $a and $b are the two elements to compare — cmp is the built-in string

compare, and is the built-in numeric compare.

There’s a variant of array assignment that is used sometimes to assign several variables

at once. If an array on the left hand side of an assignment operation contains the names

of variables, the variables are assigned the corresponding values from the right hand

side.

($x, $y, $z) = (1, 2, “hello”, 4);

## assigns $x=1, $y=2, $z=”hello”, and the 4 is discarded

This type of assignment only works with scalars. If one of the values is an array, the

wrong thing happens (see “flattening” below).

Array Add/Remove/Splice Functions

These handy operators will add or remove an element from an array. These operators change

the array they operate on…

Operating at the “front” ($array[0]) end of the array…

shift(array)
returns the frontmost element and removes it from the array. Can be used

in a loop to gradually remove and examine all the elements in an array left to right. The

foreach operator, below, is another way to examine all the elements.

unshift(array, elem)
inserts an element at the front of the array. Opposite of shift.

Operating at the “back” ($array[$len-1]) end of the array…

pop(array)
returns the endmost element (right hand side) and removes it from the

array.

push(array, elem)
adds a single element to the end of the array. Opposite of pop.

splice(array, index, length, array2)
removes the section of the array defined by index

and length, and replaces that section with the elements from array2. If array2 is omitted,

splice()
simply deletes. For example, to delete the element at index $i from an array, use

splice(@array, $i, 1).

Perl Operators

In Perl, the comparison operators are divided into two classes:

• Comparison operators that work with numbers
• Comparison operators that work with strings

Integer-Comparison Operators

  Operator Description            Greater than  ==       Equal to  =       Greater than or equal to  !=       Not equal to   Comparison returning 1, 0, or -1  

Each of these operators yields one of two values:

• True, or nonzero
• False, or zero

The operator is a special case. Unlike the other integer comparison operators, returns one of three values:

• 0, if the two values being compared are equal
• 1, if the first value is greater
• -1, if the second value is greater

String-Comparison Operators

For every numeric-comparison operator, Perl defines an equivalent string-comparison operator.

  String operator Comparison operation   lt              Less than  gt              Greater than  eq              Equal to  le              Less than or equal to  ge              Greater than or equal to  ne              Not equal to !=  cmp             Compare, returning 1, 0, or -1  

Example

  #!/usr/bin/perl   $a="coding-school.com";  $b="www.coding-school.com";  if($a eq $b)  {  print "Both are same";  }  else  {  print "Both are different";  }  

List

A list is a sequence of scalar values enclosed in parentheses. The following is a simple example of a list:

(1, 5.3, “hello”, 2)

This list contains four elements, each of which is a scalar value: the numbers 1 and 5.3, the string hello, and the number 2.

Lists can be as long as needed, and they can contain any scalar value. A list can have no elements at all, as follows:

()

This list also is called an empty list.

NOTE

A list with one element and a scalar value are different entities. For example, the list

(43.2)

and the scalar value

43.2

are not the same thing. This is not a limitation because one can be converted to or assigned to the other.

Perl Scalars

The most basic kind of variable in Perl is the scalar variable.

Scalar variables hold both strings and numbers, and are remarkable in

that strings and numbers are completely interchangable. For example,

the statement

$priority =10;

sets the scalar variable $priority to 10, but you can also assign a string

to exactly the same variable:

$priority = ‘high’;

Perl also accepts numbers as strings, like this:

$priority = ’10’;

$default = ‘0010’;

and can still cope with arithmetic and other operations quite happily.

In general variable names consists of numbers, letters and underscores, but

they should not start with a number and the variable $_ is

special, as we’ll see later. Also, Perl is case sensitive, so

$a and $A are different.

Operations and Assignment

Perl uses all the usual C arithmetic operators:

and for strings Perl has the following among others:

To assign values Perl includes

Note that when Perl assigns a value with $a = $b it makes

a copy of $b and then assigns that to $a. Therefore the next time you change

$b it will not alter $a.

Interpolation

The following code prints apples and pears using concatenation:

$a = ‘apples’;

$b = ‘pears’;

print $a.’ and ‘.$b;

It would be nicer to include only one string in the final print statement,

but the line

print ‘$a and $b’;

prints literally $a and $b which isn’t very helpful. Instead we

can use the double quotes in place of the single quotes:

print “$a and $b”;

The double quotes force interpolation of any codes, including

interpreting variables. This is a much nicer than our original statement.

Other codes that are interpolated include special characters such as

newline and tab. The code n is a newline and

t is a tab.