Perl examples tutorial - under construction but still useful

This is a compilation of examples borrowed from several tutorials or self-made. Arranged into chapters.
After you read them, copy/paste and run them - you will be able to program in Perl on an average beginner level.

- Chapter 1, 2: Numeric and String Literals
- Chapter 3: Variables
- Chapter 4: Operators
- Chapter 5: Functions
- Chapter 6: Statements
- Chapter 7: Statements
- Chapter 8: References
- Chapter 9: Using Files
- Chapter 10: Regular Expressions
- Chapter 11: Creating Reports
- Chapter 12: Using Special Variables

 

-- Chapter 1,2 --

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# --------------------------------------------------
print("My name is Yon Yonson,\n");    #    this is a comment
print("I live in Wisconsin,\n",
      "I work in a lumbermill there.\n");

print("
   I live in NY,
   I work in GS there.
 ");

#  decimal, octal, hexadecimal nubmers
 print 123, ' ', 043, ' ', 0x23, "\n";
 
#  float numbers
 print 100.5, ' ', 54.534, ' ', 3.4E-5, "\n";
 
# quotes - single and double. Note using '\' to include special
# characters into the string:
 print 'WasWaldo can\'t hit the broad side of a barn.', "\n";
 print 'Morganna said, "WasWaldo can\'t hit anything."', "\n";
 print "WasWaldo the Illusionist\n";
 print "Morganna said, \"WasWaldo can't hit anything.\"", "\n";
 
 # using embeded line-breaks:
 print 'Bill of Goods
   Bread:    $34 .45
   Fruit:    $45.00
             ======
             $79.45', "\n";
 
# ----- There are many Escape Sequences - to insert special characters into strings, capitalize, etc.:
 print "\uwas\uwaldo is \x33\x34 years old.\n";
 print "The kettle was \Uhot\E!\n";
 print "\nYet another example:\n";
 print "Bill of Goods\n
   Bread:\t\$34.45\nFruit:\t\$45.00\n\t======\n\t\$79.45\n";
 
print "Back-Quoted Strings are used to run external commands and collect their output\n";

$allfiles = `dir *.pl`;
print "$allfiles\n";
print "===== hit 'Enter' to continue";
<>; # waiting for user to enter the line

# note: you can use qx() notation instead of  back quotes, for example:
print "------------------------------\n";
$allfiles = qx(dir *.pl);
print "$allfiles\n";

# --------------------------------------------------
# ----- arrays:
@arr1 = (); # empty array
@arr1 = ("This", "is", 'an', "array", 'of', "strings");
@arr2 = qw(This is an array of strings); # qw puts quotes and commas for you
for (@arr2) { print "$_\n" }
print "Here is an empty array:" . () . "<-- Nothing there!\n";
print (12, 014, 0x0c, 34.34, 23.3E-3), "\n";
print ("This", "is", 'an', "array", 'of', "strings"), "\n";
print ("This", 30, "is", 'a', "mixed array", 'of', 0x08, "items", "\n");
print (1..15), "\n";
print (1, 2, 7..10, 14, 15, "\n");
print ("A", "B", "F".."H", "Y", "Z", "\n\n");
# ----- putting to lists into one:
@arr1 = qw( a b c d e );  @arr2 = qw( f g h i j);
@arr3 = (@arr1, @arr2);
print "@arr3 \n";
# ----- array returns its length in scalar context:
$length1 = @arr1; $length3 = @arr3;
print "\$length1 = $length1, \$length3 = $length3\n";
print "\n";

 

-- Chapter 3 -- Variables

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# --------------------------------------------------
print "\$ - scalars , \@ - arrays, \% - hashes\n";
$numberOfRooms = 23;
$bookTitle = "Perl by Example";
$numberOfRooms = $numberOfRooms + 5;     # 23 + 5 = 28
$numberOfRooms += $numberOfRooms + 5;     # 28 + 5 = 33
 
@emptyArray = ();
@numberArray = (12, 014, 0x0c, 34.34, 23.3E-3);
@stringArray = ("This", "is", 'an', "array", 'of', "strings");
@mixedArray = ("This", 30, "is", 'a', "mixed array", 'of', 0x08, "items");

print "Here is an empty array:" . @emptyArray . "<- Nothing there!\n";
print @numberArray;  print "\n";
print @stringArray;  print "\n";
print @mixedArray;   print "\n";
print "======== press 'Enter' to continue:";<>;

@smallArrayOne = (5..10);
@smallArrayTwo = (1..5);
@largeArray = (@smallArrayOne, @smallArrayTwo);
print @largeArray, "\n";
print "======== press 'Enter' to continue:";<>;
 
print "\narray indexing:\n";
@array = (1..5);   # indexes 0..4, values 1..5
print @array, "\n";
print $array[0], "\n";
print $array[1], "\n";
print $array[2], "\n";
print $array[3], "\n";
print $array[4], "\n";
print $array[-1], "\n";  # last element = $array[4]
print $array[-2], "\n";  #  = $array[3]
print $array[-3], "\n";  #  = $array[2]
print $array[-4], "\n";  #  = $array[1]
print $array[-5], "\n";  #  = $array[0]
print "======== press 'Enter' to continue:";<>;
$index = 2;
print $array[$index], "\n";
 
print "\nnumber of elements:\n";
$numberOfElements = @array;
print "$numberOfElements\n";
print "======== press 'Enter' to continue:";<>;
 
 
print "\nGrab a slice of an Array:\n";
@array  = ("One", "Two", "Three", "Four");
($first, $third) = @array[0, 2];
@arr = (0,2);
($first, $third) = @array[@arr];  # same thing
print("\$first=$first  \$third=$third\n");
print "======== press 'Enter' to continue:";<>;

print "\nswap values of 2 variables:\n";
$aa = 'mama'; $bb='papa';
($aa, $bb) = ($bb, $aa);
print "$aa $bb\n";

# swap array elements
@array[0, 3] = @array[3, 0];
print("\@array=@array\n");
 
print "======== press 'Enter' to continue:";<>;

print "\nhashes (key-value pairs) - also called assotiative arrays:\n";
%hh = ("Jack A.", "Dec 2",
        "Joe B.",  "June 2");
 
 $hh{"Jennifer S."} = "Mar 20";
 
 print "Joe's birthday is: " . $hh{"Joe B."} . "\n";
 print "Jennifer's birthday is: " . $hh{"Jennifer S."} . "\n";
 print "======== press 'Enter' to continue:";<>;
 
 print "\nprinting arrays - see the difference:\n";
   print @array, "\n";
   print scalar(@array), "\n";
 
   print "@array\n";  # by default separates by spaces
   $" = ",";
   print "@array\n";  # no separates by commas
 
 

-- 4 -- Operators

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Chapter 4 - operators

# -----------------------------------------------
Operator Description
op1 + op2 Addition
op1 - op2 Subtraction
op1 * op2 Multiplication
op1 ** op2 Exponentiation
op1 / op2 Division
op1 % op2 Modulus
Example:
  $aa = 4; print $aa ** 3, "\n";  # 64

# -----------------------------------------------

for($i=0;$i<=100;$i++) {
  if($i % 10 == 0) {
    print "$i ";
  }
}
  # 0 10 20 30 40 50 60 70 80 90 100

# -----------------------------------------------

++ and -- : pre - act before using operand in expression
            after - act after using operand in expression
For example:
$aa = 5; $bb = $aa++;   # now $bb == 5, $aa == 6;
$aa = 5; $bb = ++$aa;   # now $bb == 6, $aa == 6;

# -----------------------------------------------

$aa = 5;
$aa = $aa + 3;  # inconvenient - has to write $aa two times
$aa += 3;       # shorter way - means "increase by 3"

Other similar operators:
 +=
 -=
 *=
 /=
 %=
 **=
 .=     concatenate strings together:
        $aa= "We are "; $aa .= "friends";

NOTE: similar you can use = with other operators described later:

var  x= op1; # repetition
var <<= op1; # bit shifting
var >>= op1; # bit shifting
var  &= op1; # bitwize AND
var  |= op1; # bitwize OR
var ||= op1; # logical OR
var  ^= op1; # logical XOR

# -----------------------------------------------

Check if something is true or false

if ($aa) { print "TRUE\n"; } else { print "FALSE\n" };

Three types of false values:
  $aa = 0;     # number zero
  $aa = "";    # empty string
  $aa = undef; # undefined value

Everything else is considered to be true

# -----------------------------------------------

Comparing equality:

 ==   number values are the same
 !=   numbers are different
 eq   string values are the same
 ne   string values are different

$str1 = "1";
$str2 = "01";
if ($str1 eq $str2) { print "equal strings,"; } else { print "different
strings,"; }
if ($str1 == $str2) { print " equal numbers\n"; } else { print " different
numbers\n"; }

   #   different strings, equal numbers

# -----------------------------------------------

Comparing:

numbers:  < , <= , > , >=
strings:  lt, le , gt, ge   # strings are compared by their ascii order

# -----------------------------------------------

3-way comparing:

( op1 <=> op2 )  # this operator returns:
                 #   1 if op1 is greater than op2,
                 #   0 if op1 equals op2,
                 #  -1 if op1 is less than op2.

( op1 cmp op2 )  # similar thing for strings

# -----------------------------------------------

Logical operators:

op1 && op2 - Performs a logical AND of the two operands.
             NOTE: runs op2 only if op1 is "true"
                   returns result of the last operation

op1 || op2 - Performs a logical OR of the two operands.
             NOTE: performs op2 only if op1 is "false"
                   returns result of the last operation

!op1       - Performs a logical NOT of the operand.

$aa = "mama";
$bb = "papa";
$cc = $aa && $bb;
print "$cc\n";   # papa

# -----------------------------------------------

low-priority version of logical operators:

  and    or    not

if ($aa == 9 || $aa == 10) { print("Error!") }
if ($aa == 9 or $aa == 10) { print("Error!") }

# -----------------------------------------------

Bitwize operators:

1   00000001
2   00000010
3   00000011
4   00000100
etc.

op1 & op2   bitwize AND  : (2 & 3) == 2
op1 | op2   bitwize OR   : (2 & 3) == 3
op1 ^ op2   bitwize XOR (exclusive OR)  : (2 ^ 3) == 1
~op1        bitwize NOT (COMPLEMENT):  ~63 = 4294967232
   63  11111111 11111111 11111111 00111111
   ~63 11111111 11111111 11111111 11000000

op1 << op2  SHIFT bits LEFT (effectively multiplies by 2)
op1 >> op2  SHIFT bits RIGHT (effectively divides by 2 for positive
numbers)

print "\n", 8 << 3;  # 64
print "\n", 8 >> 1;  # 4

# -----------------------------------------------

Terniary operator:  __ ? __ : __

Example1: getting one of 2 values
$max = ( $aa >= $bb ) $aa : $bb;

Example2: assigning to one of 2 values
$a1=1;$a2=2;$a3=3;
($a3 == 0 ? $a1 : $a2) = 4;
print "$a1 $a2 $a3\n";  # 1 4 3

# -----------------------------------------------

Range operator: ( .. )

@a = (1..10);  #  $a[0] == 1, $a[1] == 2, ..., $a[9] = 10

Examples of usage:
@arr = ("AAA", 1..5, "A".."D", "ZZZ");
@arr = ("aa" .. "af");

# -----------------------------------------------

The String Operators (. and x) - concatenation and repition.

$aa = "AA";
$bb = "BB";
$cc = $aa . $bb; # "AABB";

$separator = "-" x 20 . "\n";
print $separator;  #  --------------------

NOTE: you can use repetition operator to repeat array elements as well:

@arr = ('a') x 5;
print "@arr\n";   # a a a a a
 

Excercise:
@arr = ('A'..'G');
$aa = @arr x 2;
print("@arr\n");  # A B C D E F G
print("$aa\n");   # 77

# -----------------------------------------------

Assigning using array slices:
Example1:
@arr = (0..10);
@arr[4, 7] = ("AA","BB");
print("@arr\n");   #   0 1 2 3 AA 5 6 BB 8 9 10

Example2:
@arr1 = (0..10);
@arr2 = ("A".."Z");
@arr1[1..3] = @arr2[23..25];
print "@arr1\n";   #  0 X Y Z 4 5 6 7 8 9 10

# -----------------------------------------------

@arr = ("Tom Jones", "123 Harley Lane", "Birmingham", "AR");
($name, $street, $town, $state) = @arr;
print("$name, $street, $town, $state\n");

# -----------------------------------------------

Operator precedence:

$a1 = -2 ** 4;
$a2 = -(2 ** 4);
$a3 = (-2) ** 4;
print "$a1  $a2  $a3\n";  #  -16  -16  16

# -----------------------------------------------

Questions to repeat the main topics of the chapter:

What are three arithmetic operators?
What does the x operator do?
What does it mean to pre-decrement a variable?
What is the value of 1 ^ 1?
What is the value of 1 << 3?
What is the ternary operator used for?
Can the x operator be used with arrays?
What is the precedence level of the range operator?
What is the value of 2 % 5 + 10?
What is the value of 65 >> 1?
What is the spaceship operator used for?
If an array were defined with ("fy".."gb"), what would its elements be?

 

-- 5 -- Functions

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# functions = subroutins.
# Parameters passed as aliases into one array - @_.

# ----------------------------
sub s1 {
  print "Hello, sub\n";
}

s1();  # calling this sub
# ----------------------------
sub s2 {
  my ($a1, $a2) = @_;
  print "$a $b\n";
}
 
s2("mama", 123);
# ----------------------------
sub s3 {
  my $number = @_;  # number of elements
  print "number of parameters = $number : ";
  for (@_) { print "$_ "; }
  print "\n";
}

s3(1,2,3,"mama",'papa', 'crocodil');
s3(1, 2, 3, 4, 5, 6);
s3(1..3);
s3("A".."Z");
# ----------------------------
sub s4 {
  my $x1=shift;
  my $x2=shift;
  return $x2 >= $x1 ? $x2 : $x1;
}
$max = s4(4,5);

# ----------------------------
sub s5 {
  @_[0] +=1;
  print "in sub: $_[0]\n";
}
$aa=1;
s5($aa);
print "after sub: $aa\n";
# ----------------------------
sub s6 {
  my $x = shift;
  $x++;
  print "in sub: $x\n";
}
$aa=1;
s6($aa);
print "$aa\n";
# ----------------------------
sub s7 {
  for (@_) { $_++ }
}
@arr = (1, 2, 3);
s7(@arr);
print "@arr\n";
# ----------------------------
sub areaOfCircle {
  $radius = $_[0];
  return(3.1415 * ($radius ** 2));
}
print areaOfCircle(5),"\n";
# ----------------------------
# you can call functions from other functions.
# You can even call function from itself recursively

$i=0;
sub p { print "$i "; p(++$i) if $i<100; }
p(); print "\n";

sub factorial {
  my $x = shift;
  $x= int($x);
  if ($x<1) {
    print "number should be a positive integer, exiting..";
    exit(1);
  }
  return $x * factorial($x-1) if $x >=2;
  return $x;
}

print factorial(5),"\n";

# ----------------------------

# local variables:
#   local - temporarily make/substitute a global variable
#           used to temporary change values of special variables
#           like $/, $|, etc.
#
#   my    - create a true local variable
#           which is not visible outside the body of the sub

# ----------------------------
# some standard string functions:

# chomp(STRING) OR chomp(ARRAY)
#    Uses the value of the $/ special variable to remove endings from STRING
#    or each element of ARRAY. The line ending is only removed if it matches
#    the current value of $/.

# chop(STRING) OR chop(ARRAY)  Removes the last character from a string or
#    the last character from every element in an array. The last character
#    chopped is returned.

# chr(NUMBER)  Returns the character represented by NUMBER in the ASCII table.
#    For instaNCe, chr(65) returns the letter A.

# crypt(STRING1, STRING2)  ENCrypts STRING1.
#    Unfortunately, Perl does not provide a decrypt fuNCtion.

# index(STRING, SUBSTRING, POSITION)  Returns the position of the first
#    occurreNCe of SUBSTRING in STRING at or after POSITION.
#    If you don't specify POSITION, the search starts at the beginning of STRING.
 
# join(STRING, ARRAY)  Returns a string that consists of all of the elements
#    of ARRAY joined together by STRING.
#    For instaNCe, join(">>", ("AA", "BB", "cc")) returns "AA>>BB>>cc".

# lc(STRING)  Returns a string with every letter of STRING in lowercase.
#    For instaNCe, lc("ABCD") returns "abcd".

# lcfirst(STRING)  Returns a string with the first letter of STRING in lowercase.
#    For instaNCe, lcfirst("ABCD") returns "aBCD".

# length(STRING)  Returns the length of STRING.

# rindex(STRING, SUBSTRING, POSITION)  Returns the position of the last
#    occurreNCe of SUBSTRING in STRING at or after POSITION.
#    If you don't specify POSITION, the search starts at the end of STRING.

# split(PATTERN, STRING, LIMIT)  Breaks up a string based on some delimiter.
#    In an array context, it returns a list of the things that were found.
#    In a scalar context, it returns the number of things found.
 
# substr(STRING, OFFSET, LENGTH)  Returns a portion of STRING as determined
#    by the OFFSET and LENGTH parameters.
#    If LENGTH is not specified, then everything from OFFSET to the end
#    of STRING is returned. A negative OFFSET can be used to start from
#    the right side of STRING.
#    NOTE: also can be used to change a substring inside the string:
#    for example:
     $str = "my mother is living with me";
     print "$str\n";
     substr($str, 3, 6) = "dad";
     print "$str\n";
 

# uc(STRING)  Returns a string with every letter of STRING in uppercase.
#    For instaNCe, uc("abcd") returns "ABCD".

# Ucfirst(STRING)  Returns a string with the first letter of STRING
#    in uppercase. For instaNCe, ucfirst("abcd")

# -------------------------------------------

  $pathName = "C:\\WINDOWS\\TEMP\\somefile.txt";
  $position = rindex($pathName, "\\") + 1;
  $fileName = substr($pathName, $position);
  print("$fileName\n");

# -------------------------------------------
# Some functions often used with arrays and hashes:
# -------------------------------------------

$aa = undef;
print "NOT DEFINED\n" if not defined $aa;
$aa = "";
print "DEFINED\n" if defined $aa;

# -------------------------------------------
%hh = ( k1 => 'v1', k2 => v2);
$kk = 'k3';
print "key '$kk' does not exist\n" if not exists $hh{$kk};
$kk = 'k2';
delete $hh{$kk} if exists $hh{$kk};
print "key '$kk' does not exist\n" if not exists $hh{$kk};

# -------------------------------------------
# 2 ways to go through the elements of a hash

while (my ($kk,$vv) = each %hh) {
  do something with $kk and $vv
}

for my $key (keys %hh) {
   print $hh{$key},"\n";
}

for  (sort keys %hh) {
   print $hh{$_},"\n";
}

# -------------------------------------------

# 5 functions to add/remove elements of the array:
# push, pop, shift, unshift, splice

push @arr, $var;   # add one element to the end of the array - and assign its value to be $var
push @arr, (1,2,3);  # add 3 elements
push @arr, @arr2, $var1, $var2;  # add many elements

$var = pop @arr;  # remove last element from the array and return it. Array gets shorter by 1.
$var = shift @arr;  # same as pop - but takes element from the beginning of the array

unshift @arr, $var;  # same as push, but adds elements to the beginning of the array.

splice(@arr, $ofset, $length, @arr2);
  # Replaces elements of @arr (starting at $ofset, number - $length)
  # with elements of @arr2 (number elements in it may differ $length).
  # It returns a list holding any elements that were removed.
  # Note: special variable $[  may change the base array subscript when determining the OFFSET value.

# -------------------------------------------
#  make a string from the array of words using join( ):
# split string into an array of words using split():
  @arr = qw(word1 word2 word3 word4);
  print "@arr\n";
  $string = join (":", @arr);
  print "$string\n";
  @arr2 = split (/:/, $string);
  print "@arr2\n";

# -------------------------------------------
# use undef()  to undefine variable, array or hash
$var = "";
@arr = (1,2,3);
%hh = (1=>2,3=>4);

print "defined \$var\n" if defined $var;
print "defined \@arr\n" if defined @arr;
print "defined \%hh\n"  if defined %hh;

undef $var;
undef @arr;
undef %hh;

print "NOT defined \$var\n" if not defined $var;
print "NOT defined \@arr\n" if not defined @arr;
print "NOT defined \%hh\n"  if not defined %hh;

# -------------------------------------------
$length = scalar @arr;
   #  forces to dot he same as $length = @arr;
   # that is - evaluate array in a scalar context
   # which returns the length (number of elements) of the array
# -------------------------------------------
 @arr = (1,2,3,4,'mama');
 @revarr = reverse @arr;   # reverse the order of array elements
 print "@revarr\n";
 $revstr = reverse @arr; # concatenates all elements in a string and reverses the string
 print scalar reverse @arr;  # amam4321
 print "\n";

# -------------------------------------------
@sorted = sort @arr;
@sorted = sort { $a <=> $b } $arr;

# -------------------------------------------
#  map  and   grep

map { some code with $_ } @arr;
@result = map { some code with $_ } @arr; # result elements are calculated for each element of @arr;
@filtered = grep { some code with $_ } @arr;   # filtered are those for which code returns true;
# -------------------------------------------
pack(format, @arr); # creates a binary structure from @arr using format
unpack(format, @arr); # the opposite of pack().

# -------------------------------------------

using q(), qq(), qw(), qx()
 

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