Tuesday, April 15, 2008

Perl arrays

An array in perl is an ordered collection of scalar items.While scalar data (single pieces of data) use the $ sign, arrays use the @ symbol in perl.Array indices are whole numbers and the first index is 0.

There are 3 distict characteristics for arrays in perl :

1. Perl supports only single dimentional arrays.
2. Array size cannot be fixed.
3. Collection of data items of any types.

Examples :
@strn=("abc",34,56.7,"hello"); # Declares and initialises an array
print @strn; # Print all the elements

What does the below code fragment do ?

Since we are assigning a list to a scalar,it takes the last value , ie, "hello".

PS : For the difference between arrays and lists , see here.

The syntax that is used to access arrays is closer to arrays in C. In fact, one can often treat Perl's arrays as if they were simply C arrays, but they are actually much more powerful than that.

$, is a global variable and is called the field seperator.By default its not set to anything.Therefore print @string statement above prints all the elements without any spaces.Now , we can use the field seperator variable to our own type of seperator.

Examples :
$,=" ";
print @strn;

What do you think the below code fragment should output ?
print "value of $x is ",$x,"\n" ;

Some more special global variables :
$# Gives the size of the array.
$" Special variable used when printing an array . Default is a space.
$\ Output record seperator.Default is nothing.
$/ Input record seperator.Default is \n.

Below examples show how the size of an array is referenced
$s=@strn; # Assigning the array to a scalar gives the no. of elements of the array.
print @strn>5 ; # In an scaler context we compare with the size of the array.
print scalar @strn; # Explicitly request the size of an array.
print $#strn; # Returns the last index no.

Note we can have a scalar and an array with the same name.
Examples :
print $strn[0]; # The square bracket differentiates it to be an array.
$strn[100]="rrr"; # Now the array size is 101.All the uninitialized values are 0.
# Array elements beyond the array size is undef/null.
$#strn=5; # Reduces the size of the array.
print @strn[0,4,2]; # prints 0th , 4th and 2nd element.

.. is the range operator.Range should always be positive.
Examples :
@strn[11..15]=(45,6,7,8,9); # truncates any additional values given.
print $strn[-1]; # -1 is the last index no.
print $strn[-2]; # -2 is second last index and so on.

Build-In Array functions :

1. Push ( push array,list of elements )
Push 1 or more elements.Push returns size of the new array.

Examples :
@n=qw(a b c d e f); # qw stands for quote words.
push @n,"56",33,"aa";
print push @n,"ui","ll"; # prints the size of the new array.
print push @n; # returns the size of array.
print @n;

is the same as
@n=qw(hello world);

2. Pop ( pop arrayname )
Removes the last element of an array and decrease the size of array.
Returns the element removed.

Examples :
print @ARGV;
pop; # pop looks into @ARGV & removes the last element.
print @ARGV;

3. Unshift ( unshift arrayname,list of elements )
Adds the elements at the begining of array ( the opposite of push )

Examples :
unshift @n,"first",second";
print @n;

4. Shift ( shift arrayname )
Same as pop , but removes the first element.

Examples :
my @numbers = (1 .. 10);
while(scalar(@numbers) > 0)
my $i = shift(@numbers);
print $i, "\n";

5. Splice ( splice arr,startindex,no. of elem to be removed,list of elem to add )
Overwrite/Append anywhere in an array.

Examples :
splice @cities,2,1,"mys";
print "@cities";

splice @cities,0,0,"mum","sri","bhu"; # appends at the begining.
print "@cities";

splice @cities,1,2; # remove 2 elements begining at index 1.Index starts at 0.
splice @cities,3; # removes all the elements starting from the 3rd index.
splice @cities; # deletes all the elements.

6. Sort ( sort arrayname )
Sort the array elements by ASCII ascending order ( default ).This dosen't modify the array,returns a new sorted array.By default the array elements are compared with the string comparision operator.

Examples :
$,=" ";
print sort @cities; # prints a ascii sorted list with a space inbetween.
print @cities;

@cities=sort @cities; # overwrites the array with the sorted array
print @cities;

Below examples show how to do numeric comparisions.
Examples :
print sort @nn; # o/p 1,11,20,30,45,67 ( ascii sort ).
print sort{$a <=> $b} @nn; # ascending order . Remember this construct.
print @nn;
print sort{$b <=> $a} @nn; # descending order. Remember this construct.
print sort{$b cmp $a} @cities; # string (ascii)comparision in descending order.

7. Reverse ( reverse arrayname )
Reverse the array elements.Dosen't modify the array.

Examples :
print reverse @cities; # prints reverse.
print reverse sort @cities; # descending order.

8. split ( split ,string )
Returns an array splitting on a character or string.

Examples :
@arr2=split(m/:+/,$s); # m stands for match.
# The contents between / / is the regex pattern
# $s is the string to be searched.

9. Join ( Join char/string,string )
Its the opposite of split.Returns a string.

Examples :
$st=join "-",@cities;
print $st;
print join "\n",@cities;

10. Delete ( delete array )
Deletes any element of an array.Deleting an element other than the last element of the array dosen't change the size of the array , else it changes.

Examples :
delete $cities[1];
print "@cities";
print scalar @cities;
delete $cities[$#cities];
print scalar @cities;
print "@cities";

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