Perl m03 - Arrays & Hashes

7/23/2019 Perl m03 - Arrays & Hashes

http://slidepdf.com/reader/full/perl-m03-arrays-hashes 1/30

Copyright © Amstar Technologies 1

PPracticalractical EExtraction &xtraction & RReporteportLLanguageanguage

By:B.T.R NaiduB.T.R Naidu

Email: [email protected]

Copyright © Amstar Technologies




PERL

Arrays & Hashes




Data Types Arrays

Arrays – Different types of scalarscan be stored in a single array.

Declare: @variableName;

Assign:@varName = (elem1, elem2, …, elemN);

Index: $variableName[index]• Index starts at 0 just like in C




Data Types Arrays

We can create an array by assigning a list to a variable like this@array=(1,2,3);

To see the result, we can print out the new array

print @array; # Output: 123

We can also refer to individual array elements by index usingsquare brackets and prefacing the name of the element with $,because it’s a scalar

@array = (1, 2, 3);print $array[0]; # Output: 1

Besides numbers, we can also store other types of scalars, like

strings@array = (“one”, “two”, “three”);

print @array; # Output: onetwothree




Data Types Arrays

Note: Because Perl skips over white space (including Newlines)when handling lists, we can set up our array assignment this wayas well.

@array = (

“one”, “two”, “three”,

“four”, “five”, “six”,

);

print @array; # Output: onetwothreefourfivesix

We can also use the x repetition operator, as in this case, whichcreates an array of 100 zeroes.

@array = (0) x 100;

And we can use quote operators like qw.@array = qw(one two three);

print @array; # Output: onetwothree




Data Types Arrays

Note: Although arrays are zero based by default, we canactually change the base by placing a new value in the Perlspecial variable $[.

We can vary a loop index to work through every value in anyarray

@array = (“one”, “two”, “three”);

for ($loop_index=0; $loop_index<=$#array; $loop_index++)

{print $array[$loop_index];

}

Output: onetwothree

Note: The use of the value $#array, which, in Perl, is the lastindex in the array @array.




Data Types Arrays

Besides using list assignments, we can use the push and popfunctions to work with arrays.

The push function adds a value, or values to the end of an array

push ARRAY, LIST

The length of the ARRAY increases by the length of LIST.

The pop function get a value from an array

pop ARRAY

In particular, this function pops ( removes and returns ) the lastvalue of the array, shortening the array by one element.

Example for push:

push(@array, “one”);

push(@array, “two”);

push(@array, “three”);

print $array[0]; # Output: one




Data Types Arrays

Example for pop:@array = (“one”, “two”, “three”);

$var1 = pop(@array);

print $var1; # Output: three

shift and unshift do to the left end of the array with pushand pop do to the right end.

The shift function shifts the first value of the array off and

returns it, shortening the array by one element and movingeverything down one place

shift ARRAY

This unshift function does the opposite of shift; it adds LISTto the front of the array and returns the new elements inthe array.

unshift ARRAY, LIST




Data Types Arrays

Finding The Length of ArraysWhen we have an array names, say,

@array, then the expression $#arrayholds the last index value in the array.

We can display the number of elements inthis array by adding 1 to $#array:

@array = (1, 2, 3);print “\@array has “.($#array+1).” Elements.”;

Output: @array has 3 elements




Data Types Arrays

Using an array in a scalar context also returns itslength.

To put an array in a scalar context, we can dosomething numeric that has no effect, like addinga zero to it, such as

@array + 0

or; more professionally, we can use the scalarfunction:

@array = (1, 2, 3);

print “\@array has “.scalar(@array).” Elements.”;

Output: @array has 3 elements.




Data Types Arrays

Growing Or Shrinking Arrays We can change the number of elements in an array simply by

changing the value of the last index value in the value $#array.

@array = (1, 2, 3);

$#array = 10;$array[5] = “Here is a new element!”;

print “$array[5]\n”;

Output: Here is a new element!

In fact, if you simply refer to a nonexistent element in anarray, Perl extends the array as needed, creating newelements up to and including the new element:

You can empty an array by setting its length to a negativenumber:$#array = -1;




Data Types Arrays

Merging Two Arrays We can merge two arrays with a list assignment.

@array1 = (1, 2, 3);

@array2 = (4, 5, 6);

@bigarray = (@array1, @array2);

print $bigarray[5]; #Output:6

Getting Array Slices

An array slice is a section of an array that we can createwith the range operator.

The range operator works like this: [x..y]. Here, we arereferring to the array elements x, x+1, all the way up to y.

@array = (1, 2, 3, 4, 5, 6, 7, 8, 9, 10);

@array2 = @array[2..3];

print join(“, “, @array2);

Output: 3, 4




Data Types Arrays

Looping Over Arrays Using for loop


for($loop_index = 0; $loop_index <= $#array; $loop_index++){

print $array[$loop_index];

}

Output: onetwothree

Using foreach loop@array = (1, 2, 3, 4, 5);

foreach $element (@array) {

print “$element\t”;

}

Output: 1 2 3 4 5




Data Types Arrays

In fact, you can loop over several arrays at thesame time by creating a list of arrays.@array = (1, 2, 3);

@array2 = (4, 5, 6);

foreach $element (@array, @array2) {


}

Output: 1 2 3 4 5 6

Besides foreach, you can use a for loop

@array = (1, 2, 3, 4, 5);

for $element (@array) {


}

Output: 1 2 3 4 5




Data Types Arrays

When you want to, you can use a for loop withoutspecific reference to the elements in the loop atall, using the default variable $_:

@array = (1, 2, 3, 4, 5);

for (@array) {

print;

}

Output: 12345

Printing ArraysWhen you just want to print an array, you can

pass it to the print function this way@array = (“one”, “two”, “three”);

print “Here is the array: @array.\n”;

Output: Here is the array: one two three.




Data Types Arrays

If you want to display a comma between eachelement in the array then use the join functionlike this -


print join(“, “, @array);Output: one, two, three

You are also free to explicitly loop over all theelements in the array using for or foreach -


foreach $element (@array) {

print “Current element = $element\n”;

}

Output: Current element = one

Current element = two

Current element = three




Data Types Arrays

Splicing Arrays Splicing an array means adding elements from a list to that

array, possibly replacing elements now in the array. You

use the splice function to splice arrays.Splice ARRAY,OFFSET,LENGTH,LIST

Splice ARRAY,OFFSET,LENGTH

Splice ARRAY,OFFSET

The splice function removes the elements indicated byOFFSET and LENGTH from an array and replaces them withthe elements of LIST, when you specify a list.

@array = (“one”, “two”);splice(@array, 2, 0, “three”);

print join(“, “, @array);

Output: one, two, three




Data Types Arrays

We can also splice a new array onto the end of an old one@array = (“one”, “two”);

@array2 = (“three”, “four”);

splice(@array, 2, 0, @array2);


Output: one, two, three, four

We can also replace elements in an array that we aresplicing

@array = (“one”, “zero”);

@array2 = (“two”, “three”, “four”);splice(@array, 1, 1, @array2);


Output: one, two, three, four




Data Types Arrays

Reversing Arrays To reverse an array, you simply use the reverse

function.

@New = reverse @array;

Sorting Arrays To sort an array, you use the sort function.

@new = sort {$a <=> $b} @array; Here’s an array sorted in descending order.

@new = sort {$b <=> $a} @array;




Data Types Hashes

Hashes – associative arrays; definepairs of scalars, keys then elements

Declare: %varName;Assign:• %varName = (key1, elem1, key2, elem2, … );

Index: $varName{key};




Data Types Hashes

As with arrays, we use the $ prefix dereferencers whenworking with individual hash elements.

Note that we use curly braces, {}, to dereference a hashelement, not square brackets, [], as we do with arrays.

%hash = ();$hash{fruit} = apple;

$hash{sandwich} = hamburger;

$hash{drink} = bubbly;

print $hash{sandwich}; # Output: hamburger

we can create a hash like this, specifying the key/valuepairs we want to fill the hash with:

%hash = (

fruit, apple,

sandwich, hamburger,

drink, bubbly,

);

print “$hash{fruit}\n”; # Output: apple




Data Types Hashes

In fact, there’s a synonym for a comma: =>. Using thisoperator makes the relationship between keys and valuesclearer

%hash = (fruit => apple,

sandwich => hamburger,drink => bubbly,

);

print “$hash{fruit}\n”; # Output: apple

We can also use keys with spaces in them, as in this case,which creates a hash element with the key ice cream:

$hash2{cake} = chocolate;$hash2{pie} = blueberry;

$hash2{‘ice cream’} = pecan;print “$hash{‘ice cream’}\n”; # Output: pecan

Keep in mind that we can’t reference the values in a hashdirectly with a numeric index.




Data Types Hashes

Adding elements to Hashes We can create hashes using list assignments

%hash = (

fruit => apple,sandwich => hamburger,

drink => bubbly,

);

%hash = (%hash, ‘dressing, ‘blue cheese’);print “$hash{dressing}\n”;

# Output: bleu cheese

This example works because the list operator, (),interpolates %hash into a list, and then we extend that list

by one key/value pair.




Data Types Hashes

Checking If Hash Elements Exist To check if an element exists in a hash, we can use the

exists function.

$hash{fruit} = apple;$hash{sandwich} = hamburger;


if ( exists ($hash{“vegetable”})) {

print “Element exists.”;

} else {

print “Element does not exist.”;

}

Output: Element does not exist




Data Types Hashes

Deleting Hash Elements To delete an element in a hash, just use the

delete function.

$hash{fruit} = apple;$hash{sandwich} = hamburger;


delete($hash{“fruit”});

if ( exists ($hash{“fruit”})) {

print “Element exists.”;

} else {print “Element does not exist.”;

}

Output: Element does not exist.




Data Types Hashes

Looping Over Hashes

Using each function

while(($key, $value) = each (%hash)) {

print “$key => $value\t”;

}

Output: drink=>buddly sandwich => hamburger fruit => apple

Using keys function

foreach $key (keys %hash) {

print $hash{$key} . “\t”;}

Output: bubbly hamburger apple

Using values function

foreach $value (values %hash) {print $value . “\t”;

}

Output: bubbly hamburger apple




Data Types Hashes

Printing HashesWe can print a hash by interpolating it in

double quotes

$hash{fruit} = apple;



print “@{[%hash]}\n”;

#Output: drink bubbly sandwich hamburger fruit apple

Note that this prints the hash as it list context: askey/value pairs, one after the other. A betterchoice might be to use the each function.




Data Types Hashes

Sorting Hashes We can use the sort function to sort a hash



$hash{drink} = bubbly;foreach $key (sort keys %hash) {

print “$key => $hash{$key}\t”;

}

Output: Drink => bubbly Fruit => apple Sandwich => hamburger

We can also sort a hash by value instead of by key




foreach $value (sort values %hash) {

print “$value\t”;

}

Output: apple bubbly hamburger




Data Types Hashes

Merging Two Hashes To merge two hashes, we can use a list

assignment

$hash1{fruit} = apple;

$hash1{sandwich} = hamburger;

$hash1{drink} = bubbly;

$hash2{cake} = chocolate;

$hash2{pie} = blueberry;

$hash2{‘ice cream’} = pecan;%bighash = (%hash1, %hash2);

print $bighash{‘ice cream’};

Output: pecan

Perl m03 - Arrays & Hashes

Documents

Transcript of Perl m03 - Arrays & Hashes