ch5.htm

this is a book on pearl , simple example with explanation is given here. it could be beneficial for

perform iNClude deleting elements, checking for the existeNCe

of an element, reversing all of the the elements in an array,

and sorting the elements. Table 5.2 lists the fuNCtions you can

use with arrays.<BR>

<P>

<CENTER><B>Table 5.2&nbsp;&nbsp;Array FuNCtions</B></CENTER>

<p>

<CENTER>

<TABLE BORDERCOLOR=#000000 BORDER=1 WIDTH=80%>

<TR><TD WIDTH=205><I>FuNCtion</I></TD><TD WIDTH=385><I>Description</I>

</TD></TR>

<TR><TD WIDTH=205><TT>defined(VARIABLE)</TT>

</TD><TD WIDTH=385>Returns true if <TT>VARIABLE</TT> has a real value and if the variable has not yet been assigned a value. This is not limited to arrays; any data type can be checked. Also see the <TT>exists</TT> fuNCtion for information about 
associative array keys.

</TD></TR>

<TR><TD WIDTH=205><TT>delete(KEY)</TT>

</TD><TD WIDTH=385>Removes the key-value pair from the given associative array. If you delete a value from the <TT>%ENV</TT> array, the environment of the current process is changed, not that of the parent.

</TD></TR>

<TR><TD WIDTH=205><TT>each(ASSOC_ARRAY)</TT>

</TD><TD WIDTH=385>Returns a two-element list that contains a key and value pair from the given associative array. The fuNCtion is mainly used so  you can iterate over the associate array elements. A null list is returned when the last element has been 
read.

</TD></TR>

<TR><TD WIDTH=205><TT>exists(KEY)</TT>

</TD><TD WIDTH=385>Returns true if the <TT>KEY</TT> is part of the specified associative array. For instaNCe, <TT>exists($array{&quot;Orange&quot;})</TT> returns true if the <TT>%array</TT> associative array has a key with the value of &quot;Orange.&quot;


</TD></TR>

<TR><TD WIDTH=205><TT>join(STRING, ARRAY)</TT>

</TD><TD WIDTH=385>Returns a string that consists of all of the elements of <TT>ARRAY</TT> joined together by <TT>STRING</TT>. For instaNCe, <TT>join(&quot;&gt;&gt;&quot;, (&quot;AA&quot;, &quot;BB&quot;, &quot;cc&quot;))</TT> returns 
<TT>&quot;AA&gt;&gt;BB&gt;&gt;cc&quot;</TT>.

</TD></TR>

<TR><TD WIDTH=205><TT>keys(ASSOC_ARRAY)</TT>

</TD><TD WIDTH=385>Returns a list that holds all of the keys in a given associative array. The list is not in any particular order.

</TD></TR>

<TR><TD WIDTH=205><TT>map(EXPRESSION, ARRAY)</TT>

</TD><TD WIDTH=385>Evaluates <TT>EXPRESSION</TT> for every element of <TT>ARRAY</TT>. The special variable <TT>$ </TT>is assigned each element of <TT>ARRAY</TT> immediately before <TT>EXPRESSION</TT> is evaluated.

</TD></TR>

<TR><TD WIDTH=205><TT>pack(STRING, ARRAY)</TT>

</TD><TD WIDTH=385>Creates a binary structure, using <TT>STRING</TT> as a guide, of the elements of <TT>ARRAY</TT>. You can look in <A HREF="ch8.htm" >Chapter 8</A> &quot;RefereNCes,&quot; for more information.

</TD></TR>

<TR><TD WIDTH=205><TT>pop(ARRAY)</TT>

</TD><TD WIDTH=385>Returns the last value of an array. It also reduces the size of the array by one.

</TD></TR>

<TR><TD WIDTH=205><TT>push(ARRAY1, ARRAY2)</TT>

</TD><TD WIDTH=385>Appends the contents of <TT>ARRAY2</TT> to <TT>ARRAY1</TT>. This iNCreases the size of <TT>ARRAY1</TT> as needed.

</TD></TR>

<TR><TD WIDTH=205><TT>reverse(ARRAY)</TT>

</TD><TD WIDTH=385>Reverses the elements of a given array when used in an array context. When used in a scalar context, the array is converted to a string, and the string is reversed.

</TD></TR>

<TR><TD WIDTH=205><TT>scalar(ARRAY)</TT>

</TD><TD WIDTH=385>Evaluates the array in a scalar context and returns the number of elements in the array.

</TD></TR>

<TR><TD WIDTH=205><TT>shift(ARRAY)</TT>

</TD><TD WIDTH=385>Returns the first value of an array. It also reduces the size of the array by one.

</TD></TR>

<TR><TD WIDTH=205><TT>sort(ARRAY)</TT>

</TD><TD WIDTH=385>Returns a list containing the elements of <TT>ARRAY</TT> in sorted order. See <A HREF="ch8.htm" >Chapter 8</A> &quot;RefereNCes,&quot; for more information.

</TD></TR>

<TR><TD WIDTH=205><TT>splice(ARRAY1, OFFSET, </TT>

</TD><TD WIDTH=385>Replaces elements of <TT>ARRAY1</TT> with elements 

</TD></TR>

<TR><TD WIDTH=205><TT>LENGTH, ARRAY2)</TT>

</TD><TD WIDTH=385>in <TT>ARRAY2</TT>. It returns a list holding any elements that were removed. Remember that the <TT>$[</TT> variable may change the base array subscript when determining the <TT>OFFSET</TT> value.

</TD></TR>

<TR><TD WIDTH=205><TT>split(PATTERN, STRING, LIMIT)</TT>

</TD><TD WIDTH=385>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.

</TD></TR>

<TR><TD WIDTH=205><TT>undef(VARIABLE)</TT>

</TD><TD WIDTH=385>Always returns the undefined value. In addition, it undefines <TT>VARIABLE</TT>, which must be a scalar, an entire array, or a subroutine name.

</TD></TR>

<TR><TD WIDTH=205><TT>unpack(STRING, ARRAY)</TT>

</TD><TD WIDTH=385>Does the opposite of <TT>pack()</TT>. 

</TD></TR>

<TR><TD WIDTH=205><TT>unshift(ARRAY1, ARRAY2)</TT>

</TD><TD WIDTH=385>Adds the elements of <TT>ARRAY2</TT> to the front of <TT>ARRAY1</TT>. Note that the added elements retain their original order. The size of the new <TT>ARRAY1</TT> is returned.

</TD></TR>

<TR><TD WIDTH=205><TT>values(ASSOC_ARRAY)</TT>

</TD><TD WIDTH=385>Returns a list that holds all of the values in a given associative array. The list is not in any particular order.

</TD></TR>

</TABLE>

</CENTER>

<P>

<P>

As with the string fuNCtions, only a few of these fuNCtions will

be explored. ONCe you see the examples, you'll be able to handle

the rest with no trouble.

<H3><A NAME="ExamplePrintinganAssociativeArray">

Example: Printing an Associative Array</A></H3>

<P>

The <TT>each()</TT> fuNCtion returns

key, value pairs of an associative array one-by-one in a list.

This is called <I>iterating</I> over the elements of the array.

Iteration is a synonym for looping. So, you also could say that

the <TT>each()</TT> fuNCtion starts

at the beginning of an array and loops through each element until

the end of the array is reached. This ability lets you work with

key, value pairs in a quick easy manner.

<P>

The <TT>each()</TT> fuNCtion does

not loop by itself. It needs a little help from some Perl control

statements. For this example, we'll use the <I>while</I> loop

to print an associative array. The <TT>while

(CONDITION) {}</TT> control statement continues to execute

any program code surrounded by the curly braces until the <TT>CONDITION</TT>

turns false.

<P>

<IMG SRC="pseudo.gif" BORDER=1 ALIGN=RIGHT><p>

<BLOCKQUOTE>

<I>Create an associative with number, color pairs.<BR>

Using a while loop, iterate over the array elements.<BR>

Print the key, value pair.</I>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

%array = ( &quot;100&quot;, &quot;Green&quot;, &quot;200&quot;, &quot;Orange&quot;);



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

      print(&quot;$key = $value\n&quot;);

}

</PRE>

</BLOCKQUOTE>

<P>

This program prints:

<BLOCKQUOTE>

<PRE>

100 = Green

200 = Orange

</PRE>

</BLOCKQUOTE>

<P>

The <TT>each()</TT> fuNCtion returns

false when the end of the array is reached. Therefore, you can

use it as the basis of the <TT>while</TT>'s

condition. When the end of the array is reached, the program continues

execution after the closing curly brace. In this example, the

program simply ends.

<H3><A NAME="ExampleCheckingtheExisteNCeofanElement">

Example: Checking the ExisteNCe of an Element</A></H3>

<P>

You can use the <TT>defined()</TT>

fuNCtion to check if an array element exists before you assign

a value to it. This ability is very handy if you are reading values

from a disk file and don't want to overlay values already in memory.

For instaNCe, suppose you have a disk file of customers' addresses

and you would like to know if any of them are duplicates. You

check for duplicates by reading the information one address at

a time and assigning the address to an associative array using

the customer name as the key value. If the customer name already

exists as a key value, then that address should be flagged for

follow up.

<P>

Because we haven't talked about disk files yet, we'll need to

emulate a disk file with an associative array. And, instead of

using customer's address, we'll use customer number and customer

name pairs. First, we see what happens when an associative array

is created and two values have the same keys.

<P>

<IMG SRC="pseudo.gif" BORDER=1 ALIGN=RIGHT><p>

<BLOCKQUOTE>

<I>Call the </I><TT><I>createPair()</I></TT><I>

fuNCtion three times to create three key, value pairs in the </I><TT><I>%array</I></TT><I>

associative array.<BR>

Loop through </I><TT><I>%array</I></TT><I>,

printing each key, value pair.<BR>

Define the </I><TT><I>createPair()</I></TT><I>

fuNCtion.<BR>

Create local variables to hold the key, value pair passed as parameters.

<BR>

Create an array element to hold the key, value pair.</I>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

createPair(&quot;100&quot;,  &quot;Kathy Jones&quot;);

createPair(&quot;200&quot;,  &quot;Grace Kelly&quot;);

createPair(&quot;100&quot;, &quot;George Orwell&quot;);



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

    print(&quot;$key, $value\n&quot;);

};



sub createPair{

    my($key, $value) = @_ ;



    $array{$key} = $value;

};

</PRE>

</BLOCKQUOTE>

<P>

This program prints:

<BLOCKQUOTE>

<PRE>

100, George Orwell

200, Grace Kelly

</PRE>

</BLOCKQUOTE>

<P>

This example takes advantages of the global nature of variables.

Even though the <TT>%array</TT> element

is set in the <TT>createPair()</TT>

fuNCtion, the array is still accessible by the main program. Notice

that the first key, value pair (100 and Kathy Jones) are overwritten

when the third key, value pair is eNCountered. You can see that

it is a good idea to be able to determine when an associative

array element is already defined so that duplicate entries can

be handled. The next program does this.

<P>

<IMG SRC="pseudo.gif" BORDER=1 ALIGN=RIGHT><p>

<BLOCKQUOTE>

<I>Call the </I><TT><I>createPair()</I></TT><I>

fuNCtion three times to create three key,  value pairs in the

</I><TT><I>%array</I></TT><I> associative

array.<BR>

Loop through </I><TT><I>%array</I></TT><I>,

printing each key, value pair.<BR>

Define the </I><TT><I>createPair()</I></TT><I>

fuNCtion.<BR>

Create local variables to hold the key, value pair passed as parameters.

<BR>

If the key, value pair already exists in </I><TT><I>%array</I></TT><I>,

then iNCrement<BR>

 the customer number by one. Check to see if the new key, value

<BR>

pair exists. If so, keep iNCrementing until a nonexistent<BR>

key, value pair is found.<BR>

Create an array element to hold the key, value pair.</I>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

createPair(&quot;100&quot;,  &quot;Kathy Jones&quot;);

createPair(&quot;200&quot;,  &quot;Grace Kelly&quot;);

createPair(&quot;100&quot;, &quot;George Orwell&quot;);



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

    print(&quot;$key, $value\n&quot;);

};



sub createPair{

    my($key, $value) = @_ ;



    while (defined($array{$key})) {

        $key++;

    }



    $array{$key} = $value;

};

</PRE>

</BLOCKQUOTE>

<P>

This program prints:

<BLOCKQUOTE>

<PRE>

100, George Orwell

101, Kathy Jones

200, Grace Kelly

</PRE>

</BLOCKQUOTE>

<P>

You can see that the customer number for Kathy Jones has been

changed to 101. If the array had already had an entry for 101,

the Kathy Jones' new customer number would be 102.

<H2><A NAME="Summary"><FONT SIZE=5 COLOR=#FF0000>

Summary</FONT></A></H2>

<P>

In this chapter you've learned about fuNCtions-what they are and

how to call them. You saw that you can create your own fuNCtion

or use one of Perl's many built-in fuNCtions. Each fuNCtion can

accept any number of parameters which get delivered to the fuNCtion

in the form of the <TT>@ </TT>array.

This array, like any other array in Perl, can be accessed using

the array element to access an individual element. ( For instaNCe,

<TT>$_ [0]</TT> accesses the first

element in the <TT>@ </TT>array.)

Because Perl parameters are passed by refereNCe, changing the

<TT>@</TT> array changes the values

in the main program as well as the fuNCtion.

<P>

You learned about the scope of variables and how all variables

are global by default. Then, you saw how to create variable with

local scope using <TT>local()</TT>

and <TT>my()</TT>. <TT>My()</TT>

is the better choice in almost all situations because it enforces

local scope and limits side effects from fuNCtion to inside the

fuNCtions.

<P>

Then you saw that it was possible to nest fuNCtion calls, which

means that one fuNCtion can call another, which in turn can call

another. You  also might call this a chain of fuNCtion calls.

Private fuNCtions were introduced next. A private fuNCtion is

one that only can be used inside the fuNCtion that defines it.

<P>

A list of string fuNCtions then was presented. These iNCluded

fuNCtions to remove the last character, eNCrypt a string, find

a sub-string, convert array elements into a string, change the

case of a string character, and find the length of a string. Examples

were shown about how to change a string's characters and how to

search a string.

<P>

The section on array fuNCtions showed that Perl has a large number

of fuNCtions that deal specifically with arrays. The list of fuNCtions

iNCluded the ability to delete elements, return key, value pairs

from associative arrays, reverse an array's elements, and sort

an array. Examples were shown for printing an associative array

and checking for the existeNCe of an element.

<P>

The next chapter, &quot;Statements,&quot; goes into detail about

what statements are and how you create them. The information that

you learned about variables and fuNCtions will come into play.

You'll see how to link variables and fuNCtions together to form

expressions and statements.

<H2><A NAME="ReviewQuestions"><FONT SIZE=5 COLOR=#FF0000>

Review Questions</FONT></A></H2>

<P>

Answers to Review Questions are in Appendix A.

<OL>

<LI>What is a parameter?

<LI>What two fuNCtions are used to create variables with local

scope?

<LI>What does parameter passing by refereNCe mean?

<LI>What is the <TT>@_ </TT>array

used for?

<LI>Do Perl variables have global or local scope by default?

<LI>Why is it hard to pass two arrays to a fuNCtion?

<LI>What is the differeNCe between variables created with <TT>local()</TT>

and variables created with <TT>my()</TT>?

<LI>What does the <TT>map()</TT> fuNCtion

do?

</OL>

<H2><A NAME="ReviewExercises"><FONT SIZE=5 COLOR=#FF0000>

Review Exercises</FONT></A></H2>

<OL>

<LI>Create a fuNCtion that prints its own parameter list.

<LI>Create a program that uses three fuNCtions to demonstrate

fuNCtion call nesting.

<LI>Use the <TT>chop()</TT> fuNCtion

in a program. Print both the returned character and the string

that was passed as a parameter.

<LI>Run the following program to see how many levels of recursion

your system configuration supports:<BR>

<BR>

<TT>firstSub();<BR>

<BR>

sub firstSub{<BR>

  print(&quot;$count\n&quot;);<BR>

        $count++;<BR>

        firstSub();<BR>

}</TT>

<LI>Write a fuNCtion that uses the <TT>substr()</TT>

and <TT>uc()</TT> fuNCtions to change

the tenth through twentieth characters to uppercase.

<LI>Write a fuNCtion that uses the <TT>keys()</TT>

fuNCtion to print out the values of an associative array.

<LI>Create a program that uses a private fuNCtion to subtract

two numbers and multiply the result by four.

<LI>Write a program that shows what the <TT>shift()</TT>

and <TT>unshift()</TT> fuNCtions do.

<LI>Write a program that shows what the <TT>push()</TT>

and <TT>pop()</TT> fuNCtions do.

</OL>

<HR>



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