ch2.htm

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

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<TR><TD><B>Note</B></TD></TR>

<TR><TD>

<BLOCKQUOTE>

In the next chapter, &quot;Variables,&quot; you'll see why you might need to use a backslash when using the $ and @ characters.</BLOCKQUOTE>



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<P>

<P>

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

<BLOCKQUOTE>

<I>This literal represents the following: WasWaldo is 34 years

old. The \u is used twice in the first word to capitalize the

w characters. And the hexadecimal notation is used to represent

the age using the ASCII codes for 3 and 4.<BR>

This literal represents the following: The kettle was HOT!. The

\U capital-izes all characters until a \E sequeNCe is seen.</I>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

 &quot;\uwas\uwaldo is \x33\x34 years old.&quot;

&quot;The kettle was \Uhot\E!&quot;

</PRE>

</BLOCKQUOTE>

<P>

For more information about ASCII codes, see Appendix E, &quot;ASCII

Table.&quot; 

<P>

Actually, this example isn't too difficult, but it does involve

looking at more than one literal at oNCe and it's been a few pages

siNCe our last advaNCed example. Let's look at the \t and \n escape

sequeNCes. Listing 2.2-a program displaying a bill with several

items-will produce the output shown in Figure 2.2.

<P>

<A HREF="f2-2.gif"><B>Figure 2.2 : </B><I>A bill of goods displayed using newline

and tab characters</I>.</A>

<P>

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

<BLOCKQUOTE>

<I>Display a literal as the first line, second and third of the

output.<BR>

Display literals that show what was purchased<BR>

Display a separator line.<BR>

Display the total.</I>

</BLOCKQUOTE>

<HR>

<BLOCKQUOTE>

<B>Listing 2.2&nbsp;&nbsp;02LST02.PL-Using Tabs and Newline Characters

to Print<BR>

</B>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

print &quot;Bill of Goods



Bread:\t\$34.45\n&quot;;

print &quot;Fruit:\t&quot;;

print &quot;\$45.00\n&quot;;

print &quot;\t======\n&quot;;

print &quot;\t\$79.45\n&quot;;

</PRE>

</BLOCKQUOTE>

<HR>

<p>

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<TR><TD><B>Tip</B></TD></TR>

<TR><TD>

<BLOCKQUOTE>

Notice that Figure 2.1 and 2.2 look identical. This illustrates a cardinal rule of Perl-there's <I>always</I> more than one way to do something.

</BLOCKQUOTE>



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<P>

<P>

This program uses two methods to cause a line break.

<UL>

<LI>The first is simply to iNClude the line break in the source

code.

<LI>The second is to use the \n or newline character.

</UL>

<P>

I recommend using the \n character so that when looking at your

code in the future, you can be assured that you meant to cause

a line break and did not simply press the ENTER key by mistake.

<BR>

<p>

<CENTER>

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<TR><TD><B>Caution</B></TD></TR>

<TR><TD>

<BLOCKQUOTE>

If you are a C/C++ programmer, this material is not new to you. However, Perl strings are <I>not identical</I> to C/C++ strings because they have no ending NULL character. If you are thinking of converting C/C++ programs to Perl, take care to modify any 
code that relies on the NULL character to end a string.

</BLOCKQUOTE>



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<P>

<H3><A NAME="ExampleBackQuotedStrings">

Example: Back-Quoted Strings</A></H3>

<P>

It might be argued that back-quoted strings are not really a data

type. That's because Perl uses back-quoted strings to execute

system commands. When Perl sees a back-quoted string, it passes

the contents to Windows, UNIX, or whatever operating system you

are using.

<P>

Let's see how to use the back-quoted string to display a directory

listing of all text files in the perl5 directory. 

<P>

Figure 2.3 shows what the output of such a program might look

like.

<P>

<A HREF="f2-3.gif"><B>Figure 2.3 : </B><I>Using a back-quoted string to display a

directory</I>.</A>

<P>

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

<BLOCKQUOTE>

<I>Print the directory listing.</I>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

print &quot;dir *.txt&quot;;

</PRE>

</BLOCKQUOTE>

<P>

All of the escape sequeNCes used with double-quoted strings can

be used with back-quoted strings.

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

Array Literals</FONT></A></H2>

<P>

Perl uses <I>arrays</I>-or lists-to store a series of items. You

could use an array to hold all of the lines in a file, to help

sort a list of addresses, or to store a variety of items. We'll

look at some simple arrays in this section. In the next chapter,

&quot;Variables,&quot; you'll see more examples of how useful

arrays can be.

<H3><A NAME="ExamplePrintinganArray">

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

<P>

In this section, we'll look at printing an array and see how arrays

are represented in Perl source code.

<P>

This example shows an empty array, an array of numbers and an

array of strings. 

<P>

Figure 2.4 shows the output of Listing 2.3.

<P>

<A HREF="f2-4.gif"><B>Figure 2.4 : </B><I>The output from Listing 2.3, showing different

array literals</I>.</A>

<P>

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

<BLOCKQUOTE>

<I>Print the contents of an empty array.<BR>

<I>Print the contents of an array of numbers.<BR>

<I>Print the contents of an array of strings.<BR>

<I>Print the contents of an array with different data types.</I></I></I></I>

</BLOCKQUOTE>

<HR>

<BLOCKQUOTE>

<B>Listing 2.3&nbsp;&nbsp;02LST03.PL-Printing Some Array Literals

<BR>

</B>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

print &quot;Here is an empty array:&quot; . () . &quot;&lt;-- Nothing there!\n&quot;;

print (12, 014, 0x0c, 34.34, 23.3E-3);

print &quot;\n&quot;;

print (&quot;This&quot;, &quot;is&quot;, 'an', &quot;array&quot;, 'of', &quot;strings&quot;);

print &quot;\n&quot;;

print (&quot;This&quot;, 30, &quot;is&quot;, 'a', &quot;mixed array&quot;, 'of', 0x08, &quot;items&quot;);.

</PRE>

</BLOCKQUOTE>

<HR>

<P>

The fourth line of this listing shows that you can mix single-

and double-quoted strings in the same array. You can also mix

numbers and strings interchangeably, as shown in the last line.

<BR>

<p>

<CENTER>

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<TR><TD><B>Note</B></TD></TR>

<TR><TD>

<BLOCKQUOTE>

Listing 2.3 uses the period, or <I>coNCatenation</I>, operator to join a string representation of the empty array with the string <TT>&quot;Here is an empty array:&quot;</TT> and the string <TT>&quot;&lt;-- Nothing there!\n&quot;</TT>. You can read more 
about operators in <A HREF="ch4.htm" >Chapter 4</A> &quot;Operators.&quot;

</BLOCKQUOTE>



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<p>

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

<TR><TD><B>Note</B></TD></TR>

<TR><TD>

<BLOCKQUOTE>

In this and other examples in this chapters, the elements of an array will be printed with no spaces between them. You will see how to print with spaces in the section &quot;Strings Revisited&quot; in <A HREF="ch3.htm" >Chapter 3</A> 
&quot;Variables.&quot;</BLOCKQUOTE>



</TD></TR>

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<P>

<H3><A NAME="ExampleNestingArrays">

Example: Nesting Arrays</A></H3>

<P>

Many times a simple list is not enough. If you're a painter, you

might have one array that holds the names of orange hues and one

that holds the names of yellow hues. To print them, you can use

Perl's ability to specify a sub-array inside your main array definition.

<P>

While this example is not very &quot;real-world,&quot; it gives

you the idea behind specifying an array by using sub-arrays.

<P>

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

<BLOCKQUOTE>

<I>Print an array that consists of two sub-arrays.<BR>

<I>Print an array that consists of an array, a string, and another

array.</I></I>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

print ((&quot;Bright Orange&quot;, &quot;Burnt&quot;), (&quot;Canary Yellow&quot;, &quot;Sunbeam&quot;));

print ((&quot;Bright Orange&quot;, &quot;Burnt&quot;), &quot; Middle &quot;, (&quot;Canary Yellow&quot;, 

&nbsp;&quot;Sunbeam&quot;));

</PRE>

</BLOCKQUOTE>

<P>

So far, we haven't talked about the internal representations of

data types. That's because you almost never have to worry about

such things with Perl. However, it is important to know that,

internally, the sub-arrays are merged into the main array. In

other words, the <TT>array:</TT>

<BLOCKQUOTE>

<PRE>

((&quot;Bright Orange&quot;, &quot;Burnt&quot;), (&quot;Canary Yellow&quot;, &quot;Sunbeam&quot;))

</PRE>

</BLOCKQUOTE>

<P>

is exactly equivalent to

<BLOCKQUOTE>

<PRE>

(&quot;Bright Orange&quot;, &quot;Burnt&quot;, &quot;Canary Yellow&quot;, &quot;Sunbeam&quot;)

</PRE>

</BLOCKQUOTE>

<H3><A NAME="ExampleUsingaRangeofValues">

Example: Using a Range of Values</A></H3>

<P>

At times you might need an array that consists of sequential numbers

or letters. Instead of making you list out the entire array, Perl

has a shorthand notation that you can use.

<P>

Perl uses two periods (..) to replace a consecutive series of

values. Not only is this method quicker to type-and less prone

to error-it is easier to understand. Only the end points of the

series are specified; you don't need to manually verify that every

value is represented. If the .. is used, then automatically you

know that a range of values will be used.

<P>

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

<BLOCKQUOTE>

<I>Print an array consisting of the numbers from 1 to 15.<BR>

<I>Print an array consisting of the numbers from 1 to 15 using

the shorthand method.</I></I>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

print (1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15);

print &quot;\n&quot;;

print (1..15);

</PRE>

</BLOCKQUOTE>

<P>

The two arrays used in the previous example are identical, but

they were specified differently.<BR>

<p>

<CENTER>

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

<TR><TD><B>Note</B></TD></TR>

<TR><TD>

<BLOCKQUOTE>

The double periods in the array specification are called the <I>range </I>operator. The range operator is also discussed in <A HREF="ch4.htm" >Chapter 4</A> &quot;Operators.&quot;

</BLOCKQUOTE>



</TD></TR>

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<P>

<P>

You can also use the shorthand method to specify values in the

middle of an array.

<P>

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

<BLOCKQUOTE>

<I>Print an array consisting of the numbers 1, 2, 7, 8, 9, 10,

14, and 15.Print an array consisting of the letters A, B, F, G,

H, Y, Z</I>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

print (1, 2, 7..10, 14, 15);

print &quot;\n&quot;

print (&quot;A&quot;, &quot;B&quot;, &quot;F&quot;..&quot;H&quot;, &quot;Y&quot;, &quot;Z&quot;);

</PRE>

</BLOCKQUOTE>

<P>

The range operator works by taking the lefthand value, adding

one to it, then appending that new value to the array. Perl continues

to do this until the new value reaches the righthand value. You

can use letters with the range operator because the ASCII table

uses consecutive values to represent consecutive letters.

<P>

For more information about ASCII codes, see Appendix E, &quot;ASCII

Table.&quot;

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

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

<P>

This chapter introduced you to both numeric and string literals.

You learned that literals are values that are placed directly

into your source code and never changed by the program. They are

sometimes referred to as hard-coded values.

<P>

You read about numbers and the three different bases that can

be used to represent them-decimal, octal, and hexadecimal. Very

large or small numbers can also be described using scientific

notation.

<P>

Strings were perhaps a bit more involved. Single-, double-, and

back-quoted strings are used to hold strings of characters. Back-quoted

strings have an additional purpose. They tell Perl to send the

string to the operating system for execution.

<P>

Escape sequeNCes are used to represent characters that are difficult

to enter through the keyboard or that have more than one purpose.

For example, using a double quote inside a double-quoted string

would end the string before you really intended. The backslash

character was introduced to escape the double quote and change

its meaning.

<P>

The next chapter, &quot;Variables,&quot; will show you how Perl

uses your computer memory to store data types and also will show

you ways that you can manipulate data.

<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 are the four types of literals?

<LI>What is a numeric literal?

<LI>How many types of string literals are there?

<LI>What is the major differeNCe between single- and double-quoted

strings?

<LI>What are three escape sequeNCes and what do they mean?

<LI>What would the following one-line program display?<BR>

<BR>

<TT>print 'dir /*.log';<BR>

</TT>

<LI>What is scientific notation?

<LI>How can you represent the number 64 in hexadecimal inside

a double-quoted string?

<LI>What is the easiest way to represent an array that iNCludes

the numbers 56 to 87?

</OL>

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

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

<OL>

<LI>Write a program that prints the decimal number 32. However,

in the print command, specify the value of 32 using hexadecimal

notation.

<LI>Create program that uses the tab character in three literals

to align text.

<LI>Write a program that prints using embedded new lines in a

single-quoted literal.

<LI>Convert the number 56,500,000 into scientific notation.

<LI>Write a program that prints an array that uses the range operator.

The left value should be AA and the right value should be BB.

What happens and why?

<LI>Write a program that prints its own source code using a back-quoted

string.

</OL>

<HR>



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