ch7.htm

prrl 5 programs codes in the book

Start the </I><TT><I>while</I></TT><I>

loop and test the condition. If false, don't execute the statement

block.<BR>

Print the value of </I><TT><I>$firstVar</I></TT><I>.

<BR>

INCrement </I><TT><I>$firstVar</I></TT><I>.

<BR>

Jump back to the start of the statement block and test the condition

again.<BR>

Print the value of </I><TT><I>$firstVar</I></TT><I>.</I>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

$firstVar = 10;

while ($firstVar &lt; 2) {

    print(&quot;inside:  firstVar = $firstVar\n&quot;);

    $firstVar++;

};

print(&quot;outside: firstVar = $firstVar\n&quot;);

</PRE>

</BLOCKQUOTE>

<P>

This program displays:

<BLOCKQUOTE>

<PRE>

outside: firstVar = 10

</PRE>

</BLOCKQUOTE>

<P>

This example shows that the statement block is never evaluated

if the condition is false when the <TT>while</TT>

loop starts. Of course, it's more common to use <TT>while</TT>

loops that actually execute the statement block-like the following:

<P>

<IMG SRC="pseudo.gif" tppabs="http://cheminf.nankai.edu.cn/~eb~/Perl%205%20By%20Example/pseudo.gif" BORDER=1 ALIGN=RIGHT><p>

<BLOCKQUOTE>

<I>Initialize </I><TT><I>$firstVar</I></TT><I>

to 10.<BR>

Start the </I><TT><I>while</I></TT><I>

loop and test the condition.<BR>

Print the value of </I><TT><I>$firstVar</I></TT><I>.

<BR>

INCrement </I><TT><I>$firstVar</I></TT><I>.

<BR>

Jump back to the start of the statement block and test the condition

again.<BR>

Print the value of </I><TT><I>$firstVar</I></TT><I>.</I>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

$firstVar = 10;

while ($firstVar &lt; 12) {

    print(&quot;inside:  firstVar = $firstVar\n&quot;);

    $firstVar++;

};

print(&quot;outside: firstVar = $firstVar\n&quot;);

</PRE>

</BLOCKQUOTE>

<P>

This program displays:

<BLOCKQUOTE>

<PRE>

inside:  firstVar = 10

inside:  firstVar = 11

outside: firstVar = 12

</PRE>

</BLOCKQUOTE>

<P>

It's important to note that the value of <TT>$firstVar</TT>

ends up as 12 and not 11 as you might expect upon casually looking

at the code. When <TT>$firstVar</TT>

is still 11, the condition is true, so the statement block is

executed again, thereby iNCrementing <TT>$firstVar</TT>

to 12. Then, the next time the condition is evaluated, it is false

and the loop ends with <TT>$firstVar</TT>

equal to 12.

<H3><A NAME="ExampleIUntilILoops">

Example: <I>Until</I> Loops</A></H3>

<P>

<TT><I>Until</I></TT> loops are used

to repeat a block of statements while some condition is false.

Like the previous <TT>while</TT> loop,

there are also two forms of the <TT>until</TT>

loop: one where the condition is checked before the statements

are executed (the <TT>do...until</TT>

loop), and one in which the condition is checked after the statements

are executed (the <TT>until</TT> loop).

<P>

The <TT>do...until</TT> loop has this

syntax:

<BLOCKQUOTE>

<PRE>

do {

    STATEMENTS

} until (CONDITION);

The until loop has this syntax:



until (CONDITION) {

    STATEMENTS

}

</PRE>

</BLOCKQUOTE>

<P>

Again, the loop type you use is dependent on your needs at the

time. Here is an example of the <TT>do...until</TT>

loop.

<P>

<IMG SRC="pseudo.gif" tppabs="http://cheminf.nankai.edu.cn/~eb~/Perl%205%20By%20Example/pseudo.gif" BORDER=1 ALIGN=RIGHT><p>

<BLOCKQUOTE>

<I>Initialize </I><TT><I>$firstVar</I></TT><I>

to 10.<BR>

Start the </I><TT><I>do..until</I></TT><I>

loop.<BR>

Print the value of </I><TT><I>$firstVar</I></TT><I>.

<BR>

INCrement </I><TT><I>$firstVar</I></TT><I>.

<BR>

Check the </I><TT><I>until</I></TT><I>

condition; if false, jump back to the start of the statement block.

<BR>

Print the value of </I><TT><I>$firstVar</I></TT><I>.</I>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

$firstVar = 10;

do {

    print(&quot;inside:  firstVar = $firstVar\n&quot;);

    $firstVar++;

} until ($firstVar &lt; 2);



print(&quot;outside: firstVar = $firstVar\n&quot;);

</PRE>

</BLOCKQUOTE>

<P>

This program displays:

<BLOCKQUOTE>

<PRE>

inside:  firstVar = 10

inside:  firstVar = 11

inside:  firstVar = 12

inside:  firstVar = 13

inside:  firstVar = 14

...

</PRE>

</BLOCKQUOTE>

<P>

This loop continues forever because the condition can never be

true. <TT>$firstVar</TT> starts out

greater than 2 and is iNCremented inside the loop. Therefore,

this is an <I>endless </I>loop.<BR>

<p>

<CENTER>

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

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

<TR><TD>

<BLOCKQUOTE>

If you ever find it hard to understand a conditional expression in a loop statement, try the following: Wrap the entire condition expression inside paren-theses and add == 1 to the right-hand side. The above loop then becomes</BLOCKQUOTE>



</TD></TR>

</TABLE>

</CENTER>

<P>

<BLOCKQUOTE>

<PRE>

do {

    ...

} until (($firstVar &lt; 2) == 1);

</PRE>

</BLOCKQUOTE>

<P>

This example shows that the statement block is executed even though

the condition <TT>$firstVar &lt; 2</TT>

is false when the loop starts. The next example shows the <TT>until</TT>

loop in action, which does not execute the statement block when

the conditional expression is false when the loop starts.

<P>

<IMG SRC="pseudo.gif" tppabs="http://cheminf.nankai.edu.cn/~eb~/Perl%205%20By%20Example/pseudo.gif" BORDER=1 ALIGN=RIGHT><p>

<BLOCKQUOTE>

<I>Initialize </I><TT><I>$firstVar</I></TT><I>

to 10.<BR>

Start the </I><TT><I>until</I></TT><I>

loop and test the condition. If true, don't execute the state-ment

block.<BR>

print the value of </I><TT><I>$firstVar</I></TT><I>.

<BR>

INCrement </I><TT><I>$firstVar</I></TT><I>.

<BR>

Jump back to the start of the statement block and test the condition

again.<BR>

Print the value of </I><TT><I>$firstVar</I></TT><I>.</I>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

$firstVar = 10;

until ($firstVar &lt; 20) {

    print(&quot;inside:  firstVar = $firstVar\n&quot;);

    $firstVar++;

};

print(&quot;outside: firstVar = $firstVar\n&quot;);

</PRE>

</BLOCKQUOTE>

<P>

This program displays:

<BLOCKQUOTE>

<PRE>

outside: firstVar = 10

</PRE>

</BLOCKQUOTE>

<P>

This example shows that the statement block is never evaluated

if the condition is true when the <TT>until</TT>

loop starts. Here is another example of an <TT>until</TT>

loop that shows the statement block getting executed:

<P>

<IMG SRC="pseudo.gif" tppabs="http://cheminf.nankai.edu.cn/~eb~/Perl%205%20By%20Example/pseudo.gif" BORDER=1 ALIGN=RIGHT><p>

<BLOCKQUOTE>

<I>Initialize </I><TT><I>$firstVar</I></TT><I>

to 10.<BR>

Start the </I><TT><I>while</I></TT><I>

loop and test the condition.<BR>

Print the value of </I><TT><I>$firstVar</I></TT><I>.

<BR>

INCrement </I><TT><I>$firstVar</I></TT><I>.

<BR>

Jump back to the start of the statement block and test the condition

again.<BR>

Print the value of </I><TT><I>$firstVar</I></TT><I>.</I>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

$firstVar = 10;

until ($firstVar &gt; 12) {

    print(&quot;inside:  firstVar = $firstVar\n&quot;);

    $firstVar++;

};

print(&quot;outside: firstVar = $firstVar\n&quot;);

</PRE>

</BLOCKQUOTE>

<P>

This program displays:

<BLOCKQUOTE>

<PRE>

inside:  firstVar = 10

inside:  firstVar = 11

inside:  firstVar = 12

outside: firstVar = 13

</PRE>

</BLOCKQUOTE>

<H3><A NAME="ExampleIForILoops">

Example: <I>For</I> Loops</A></H3>

<P>

One of the most common tasks in programming is looping a specific

number of times. Whether you need to execute a certain fuNCtion

for every customer in your database or print a page in a report,

the <TT><I>for</I></TT> loop can be

used. Its syntax is:

<BLOCKQUOTE>

<PRE>

for (INITIALIZATION; CONDITION; INCREMENT/DECREMENT) {

    STATEMENTS

}

</PRE>

</BLOCKQUOTE>

<P>

The <I>initialization</I> expression is executed first-before

the looping starts. It can be used to initialize any variables

that are used inside the loop. Of course, this could be done on

the line before the <TT>for</TT> loop.

However, iNCluding the initialization inside the <TT>for</TT>

statement aids in identifying the loop variables.

<P>

When initializing variables, be sure not to confuse the equality

operator (<TT>==</TT>) with the assignment

operator (<TT>=</TT>). The following

is an example of what this error could look like:

<BLOCKQUOTE>

<PRE>

for ($index == 0; $index &lt; 0; $index++)

</PRE>

</BLOCKQUOTE>

<P>

One of the equal signs should be removed. If you think you are

having a problem with programming the <TT>for</TT>

loop, make sure to check out the operators.

<P>

The <I>condition</I> expression is used to determine whether the

loop should continue or be ended. When the condition expression

evaluates to false, the loop will end.

<P>

The <I>iNCrement/decrement </I>expression is used to modify the

loop variables in some way each time the code block has been executed.

Here is an example of a basic <TT>for</TT>

loop:

<P>

<IMG SRC="pseudo.gif" tppabs="http://cheminf.nankai.edu.cn/~eb~/Perl%205%20By%20Example/pseudo.gif" BORDER=1 ALIGN=RIGHT><p>

<BLOCKQUOTE>

<I>Start the for loop by initializing </I><TT><I>$firstVar</I></TT><I>

to zero. The </I><TT><I>$firstVar</I></TT><I>

variable will be iNCremented each time the statement block is

executed. The statement block will be executed as long as </I><TT><I>$firstVar</I></TT><I>

is less than 100.<BR>

Print the value of </I><TT><I>$firstVar</I></TT><I>

each time through the loop.</I>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

for ($firstVar = 0; $firstVar &lt; 100; $firstVar++) {

    print(&quot;inside:  firstVar = $firstVar\n&quot;);

}

</PRE>

</BLOCKQUOTE>

<P>

This program will display:

<BLOCKQUOTE>

<PRE>

inside:  firstVar = 0

inside:  firstVar = 1

...

inside:  firstVar = 98

inside:  firstVar = 99

</PRE>

</BLOCKQUOTE>

<P>

This program will display the numbers 0 through 99. When the loop

is over, <TT>$firstVar</TT> will be

equal to 100.

<P>

<TT>For</TT> loops also can be used

to count backwards.

<P>

<IMG SRC="pseudo.gif" tppabs="http://cheminf.nankai.edu.cn/~eb~/Perl%205%20By%20Example/pseudo.gif" BORDER=1 ALIGN=RIGHT><p>

<BLOCKQUOTE>

<I>Start the </I><TT><I>for</I></TT><I>

loop by initializing </I><TT><I>$firstVar</I></TT><I>

to 100. The </I><TT><I>$firstVar</I></TT><I>

variable will be decremented each time the statement block is

executed. And the statement block will be executed as long as

</I><TT><I>$firstVar</I></TT><I> is

greater than 0.<BR>

Print the value of </I><TT><I>$firstVar</I></TT><I>

each time through the loop.</I>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

for ($firstVar = 100; $firstVar &gt; 0; $firstVar--) {

    print(&quot;inside:  firstVar = $firstVar\n&quot;);

}

</PRE>

</BLOCKQUOTE>

<P>

This program will display:

<BLOCKQUOTE>

<PRE>

inside:  firstVar = 100

inside:  firstVar = 99

...

inside:  firstVar = 2

inside:  firstVar = 1

</PRE>

</BLOCKQUOTE>

<P>

You can use the comma operator to evaluate two expressions at

oNCe in the initialization and the iNCrement/decrement expressions.

<P>

<IMG SRC="pseudo.gif" tppabs="http://cheminf.nankai.edu.cn/~eb~/Perl%205%20By%20Example/pseudo.gif" BORDER=1 ALIGN=RIGHT><p>

<BLOCKQUOTE>

<I>Start the </I><TT><I>for</I></TT><I>

loop by initializing </I><TT><I>$firstVar</I></TT><I>

to 100 and </I><TT><I>$secondVar</I></TT><I>

to 0. The </I><TT><I>$firstVar</I></TT><I>

variable will be decremented and </I><TT><I>$secondVar</I></TT><I>