This is a relatively long example, but you'll work through most of it one step at a time. By the time you're done with this section, you should feel comfortable with the code here.

Let's start with lines 3-7. This is really just one line of code. The lines beginning with the number sign (#) are comments and are not executed by the computer. Comments are used to help the programmer remember what is going on. Lines 3-7 could be rewritten, using shorter variable names and without comments, like this: ($sec, $min, $hr, $mntdy, $mon, $yr, $wk, $yrdy, $stm) = localtime(time);

Both this line and lines 3-7 do exactly the same thing. I have problems with this line, though. Even with more readable variable names, I would have a hard time remembering which variables start at 0 and what each variable's range is. I like comments; they help me a lot after I have slept for an hour or two. Lines 3-7 are kind of like assigning data to the array cells of a regular array, which you learned about in Exercise 2.1. In that exercise, line 3 assigned a list to a regular array: @my_name = ("Eric ","C. ","Herrmann");

The localtime function on line 7 creates a nine-element array. Instead of assigning the data to an array, however, you assign the data to a list of scalar variables: ($second, $minute, $hour, $day_of_month, $month, $year, $weekday, $day_of_year, $daylight_standard_time)

All the data returned by localtime is integer data. Most of the values start at 0. This is supposed to help you index through arrays of the names of months or days of the week. It seems kind of weird that December is the 11th month of the year. But that's how it works, because the first month of the year, January, is month zero.

The function localtime() is part of the Perl distribution library. The parameter time in localtime(time) is actually another Perl function that returns the current time in number of seconds since January 1, 1970. The function localtime() then converts that time into the nine elements of data on the left-hand side of the equal sign.

The complex if statement on lines 10 and 11 could be read as the following:

If the month is December and the day is the 24th or the 25th, then say Merry Christmas, World

The condition expression ($month == 11) is evaluated or read first. The computer checks to see whether the variable $month equals 11 (because the month of December is represented by the value 11).

The operator && is read as AND by your computer. The operator || in the next portion of the condition expression is read as OR by your computer. There is a set of parentheses around the $day_of_month checks so that the computer evaluates the conditional expression to True only if both the month is December (11) and the day is either the 24th or the 25th. So take a moment to reread line 10. You should be able to follow it now: If the $month variable is equal to December, and the $day_of_month variable is 24 or 25. The parentheses around the $day_of_month expression can be read as the either part (either this or that).

If it is December 24th or 25th, your computer prints Merry Christmas, World. Otherwise, the condition expression on line 10 evaluates to False and the computer skips the block of statements that begins with the opening curly brace ({) at the end of line 10 and ends with the closing curly brace (}) on line 12.

If the conditional expression evaluates to False, the next block of statements the computer executes is the else block from lines 13-35. An else block does not have a condition expression; it is executed whenever the if statement evaluates to False. An if statement does not require an else block.

An if statement can look like this: if (condition expression){block of statements}

An if statement like this is evaluated and, if it is True, the block of statements is executed and then the statements following the block of statements are executed.

In an if else clause such as if (condition expression){block of statements}else{block of statements}

if the if (condition expression) evaluates to True, the computer executes the block of statements following the if (condition expression). If the if (condition expression) evaluates to False, the computer skips the block of statements following the if (condition expression) and executes the block of statements following the else statement.

So, in Listing 3.9, only one of the print statements is printed each time you run the program.

Let's assume that it is not December 24th or 25th, which means that you will skip the block of statements beginning on line 11 and begin to execute the block of statements that begins on line 14. (Actually, the block of statements begins with the opening curly brace ({) on line 13. Each of these condition expressions is very similar, so after you get the first one, you should understand the rest.

The first statement inside the else block of statements is another if statement. You can have any type of statements inside a block of statements. The if statement checks whether the $hour variable is greater than 18 by using the greater than (>) operator. The block of statements following this if statement extends from lines 16-24 and includes another if check. Read the comments that go along with the code to follow along and understand what is happening.

If the condition expression on line 15 evaluates to False, the next statement to execute is line 25-an elsif expression. elsif statements are really the equivalent of an if else statement but sometimes make your code more readable; this is a good example of such a time. In this case, the hour is not greater than 18, so it must be between 0 and 18, so now you check for each block of time that usually is associated with a greeting. Is it after noon? If not, is it after 6 a.m.? If not, it must be between 0 and 6 a.m. I usually take time to write down in English what is happening in a complex if, elsif, or else expression. It helps me make sure that my code is doing what I really want it to do.

Starting from the else block beginning on line 13, the English that describes what you want this code to do follows: otherwise if the hour is greater than 6pm then if the hour is less than 9pm print Good Evening, World otherwise the hour must be between 9pm and midnight so print Good Night, World. otherwise if the hour is greater than noon but less than 6pm print Good Afternoon, World otherwise if the hour is greater than 6am but less than 6pm and less than noon print Good Morning, World otherwise the hour must be between midnight and 6am so print Go to BED already!

If this program were written as blocks of statements and conditional expressions, it would look like this: if (condition expression) {block of statements1} else {block of statements2}

{block of statments2} looks like this: if (condition expression){block of statements3} elsif(condition expression){block of statements4} elsif(condition expression){block of statements5} else{block of statements6} {block of statments3} looks like this: if(condition expression){block of statements7} else{block of statements8}

The unexpanded {block of statements} are simple print statements with no condition expression inside them.

During this exercise, you learned about statements that cause the computer to choose between different blocks of statements to execute. The basic syntax of those statements follows:

• if (condition expression){block of statements}
• if (condition expression){block of statements} else {block of statements}
• if (condition expression){block of statements} elsif {block of statements} ...

The elsif statement can be repeated as often as desired.

• if (condition expression){block of statements} elsif {block of statements} ... else {block of statements}

Exercise 3.3. The Perl loop statements While and Until

In Perl, as in other languages, you actually can build more than the basic looping constructs, but the four basic loop constructs will satisfy all your programming needs. Even these four do basically the same thing:

1. Checks for some condition
2. If True, executes the next block of statements
3. Repeats from step 1

You will find yourself using at least two of the four constructs on a regular basis. It's kind of like the different knifes you have in your kitchen. They all do basically the same thing, but you use different knifes to do slightly different tasks. The four basic loop control structures follow: while (conditional expression) {block of statements} until (conditional expression) {block of statements} for (conditional expression) {block of statements} foreach variable (array) {block of statements}

There is a fifth construct that really isn't a loop control construct but sometimes is used with loops to change when the conditional expression is evaluated. This fifth construct is called the do statement. In this exercise, you will learn about the while and until loops and the effect the do statement has on these two loop constructs.

The while and until loop constructs first check the conditional expression before they execute their block of statements. The do statement has the following syntax: 1: do {block of statements} while (conditional expression) 2: do {block of statements} until (conditional expression)

This makes the do looping constructs unique among the other looping constructs. The do until/while loop construct always executes the {block of statements} at least once. The (conditional expression) of the while/until statement is evaluated after the {block of statements} of the do statement. Each of the other loop control constructs evaluate the (conditional expression) first and might not ever execute their {block of statements}. The for and foreach statements are covered in the exercises in Chapter 4. Listing 3.10 illustrates using the while, until, and do statements using different looping variations.

01: #!/usr/local/bin/perl
02:
03: while($count < 5){
04:     print "the count is $count\n";
05:     $count++
06: }
07:
08: print "The count AFTER THE WHILE is now $count\n";
09:
10: until ($count > 9) {
11:     print "the count in the until loop is $count\n";
12:     $count++;
13: } ;
14:
15: print "The count AFTER THE UNTIL is now $count\n";
16:
17: do {
18:     print "the count in the do while loop is $count\n";
19:     $count++;
20: } while ($count < 9);
21:
22: print "The count AFTER THE DO WHILE is now $count\n";
23:
24: do {
25:     print "the count in the do until loop is $count\n";
26:     $count++;
27: } until ($count > 14);
28:
29: print "The count AFTER THE DO UNTIL is now $count\n";
30:
31: while(@pwdlist = getpwent){
32:     $user = $pwdlist[0];
33:     $shelltype = $pwdlist[8];
34:     print "$user uses the $shelltype shell\n";
35: }