WDVL: Perl Program Flow: Conditionals and Loops

By default, Perl programs, like many other languages, execute in the order the code is written, from top to bottom. This order of execution is known as "program flow" in textbooks, and there are a variety of reasons why we might want to alter the program flow under certain circumstances. Again, consider the parallels to real life -- you may plan out your day in a certain order, but variable circumstances may change this order. For instance, your day plan might look like:

The use of conditional logic allows your schedule to become more flexible -- go to the bank first if it is open, otherwise go to the grocery and then go to the bank. Similarly, we apply the same sort of logic to the flow of Perl programs. The two main controls which alter program flow are conditionals and loops.

Conditionals, like the if...else statement seen in the above example, determine what actions to take when certain conditions are met or are not met.

Loops are used to repeat one or more actions a certain number of times based upon certain conditions.

if and unless statements

One of the most intuitive conditionals is the if statement. You can read this example aloud to make the logic perfectly clear:

Notice that the actions clause is enclosed within curly braces. A set of curly braces is known as a statement block, and any number of Perl statements can appear here, separated by semicolons. Recalling our currency conversion example, we can output a message sensitive to which currency is more valuable than the other:

if ($USTotal>$CDNTotal)
 { print "Presently, the US dollar is ".
  "worth more than the Canadian dollar.\n" }

The else clause of an if statement lets you define a statement block in case the conditional test is false:

if ($USTotal>$CDNTotal)
 { print "Presently, the US dollar is ". 
 "worth more than the Canadian dollar.\n" }
else
 { print "Presently, the Canadian dollar is ". 
 "worth more than the US dollar.\n" }

Unfortunately, the above example is flawed. It is theoretically possible that both currencies are of equal value. The elsif clause acts as a combined else and if clause, allowing an else clause which tests an additional condition. This may be clearer by example:

if ($USTotal>$CDNTotal)
 { print "Presently, the US dollar is ".
  "worth more than the Canadian dollar.\n" }
elsif ($USTotal==$CDNTotal)
 { print "Presently, both the US dollar and ".
  "the Canadian dollar are of equal value.\n" }
else 
 { print "Presently, the Canadian dollar is ".
  "worth more than the US dollar.\n" }

Note the way the output strings have been broken up into multiple segments. Although we could have simply typed one long line, it would have been difficult to read within this article. To make the line more legible, the output string has been broken into portions, with a concatenation operator, the ".", between each portion. The result is exactly the same as if we typed one long line.

The statement above actually tests two conditions: if the US dollar is worth more then the first message is output; if the two are equal then the second message is output; the process of elimination tells us that if neither of these were true than the US dollar must be worth less, so the third message it output.

A few notes on syntax: the formatting and indentation used in these examples is merely one style, albeit a style this author finds intuitive and legible. Other programmers may favor different styles of indentation -- Perl is very flexible and essentially lets you format statements and statement blocks any way you like. Also, note that the elsif clause of this statement is spelled strangely, missing the second "e" that it seems it should have. Perl can be quirky.

A related variation of the if statement is the unless statement. This statement merely tests if the given condition is not true:

In this logic, if the bank is not open then we go shopping first; otherwise the bank is open and we go to the bank first. The unless statement can sometimes be confusing to work through mentally, though it might help to think of it as meaning "if the condition is not true". In every other way unless behaves like the if statement, and can take additional elsif and/or else clauses.

while, for, and foreach loops

"Lather, rinse, repeat" -- the insightful instructions on the label of shampoo bottles. Yet, this bit of cleansing wisdom sheds light on one of the most fundamental techniques in Perl programming -- the loop. In fact, we can code the shampoo mantra using Perl's while loop:

The shampoo loop starts with a conditional test: is the hair greasy? A statement block follows this test, and this block is executed if the condition is true. Thus, if the hair is greasy, lather and rinse. When the statement block is completed, the while loop control returns to the test at the top of the loop. Is the hair greasy? If yes, the loop iterates again, once more lathering and rinsing. This loop will iterate ad infinitum until the while condition evaluates to false. If, on the third iteration, the hair is no longer greasy, the loop ends -- the statement block is not executed and program flow drops to the next statement after the while loop's statement block. If the while condition is false the first time the loop is evaluated, the statement block is never executed.

The for loop is similar in nature to the while loop but tends to be used when counting is important. A basic example would be a for loop which counts from 1 to 10:

The for statement takes a three-part control:. First, $j=1 sets the counting variable for this loop, $j, to a value of 1. You can use any variable name you wish, but $i and $j are historically traditional loop counters. The second part of the for loop's control is the conditional -- in this case, is $j less-than or equal to the value 10? The third part of the control advances the counter. Here we use the common autoincrement operator which simply adds 1 to the current value of $j.

The first time through the loop, $j contains 1, and thus 1 is output by the print function. The second time through the loop $j is incremented to 2, is compared against 10, and again is output by the print function. This process simply repeats until $j increments to 11, which of course is greater than 10. At that point the loop exits and Perl passes over the statement block and moves onto the next statement in the program.

When dealing with lists, the foreach loop provides a simple way to iterate through each item in the list. Recall our grocery list from earlier, @shopping. Suppose you wanted to work with each item in @shopping:

This foreach loop does not take any conditional. Rather, it iterates for each item in @shopping. In each iteration, the variable $shopping_item contains the next value in @shopping. Thus, the first time through the loop $shopping_item is "milk"; the second time through $shopping_item is "bread", and so on. Within the statement block of the foreach loop you can modify the value of $shopping_item, if need be, without affecting the original value in @shopping.