Flow-Control Constructs.

Flow-control constructs are very familiar. An important thing about these constructs in Perl is that the curly braces, {}, are required. Omitting them is a syntax error.

Conditional Constructs

`if` statement

The if statement is the most common conditional construct. The general syntax for it is:

if( condition ){
  statements;
}else{
  statements;
}

The else part is optional. The condition cab be any logical expression that evaluates to true or false. These are two examples of if statements:

if( $#ARGV > -1 ){
   print "The number of arguments is:", $#ARGV+1, "\n";
}
else{
   print "No arguments\n";
}
...
if( $#ARGV == -1 ){
   print "Please specify arguments\n";
   exit 1;
}

`unless` statement

The unless statement is like if, except the logic is reversed. The basic syntax is:

unless( condition ){
   statements;
}

If the condition is false the statements are executed. Thus, the following two examples are equivalent:

unless( $done ){
   print "Keep on working ...\n";
}
if( not $done ){
   print "Keep on working ...\n";
}

Looping Constructs

`while` loop

The while loop has the following syntax:

while( condition ){
   statements;
}

The condition is tested, and if true, the statements are executed; then the condition is tested again, and if true, the statements are executed; etc. The loop terminates when the condition is false. The following example prints all fields of a hash variable:

%student = (
  name   => "Bob",
  phone  => "123-4567",
  street => "Hal Greer Blvd, #1",
  city   => "Huntington",
  state  => "WV",
  zip    => 25755,
  gpa    => 3.75 
);
@ks = keys(%student);
$i = 0;
while( $i <= $#ks ){
   print "$ks[$i] = $student{$ks[$i]}\n";
   $i++;
}

`until` loop

The until loop is like the while, except the logic is reversed. The syntax of the loop is basically the same:

until( condition ){
   statements;
}

The condition is tested, and if it's false, the statements are executed. Then the condition is tested again, ....

`for` loop

The for loop is mostly used in situations when we know how many times we need to repeat the statements inside. The general syntax is:

for(initialization;condition;expression){
   statements;
}

The initialization is executed before the loop and only once. Then the condition is tested, if it's true, the statements are executed, then the expression is executed, and then the condition is tested again. The loop stops when the condition is false. The following example prints all elements of an array:

@data = (1, 1, 2, 3, 5, 8, 13, 21, 34);
for($i=0;$i<=$#data;$i++){
   print "$i: \t$data[$i]\n";
}

`foreach` loop

Besides usual loops described above Perl has a special loop to process lists and arrays called foreach loop. It has the following syntax:

foreach scalar_variable (list){
   statements;
}

The statements are executed for each item in the list; that is, we don't have to know how long the list is or check whether the end has been reached. For example:

@data = (1, 1, 2, 3, 5, 8, 13, 21, 34);
foreach my $i (@data){
   print "$i\n";
}
for my $i (1..10){
   print "Hello!\n";
}

Expression Modifiers

Expression modifiers in Perl are also known as the backward form. For example, the following if statement:

if( $#ARGV == -1 ){
   die "You did not provide enough arguments";
}

can be written backward as:

die "You did not provide enough arguments" if $#ARGV == -1;

Expression modifiers can be used only when the body of the construct is a simple statement. In the other words, the body cannot be an if or a while or any other multistatement body.

All the following examples are legal:

complain() unless $happy;
sleep() while $tired && $there_is_time;
cook() until $done;

Reference: Open Source Web Development with LAMP by James Lee and Brent Ware. ISBN: 9-780201=770612. Chapter 4.