Compound Statements in PERL, Practical Extraction and Report Language @ DomainAvenue.com | Domain Name Registration and Web Hosting on IBM/Hewlett-Packard (HP) Compaq Servers running Red Hat Linux 7.x w/Apache

Compound statements in PERL

In perl, a sequence of commands may be treated as one command by enclosing it in curly brackets. We will call this a BLOCK.

The following compound commands may be used to control flow:

   if (EXPR) BLOCK
   if (EXPR) BLOCK else BLOCK
   if (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK
   LABEL while (EXPR) BLOCK
   LABEL while (EXPR) BLOCK continue BLOCK
   LABEL for (EXPR; EXPR; EXPR) BLOCK
   LABEL foreach VAR (ARRAY) BLOCK
   LABEL BLOCK continue BLOCK

Note that, unlike C and Pascal, these are defined in terms of BLOCKs, not statements. This means that the curly brackets are required -- no dangling statements allowed. If you want to write conditionals without curly brackets there are several other ways to do it. The following all do the same thing:

   if (!open(foo)){
      die "Can't open $foo: $!";
   }
   die "Can't open $foo: $!" unless open(foo);
   open(foo) || die "Can't open $foo: $!"; # foo or bust!
   open(foo) ? 'hi mom' : die "Can't open $foo: $!";
	# a bit exotic, that last one

The if statement is straightforward. Since BLOCKs are always bounded by curly brackets, there is never any ambiguity about which if and else goes with. If you use unless in place of if, the sense of the test is reversed.

The while statement executes the block as long as the expression is true (does not evaluate to the null string or 0). The LABEL is optional, and if present, consists of an identifier followed by a colon. The LABEL identifies the loop for the loop control statements next, last, and redo. If there is a continue BLOCK, it is always executed just before the conditional is about to be evaluated again, similarly to the third part of a for-loop in C. Thus it can be used to increment a loop variable, even when the loop has been continued via the next statement (similar to the C "continue" statement).

If the word while is replaced by the word until, the sense of the test is reversed, but the conditional is still tested before the first iteration.

In either the if or the while statement, you may replace "(EXPR)" with a BLOCK, and the conditional is true if the value of the last command in that block is true.

The for loop works exactly like the corresponding while-loop:

   for ($i = 1; $i < 10; $i++) {
      ...
   }

is the same as

   $i = 1;
   while ($i < 10) {
      ...
   } continue {
      $i++;
   }

The foreach> loop iterates over a normal array value and sets the variable VAR to be each element of the array in turn. The variable is implicitly local to the loop, and regains its former value upon exiting the loop. The "foreach" keyword is actually identical to the "for" keyword, so you can use "foreach" for readability or "for" for brevity. If VAR is omitted, $_ is set to each value. If ARRAY is an actual array (as opposed to an expression returning an array value), you can modify each element of the array by modifying VAR inside the loop. Examples:

   for (@ary) { s/foo/bar/; }

   foreach $elem (@elements) {
      $elem *= 2;
   }

   for ((10,9,8,7,6,5,4,3,2,1,'BOOM')) {
      print $_, "\n"; sleep(1);
   }

   for (1..15){
      print "Merry Christmas\n";
   }

   foreach $item (split(/:[\\\n:]*/, $ENV{'TERMCAP'})){
      print "Item: $item\n";
   }

The BLOCK by itself (labeled or not) is equivalent to a loop that executes once. Thus you can use any of the loop control statements in it to leave or restart the block. The continue-block is optional. This construct is particularly nice for doing case structures.

   foo: {
      if (/^abc/) { $abc = 1; last foo; }
      if (/^def/) { $def = 1; last foo; }
      if (/^xyz/) { $xyz = 1; last foo; }
      $nothing = 1;
   }

There is no official switch statement in perl, because there are already several ways to write the equivalent. In addition to the above, you could write

   foo: {
      $abc = 1, last foo  if /^abc/;
      $def = 1, last foo  if /^def/;
      $xyz = 1, last foo  if /^xyz/;
      $nothing = 1; }

   foo: {
      /^abc/ && do { $abc = 1; last foo; };
      /^def/ && do { $def = 1; last foo; };
      /^xyz/ && do { $xyz = 1; last foo; };
      $nothing = 1; }

   foo: {
      /^abc/ && ($abc = 1, last foo);
      /^def/ && ($def = 1, last foo);
      /^xyz/ && ($xyz = 1, last foo);
      $nothing = 1; }

or even

   if (/^abc/){
      $abc = 1;
   } elsif (/^def/){
      $def = 1;
   } elsif (/^xyz/){
      $xyz = 1;
   } else {
      $nothing = 1;
   }

As it happens, these are all optimized internally to a switch structure, so perl jumps directly to the desired statement, and you needn't worry about perl executing a lot of unnecessary statements when you have a string of 50 elsifs, as long as you are testing the same simple scalar variable using ==, eq, or pattern matching as above. (If you're curious as to whether the optimizer has done this for a particular case statement, you can use the -D1024 switch to list the syntax tree before execution.)

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