notes_part_1.txt

---------- Notes on Chapter 4 of Beginning Perl (Simon Cozens, 2000). ----------

To check if something exists in a data set, simply use the "exists" keyword:
   if (not exists $rates{$to}) {
      die "That input is invalid!\n"; }
________________________________________________________________________________

Boolean logic:
        -An empty string, "", is false.
        -The number zero and the string "0" are both false.
        -0.0, or any variation of 0 other than simply 0, is true.
        -An empty list, (), is false.
        -The undefined value is false.
        -Everything else is true.
________________________________________________________________________________

String comparison:
       -($a gt $b) $a sorts alphabetically after $b
       -($a le $b) $a sorts alphabetically before $b
       -($a eq $b $)a is the same as $b
       -($a ne $b) $a is not the same as $b
________________________________________________________________________________

Check if a variable is defined with the "defined" keyword:
   if (defined $a);
________________________________________________________________________________

Perl features an "unless" command:
   unless (exists $rates($to)) {
      die "That input is invalid!\n";
________________________________________________________________________________

Else/elseif syntax:
   if (condition){
      action
   } elseif (condition2){
      action2
   } else {
      # everything else failed
      action3
   }
________________________________________________________________________________

While loop syntax:
   while (condition) {action};
   Also valid:
   {action} while (condition)

Example:
   my $count = 5;
   print "Counting down: ";
   while ($count > 0) {
      print "$count ";
      count--;
   } # prints "5 4 3 2 1 "
________________________________________________________________________________

ARGV: Argument vector, takes arguments passed via command line
________________________________________________________________________________

Do-while loops also valid a la C/C++.
   Syntax:
   do {actions} while (condition)
________________________________________________________________________________

"Until" loops - unique to Perl:
   until (condition) {statement};
   Example:
   my $count = 5;
   until ($count-- == 0) {
      print "$count ";
   } # prints "5 4 3 2 1 "
________________________________________________________________________________

To break out of a loop, use the keyword "last".
   Example:
   my @array = ("a","b","c","stop","d");
   for (@array) {
      if ($_ eq "stop") {last;}
      print "$_ ";
   } # prints "a b c "
________________________________________________________________________________

To skip an iteration of a loop, use the "next" keyword.
   Example:
   my @array = ("a","b","c","skip","d");
   for (@array) {
      if ($_ eq "skip") {next;}
      print "$_ ";
   } # prints "a b c d "
________________________________________________________________________________

To redo an iteration of a loop, use the "redo" keyword.
________________________________________________________________________________

Loops can be labeled, ie. given a name before the for, while, or until.
The convention is that these labels are formed with all uppercase letters.
   Example:
   my @array = ("keep", "going", "stop", "now", "what");
   OUTERLOOP: for (@array) {
      print "$_ ";
      last OUTERLOOP if ($_ eq "stop");
   } # prints "keep going stop "
________________________________________________________________________________

Goto LABEL:
   Valid command, but - as in other languages - not good practice.
________________________________________________________________________________