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perl/Examples/Chap2/rpn-table

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+
+
+###
+### rpn_table
+###
+
+## Chapter 2 section 2
+
+my @stack;
+my $actions = {
+  '+' => sub { push @stack, pop(@stack) + pop(@stack) },
+  '*' => sub { push @stack, pop(@stack) * pop(@stack) },
+  '-' => sub { my $s = pop(@stack); push @stack, pop(@stack) - $s },
+  '/' => sub { my $s = pop(@stack); push @stack, pop(@stack) / $s },
+  'NUMBER' => sub { push @stack, $_[0] },
+  '_DEFAULT_' => sub { die "Unrecognized token `$_[0]'; aborting" }
+};
+
+my $result = evaluate($ARGV[0], $actions);
+print "Result: $result\n";
+
+sub evaluate {
+  my ($expr, $actions) = @_;
+  my @tokens = split /\s+/, $expr;
+  for my $token (@tokens) {
+    my $type;
+    if ($token =~ /^\d+$/) {   # It's a number
+      $type = 'NUMBER'; 
+    }
+  
+    my $action = $actions->{$type} 
+              || $actions->{$token} 
+              || $actions->{_DEFAULT_};
+    $action->($token, $type, $actions);
+  }
+  return pop(@stack);
+}