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Task/Topic-variable/0DESCRIPTION

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+Several programming languages offer syntax shortcuts to deal with the notion of "current" or "topic" variable.  A topic variable is a [[Special variables|special variable]] with a very short name which can also often be omitted.
+
+Demonstrate the utilization and behaviour of the topic variable within the language and explain or demonstrate how the topic variable behaves under different levels of nesting or scope, if this applies, within the language.
+
+For instance you can (but you don't have to) show how the topic variable can be used by assigning the number <math>3</math> to it and then computing its square and square root.

Task/Topic-variable/J/topic-variable.j

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+   example=: *:, %:  NB. *: is square, %: is square root
+   example 3
+9 1.73205

Task/Topic-variable/Perl-6/topic-variable.pl6

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+given 3 {
+    .say for $_**2, .sqrt;
+}

Task/Topic-variable/Perl/topic-variable-1.pl

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+my $_ = 3;
+print for $_**2, "\n", sqrt;

Task/Topic-variable/Perl/topic-variable-2.pl

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+for ($_ = 0; $_ <= 9; $_++) {
+  print "Outer";
+  print "$_\n";
+  # The inner loop will not nest properly unless
+  # it is preceded by a my statement
+  my $_;    # This is required to nest the inner loop
+  for ($_ = 0; $_ <= 9; $_++) {
+    print "Inner";
+    print "$_\n";
+  }
+  # Alternatively we can use a local keyword in the
+  # inner loop declaration instead of a my statement
+  for (local $_ = 0; $_ <= 9; $_++) {
+    print "Alternative";
+    print "$_\n";
+  }
+}

Task/Topic-variable/UNIX-Shell/topic-variable.sh

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+multiply 3 4    # We assume this user defined function has been previously defined
+echo $?    # This will output 12, but $? will now be zero indicating a successful echo