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# Copyright (C) 2007-2012, Parrot Foundation.
=pod
=head1 DESCRIPTION
A tutorial lesson about Parrot's exists opcode.
=head1 EXISTS
The C<exists> opcode tells you whether an element of a container PMC
(array or hash for example) exists. It differs from C<defined>, because
C<defined> will return false on undefined values, which are valid entries.
It's important to think of an array or hash as being a collection of data
buckets. That bucket can exist but not contain a defined value. If the
bucket does not exist, there is no value to check.
In the example below, C<my_array[0]> exists and is defined, but
C<my_array[1]> only exists without ever being given a defined value.
C<my_array[2]> does not exist and is thus not defined. Similar to
C<defined>, the behavior of C<exists> for a given PMC depends on how
that PMC implements the related vtable functions.
=cut
.sub main :main
# set up an array with two elements
.local pmc my_array
my_array = new ['ResizablePMCArray']
$P0 = new ['String']
$P0 = "Hello"
$P1 = new ['Undef']
push my_array, $P0
push my_array, $P1
# looking at the first element
$I1 = defined my_array[0]
unless $I1 goto not_def_0
say "my_array[0] is defined"
goto end_check_defined_0
not_def_0:
say "my_array[0] is not defined"
end_check_defined_0:
$I2 = exists my_array[0]
unless $I2 goto not_exists_0
say "my_array[0] exists"
goto end_check_exists_0
not_exists_0:
say "my_array[1] does not exist"
end_check_exists_0:
# looking at the second element
$I3 = defined my_array[1]
unless $I3 goto not_def_1
say "my_array[1] is defined"
goto end_check_defined_1
not_def_1:
say "my_array[1] is not defined"
end_check_defined_1:
$I4 = exists my_array[1]
unless $I4 goto not_exists_1
say "my_array[1] exists"
goto end_check_exists_1
not_exists_1:
say "my_array[1] does not exist"
end_check_exists_1:
# looking at the third element
$I5 = defined my_array[2]
unless $I5 goto not_def_2
say "my_array[2] is defined"
goto end_check_defined_2
not_def_2:
say "my_array[2] is not defined"
end_check_defined_2:
$I6 = exists my_array[2]
unless $I6 goto not_exists_2
say "my_array[2] exists"
goto end_check_exists_2
not_exists_2:
say "my_array[2] does not exist"
end_check_exists_2:
.end
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