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Referring to an Array Element

   The principal way to use an array is to refer to one of its elements.
An array reference is an expression as follows:


Here, ARRAY is the name of an array.  The expression INDEX is the index
of the desired element of the array.

   The value of the array reference is the current value of that array
element.  For example, `foo[4.3]' is an expression for the element of
array `foo' at index `4.3'.

   A reference to an array element that has no recorded value yields a
value of `""', the null string.  This includes elements that have not
been assigned any value as well as elements that have been deleted
(Note: The `delete' Statement.).  Such a reference
automatically creates that array element, with the null string as its
value.  (In some cases, this is unfortunate, because it might waste
memory inside `awk'.)

   To determine whether an element exists in an array at a certain
index, use the following expression:


This expression tests whether or not the particular index exists,
without the side effect of creating that element if it is not present.
The expression has the value one (true) if `ARRAY[INDEX]' exists and
zero (false) if it does not exist.  For example, this statement tests
whether the array `frequencies' contains the index `2':

     if (2 in frequencies)
         print "Subscript 2 is present."

   Note that this is _not_ a test of whether the array `frequencies'
contains an element whose _value_ is two.  There is no way to do that
except to scan all the elements.  Also, this _does not_ create
`frequencies[2]', while the following (incorrect) alternative does:

     if (frequencies[2] != "")
         print "Subscript 2 is present."

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