The principal way to use an array is to refer to one of its elements. An array reference is an expression as follows:
array[index]
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 (see Delete). 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:
index in array
This expression tests whether 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."