The principal way of using an array is to refer to one of its elements. An array reference is an expression which looks like this:
array[index]
Here, array is the name of an array. The expression index is the index of the element of the array that you want.
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'.
If you refer to an array element that has no recorded value, the value
of the reference is "", the null string. This includes elements
to which you have not assigned any value, and elements that have been
deleted (see section 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.)
You can find out if an element exists in an array at a certain index with the expression:
index in array
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, to test whether the array frequencies contains the
index `2', you could write this statement:
if (2 in frequencies)
print "Subscript 2 is present."
Note that this is not a test of whether or not 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 would do so:
if (frequencies[2] != "")
print "Subscript 2 is present."
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