Visual Basic.NET How to Program (PDF)

290 Arrays Chapter 7
The declaration of an array creates a variable th at can store a reference to an array but does not
create the array in memory.
Arrays can be declared to contain elements of any data type.
Arrays are represented as objects in Visual Ba sic, so they must also be allocated with keyword
New. The value stored in the array variable is a reference to the location in the computer’s memory
where the array object is created.
Array bounds determine what indices can be used to access an element in the array.
The initializer list enclosed in braces ( { and }) specifies the initial values of the elements in the
array. The initializer list can contain a comma-separated list specifying the initial values of the elements
in the array. If the initializer list is empty, the elements in the array are initialized to the
default value for the data type of the array.
Keyword Nothing denotes an empty reference (i.e., a value indicating that a reference variable
has not been assigned an address in the computer’s memory).
Unlike languages such as C and C++, Visual Basic provides mechanisms to prevent the accessing
of elements that are outside the bounds of an array.
If a program attempts to use an invalid index (i.e., an index outside the bounds of an array), Visual
Basic generates an exception.
To pass an array argument to a procedure, specify the name of the array and do not include parentheses.
Although entire arrays are passed by reference, individual array elements of primitive data types
can be passed by value.
To pass an array element to a procedure, use the indexed name of the array element as an argument
in the procedure call.
The sorting of data (i.e., the arranging of data into some particular order, such as ascending or descending
order) is one of the most important computing applications.
A bubble sort makes several passes through the array. Each pass compares successive pairs of elements.
On an ascending bubble sort, if a pair is in increasing order (or the values are equal), the
bubble sort leaves the values as they are; if a pair is in decreasing order, the bubble sort swaps their
values in the array.
The advantage of the bubble sort is that it is easy to program. However, the bubble sort runs slowly,
as becomes apparent during the sorting of large arrays.
The linear search algorithm compares each element of an array against a search key. If the elements
of the array being searched are not in any particular order, it is just as likely that the value
will be found in the first element as in the last. Thus, the procedure compares the search key with
half the elements of the array, on average. Linear search works well for small arrays and is acceptable
even for large unsorted arrays.
For sorted arrays, the binary search algorithm eliminates from consideration half the elements in
the array after each comparison. The algorithm locates the middle array element and compares it
with the search key. If they are equal, the search key has been found, and the index of that element
is returned. Otherwise, the problem is reduced to searching half of the array. If the search key is
less than the middle array element, the first half of the array is searched; otherwise, the second half
of the array is searched.
In a worst-case scenario, searching an array of 1024 elements via binary search requires only 10
comparisons. The maximum number of comparisons needed to complete a binary search of any
sorted array is indicated by the exponent of the first power of two that is greater than or equal to
the number of elements in the array.