Lecture Learning programming using Visual Basic Net – Chapter 10 Working with arrays and collections

After studying this chapter you should be able to Explain why arrays are needed to solve many types of problems; construct an array to store multiple related data values; use ArrayList, Hashtable, and SortedList collections to store and process data values.

Collections

• An array is a variable with a single symbolic

name that represents many different data items

• Visual Basic NET provides a number of classes

that manage collections of objects

• A collection of objects is like an array in that one

collection is associated with many objects

• This chapter covers the ArrayList, Hashtable,

and SortedList collections

• Explain why arrays are needed to solve many

types of problems

• Construct an array to store multiple related data

values

• Use ArrayList, Hashtable, and SortedList

collections to store and process data values

10.1 Solving Problems with Arrays

• Simple variable means a variable that can store

only one value

• The Problem

– Calculate the rate of return for 10 different companies and

those companies who exceeded the average of the group.

• The Solution Using Simple Variables

– Write the pseudocode for the problem solution.

• Two passes through the data:

– One to compute the average.

– Two to determine who exceeds the average.

– Not a good way to solve the problem.

10.1 Solving Problems with Arrays (cont.)

• The Structure of an Array

– Each element of an array is like a simple variable.

– Starting at zero, the elements in an array are numbered.

– The element number is called the subscript.

– Another name for the element number is index value.

– Arrays are also called subscripted or indexed variables.

• In a one-dimensional array, each element is

referenced using a single subscript value

• Higher-dimensional arrays are called

multidimensional arrays

• Syntax for a one-dimensional array:

– ArrayName(SubscriptValue)

10.1 Solving Problems with Arrays (cont.)

• The Solution Using Arrays

– An array will provide a better problem solution.

– Use a numeric variable for a subscript.

– The subscript begins at zero.

• Storing Data in Arrays versus Databases

– Reading data from a database is an option.

– Arrays are variables that are stored in RAM.

– Data stored in RAM can be accessed quickly.

– Storage for arrays in RAM is valuable space.

– Database are stored on disk and retrieval is relatively slow.

10.1 Solving Problems with Arrays (cont.)

• Factors to consider for array versus database

are:

1 Execution speed.

2 Volume of data.

3 Clarity of the code that accomplishes the task.

• As RAM gets less expensive, the distinction

between databases and arrays gets less clear

• Multidimensional Arrays

– The number of dimensions in an array is known as

dimensionality.

– A two-dimensional array uses two subscripts.

• It is also referred to as a matrix or table.

• Syntax: ArrayName(RowSubscriptValue,

ColumnSubscriptValue).

10.2 Declaring Arrays

• Arrays can store different data types just as

simple variables can

• General syntax of the Dim statement:

– Dim ArrayName(subscripts) As Type

• Subscript bounds are specified by the Dim

statement

• Complete the examples listed below in your

textbook:

– Example 10.1 Populating an Array from a Database.

– Example 10.2 Sequentially Searching an Unordered Array.

10.3 The ArrayList Collection

• The ArrayList collection class is like a

one-dimensional array

• It is indexed using a zero-based indexing

system

• Has a dynamic resizing ability

• Its capacity automatically grows by adding items

– Properties

• Capacity

• Count

• Item

10.3 The ArrayList Collection (cont.)

– Example 10.3 Populating an ArrayList from a Database.

– Example 10.4 Performing a Binary Search.

– Example 10.5 Performing Multiple Result Search.

10.4 The Hashtable Collection

• A Hashtable is a data structure that supports

very fast access to information based on a key

field

• The hash function takes a unique key field and

transforms it into a new value called a bucket

number

– Important Points about Hashtables:

• Support very fast access.

• Store the records randomly.

• Sequential access to the entire set of records is

difficult.

• Exact key must be used for access

10.4 The Hashtable Collection (cont.)

10.4 The Hashtable Collection (cont.)

• Complete the examples listed below in your

an ArrayList.

• A SortedList maintains an array for the keys and

another array for the associated values

• The elements of a SortedList are ordered by the

value of the keys

• Operations on a SortedList tend to be slower

than operations on a Hashtable

10.5 The SortedList Collection (cont.)

10.5 The SortedList Collection (cont.)

• Complete the examples listed below in your

single value.

• Arrays must be declared using a Dim, Static,

or Public statement

• Arrays also need to be dimensioned

• To access an element of an array, the

programmer specifies both the array name and

Chapter Summary (cont.)

• The declaration statement that creates an array

initializes all of its elements to zero or the

zero-length string

• We start by populating the array

• Then, we can use the array to process data

• An ArrayList collection is an index-based data

structure like an array

• Unlike an array, a Hashtable collection uses a

key field to store and retrieve elements

• Hashing has a randomizing effect

• A SortedList provides both index-based and

key-based access to its elements