Lecture Outline. Rainfall Solution Attempt 1. Motivation for arrays. Introducing the Array. Motivation

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1 Lecture Outline IMS1906 Programming in VB.NET Week 7 Lecture 1 & 2 Introduction to Arrays Arrays Part 1 and II Angela Carbone Monash University School of Information Management and Systems Defining Arrays (VB.NET) Declaring and Initialising Array Assigning values to Arrays Properties and methods of Array Class 2 Motivation for arrays Rainfall Solution Attempt 1 Much computer processing applies to sets of data Sets can be quite large Example: Calculate average rainfall Write a program which reads in the amount of rainfall for each of 6 months, displays the average rainfall followed by the individual monthly rainfall amounts. Read rainfall1 Read rainfall2 Read rainfall6 Average (rainfall1 + rainfall2 + rainfall + rainfall4 + rainfall5 + rainfall6) / 6 Display Average Display rainfall1 Display rainfall2 Display rainfall6 4 Motivation Introducing the Array To calculate the average we need to read all the data To be able to display the average before displaying the rainfall amounts requires we retain the data Six months of rainfall Six individual variables each has different name, same type Can t use a loop to read them, because of the different names We Desire: An efficient means of storing data A generic approach to solving the problem, so that if it changes to 12 months we don t have to add more code An array stores a group of values of the one data type E.G. a group of integers, a group of Strings Individual elements of an array are distinct things As though they were a separate variable Identified by an index in addition to the name An Array is a data structurethat: Allows us to collect together similar data Has a single name Has multiple containers, each have unique index. 5 6

2 Introducing the Array Visualising an Array of Strings days of the week: DayNames: Sunday Monday Tuesday Wednesday Thursday Friday Saturday DayNames() The array has a name: DayNames Each element has an indexor subscript (an integer) The name and index uniquely identifies an element E.g. DayNames(2) has the value Tuesday All elements in the array have the same data-type Introducing the Array Number of elements grouped into array = Size of array Use a number inside brackets after array name to specify a particular element: e.g. DayNames(4) is the fifth element of array A Change to the value of one element does not affect any of the other elements The name combined with an index refers to an element its type is integer or string or some other type The name by itself refers to the array as a whole its type is array of integers, array of Strings, etc. 7 8 Rainfall Solution with Arrays Array Solution - Comments Process_Rainfall Set sum to zero DO index = 0 to 5 Read rainfall_amount (index) Set sum = sum + rainfall_amount (index) END DO Average = sum / 6 Display Average DO index = 0 to 5 Display rainfall_amount (index) END DO END Solution is able to use a loop structure Simple to increase number of months Solution is easier to understand Read a value, add that value to running total (sum) Solution is more concise This solution is more efficient than the original solution (Original solution would lose marks for efficiency) 9 10 Using Arrays Declaring Arrays in VB.NET Arrays are useful in many cases: Storing lists of related data Affords the use of loop structures to process the data Need to declare arrays Either at the module level or procedure level Need to initialise arrays Populate them with initial data (from file, from user, etc.) Once initialised, can perform processing on arrays Maybe store array elements out to a file Maybe pass arrays between modules Dim strdaynames (6) As String Name for the array The upper bound of valid subscripts The Data Type for items in array This declares a procedure-level Array named strdaynames that can store String elements. The elements are named daynames(0) daynames(6) 11 12

3 Declaring and Initialising Arrays (Examples) Dim intrainfallamounts(11) As Integer declares a 12-element procedural level array named intrainfallamounts; each element is automatically initialised to the number zero Dim decsales() As Decimal = {75.0D, 9.65D} declares and initialises a 2-element procedural level array named decsales; Private stritems(9) As String declares a 10-element module level array named stritems ; each element is automatically initialised to the keyword Nothing [Note variable initialised to nothing do not actually contain the word nothing, rather they contain no data at all] Private mstrstates() As String = { Vic, Qld, NSW } declares and initialises a -element module level array named mstrstates; 1 Declaring and Initialising Arrays When you declare an array, a default value is given to all elements Numeric data types value 0 String data type Empty String It is possible to list the desired initial values when declaring the array: No need to specify upper bound Example: Value for strdaynames(0) Dim strdaynames() As String = {, Sunday, _ Monday, Tuesday, Wednesday, Thursday, _ Friday, Saturday } Value for strdaynames(4) 14 Declaring Arrays Local Declaration procedure/function-level Dim appears inside sub procedure (or event procedure or function) values can be assigned only inside the procedure memory is freed for re-use as soon as procedure is completed Global Declaration form-level Private appears in the (Form) (Declarations) section of code values can be assigned anywhere in the form > Typically initialise in the Form s Load event procedure memory allocation (and values) are preserved between each event procedure call Variable Array Properties and Methods The length property tells how many items can be stored in an array: Consider this array: Dim intnumbers(10) As Integer intnumbers.length property will give the value 11 (index 0 is included) The GetUpperBound method tells the highest valid index for the array: The Statement: intnumbers.getupperbound(0) will report the value 10, because this is the last valid index For 1D arrays, you must always include 0 in the GetUpperBound method call Array Class Rainfall Example in VB.NET Array.Sort method Syntax > Array.Sort(arrayname ), where array name is the name of the one-dimensional array to be sorted > Eg: Array.Sort(intNumbers ) sorts numbers in ascending order Array.Reversemethod Syntax > Array.Reverse(arrayname ), where array name is the name of the one-dimensional array whose elements you want reversed > Eg: Array.Reverse(intNumbers ) organises array elements in reverse order [See Zak, p7] Dim intsum As Integer = 0, sngaverage As Single Dim intmonth As Integer ' The following line creates an array that can contain ' 12 values of rainfall Dim intrainfall(11) As Integer ' Read and store each month's rainfall ' and add to intsum For intmonth = 0 To 11 intrainfall(intmonth) = InputBox("Enter Rainfall " & intmonth) intsum = intsum + intrainfall(intmonth) Next intmonth 17 18

4 Rainfall Example in VB.NET (cont) Summary ' Calculate Average, and display message sngaverage = intsum / 12 OutputListbox.Items.Add( "The average rainfall was " & _ Format(sngAverage, "Fixed")) ' Display each month s rainfall amount in a listbox. Use a new loop, to step through each element. For intmonth = 0 To 11 OutputListbox.Items.Add( _ "Month " & CStr(intMonth) & _ ", Rainfall: " & CStr( intrainfall(intmonth)) ) Next intmonth Declaring and Initialising Arrays Dim statement procedure level Private statement module level Assigning values to arrays Array properties and methods Length property GetUpperBound(0) method Array.Sort(arrayname) method Array.Reverse(arrayname) method Summary/Reading An Array is a data structure that: Allows us to collect together similar data Has a single name Has multiple slots, each with a unique index or subscript The values in the slots are called elements All elements are of the same data type A one-dimensional array is an array where we can specify any valid element by giving: The array s name A single index, inside round-brackets. Eg: dayofweek(4) refers to the fifth element in dayofweek Reading Zak, Chapter 9, p24-4 Unit Guide, Study Guide 8 Robertson, Chapter 7, p Review Questions Zak, p58 Q1, Q-Q6, Q12 Zak, p62 Q1-Q5 21 IMS1906 Programming in VB.NET Week 10 Lecture 2 Arrays Part II Angela Carbone Monash University School of Information Management and Systems Outline Motivation to resize the array Resizing arrays Searching an unsorted array Scenario: Write a program that asks for the number of students in a class, then reads the names of those students into an array. Problems: How do we know how much space the array needs? How can we cope with larger amounts of data than expected? 2 24

5 Dynamic Sizing of Arrays The ReDim statement Arrays can have their size changed Called Re-Dimensioning Arrays do not need to be given an initial size when declared: Dim strnames( ) As String Specifies that strnames is an Array variable Specifies that the elements of strnames must be Strings But does not say how many elements there will be > Initially, there are no elements Size of array can be changed more than once Dynamic means decided at run-time instead of coding -time To change the size of a declared array, use ReDim Do not need to specify As Datatype The data type of the elements cannot be changed Example: ReDim strnames(19) Allocates enough space for 20 names (indexes 0 to 19) ReDim will then initialise all values to default values 0 or empty string ReDim can only appear inside Sub or Function procedures Dim statements can appear outside of Sub/Functions, in Form level ReDim Preserving existing data Motivation for searching through array To preserve any existing data elements, must include the Preserve clause: ReDim Preserve strnames(29) Expands the array to contain a total of 0 elements Keeps the values of elements 0-19 Initialises the values of elements to default value (empty) To shrink the array: ReDim Preserve strnames(15) Imagine you have an array of temperatures recorded over the period of a month Things you may want to do with the temperatures: Determine what the highest temperature was Determine when the highest temperature was Similar for the lowest temperature Determine when (or indeed if) it was 22 degrees. Determine how many times it was 18 degrees. These are typical array problems which can be classified as search problems Finding a maximum value Algorithm Finding Maximum To find the highest temperature, need to consider all elements of the array Initially, only first temperature known, so it is the highest we know of Consider each element in turn: If it is lower than the highest we ve seen so far, ignore it If it is higher than the highest we had previously seen, take note of this new element. After considering all elements, we will have found the maximum Assume elements on the array are stored starting at position index 1 posofhighestvalue = 1 highestvalue = array(1) curelementpos = 2 (we ve already looked at pos 1) DO WHILE curelementpos <= numofelements IF array(curelementpos ) > highestvalue THEN highestvalue = array(curelementpos ) posofhighestvalue = curelementpos END IF increment curelementpos END DO 29 0

6 Temperature(curElementPos): 16 Temperature(curElementPos): 1 No, so move to next position Temperature(curElementPos): Temperature(curElementPos): 18 Yes, so update LHS variables Yes, so update LHS variables Then move to next position Temperature(curElementPos): No, so move to next position But no more positions, so we are finished Searching Arrays Simple/Linear Search: When we consider all elements one by one in an unsorted array (just shown) Consider elements one by one (iterative process) Perform a comparison to try and get a match To find largest/smallest, must go through whole array 6

7 Summary/Reading Re-dimensioning array Sometimes size of array not known at coding time VB allows you to specify size at runtime dynamic Linear Searching Arrays Reading Zak, Chapter 9, p2-7-4 Unit Guide, Study Guide 8 [NOTE- Chapter 9 of the Unit Guide is NOT covered in ims1906.] Robertson, Chapter 7, p