Understanding Array Declaration and Dynamic Arrays in Programming

Introduction to Programming Lecture 5 Msury Mahunnah, Department of Informatics, Tallinn University of Technology

Storing set of data Arrays ArrayLists

Variable vs Array Variable consists of a cell to keep (store) the value, provided with the name Array consists of cells to keep (store) the values, provided with the name and the indexes.

Why? Let s set up an assignment to find maximum salary among the 5 employee. Dim salary1%, salary2%, salary3%, salary4%, salary5% or Dim salaries%(4) Example on the whiteboard!

Main properties 1. Name For names, same rules as variables 2. Dimensions It will determine how the items are organised? 3. Boundaries of indexes Boundaries for every dimension 4. Dynamism Static memory (Dim) and Dynamic memory (ReDim) Dim allocate memory before the work of a program Redim allocate memory during the program work 5. Type Integer, String, Date, Boolean, ... Every property is determined directly or indirecty with an array declaration!!!

Main properties - example Dim A(5) As Integer Name A Dimesions 1 (how many boundaries do we have?) Boundaries of indexes (for first dimension) 0 to 5 Dynamism static (Dim) Type Integer

Main properties - example Dim B( , ) As String Redim B(3, 7) Name B Dimesions 2 (how many boundaries do we have?) Boundaries of indexes for first dimension 0 to 3 for second dimension 0 to 7 Dynamism dynamical (ReDim) Type String

Declaring a dynamic array At first declare it as usual with Dim statement (or Public or Private), but do not specify boundaries for its dimensions: Dim DynArray() As Integer Later in the program, when you know how many elements you want to store in the array, use the ReDim statement: ReDim DynArray(UserCount)

Declaring a dynamic array Dim A( , , ) As Integer ReDim A(3, 7, 4) How to know some essentials features? Dimensions in array A.Rank - > 3 Total elements in array A.Length - > 160 Elements in first dimension A.GetLength(0) -> 4 Elements in second dimension A.GetLength(1) -> 8 Elements in third dimension A.GetLength(2) -> 5 Last index in the first dimension A.GetUpperBound(0) - > 3 Last index in the second dimension A.GetUpperBound(1) - > 7 Last index in the second dimension A.GetUpperBound(2) - > 4

Initializing Arrays General constructor: Dim arrayname() As type = {entry0, entry1, ... , entryN} Two ways to do this (real example): Dim Names() As String = { Peter , Kathy } Dim Names(1) As String Names(0)= Peter Names(1)= Kathy

Initializing Arrays real example Dim Matrix ( , ) As Integer = {{10, 20, 30}, {40, 50, 60}, {70, 80, 90}} 0 1 2 Or 0 10 20 30 1 40 50 60 Dim Matrix ( , ) As Integer = 2 70 80 90 {{10, 20, 30}, _ {40, 50, 60}, _ {70, 80, 90}}

Repeated ReDim declaration Each time you execute the ReDim statement, all the values currently stored in the array are lost!!! Q: But If I need to do it for enlarging an array? A: Use keyword Preserve

Preserve example Dim Vector() As Byte Redim Vector(7) To enlarge Vector: ReDim Preserve Vector (12) Or ReDim Preserve (Vector.GetUpperBound(0)+5)

Initializing array with random number Const max%=3, a%=1, b%=9 random numbers from 1 to 9 Dim V%(max), i% Before (for cycle) -> V={0, 0, 0, 0} For i=0 to max V(i)=Int(Rnd()*(b-a+1)-a) Next i After (for cycle) - > V = {6, 2, 5, 8}

An additional possibility to store data ... The ArrayList Collection ... ... is a dynamical structure Advantages: Adding new items simply Removing items (by index and also by value) Storing there different type of values More convinient - grow automatically as you add elements 1. 2. 3. 4.

The ArrayList Collection Declaration: Dim aList As New ArrayList Adding items: Dim words() As String = {"Just", "a", "few", "words"} aList.Add(words): aList.Add("Hello") aList.Add(777): aList.Add(True)

The ArrayList Collection First item index (sequence number) is 0 Removing items: By value: aList.Remove( Hello ) By index: aList.RemoveAt(2) All items at the same time: aList.Clear()