BACS 287

1. BACS 287 Arrays, Collections, Generics, and other Data Structures BACS 287

2. Data Structures • All programming languages provide a means to structure data in your programs. • Visual Basic provides three primary memory based data structures: • Arrays • Collections • Generics BACS 287

3. Arrays • An array is a group of related variables that share the same name. • The individual variables in the array are called elements. Elements hold values. • Each element is identified by an index number (subscript). • Arrays may have more than one subscript, each specifying the range of a different dimension. BACS 287

4. One-Dimension Arrays Dim Weekdays(6) as String Total of 7 elements (0 to 6) 0 1 2 3 4 5 6 Mon Tue Wed Thu Fri Sat Sun Weekdays(2) = “Wed” Weekdays(4) = “Fri” Element Subscript BACS 287

5. Arrays • Arrays are useful when you want to store several similar data values and do not want to create a unique name for each one. • Arrays store “strongly typed” data, meaning that you declare the datatype and that is the only type of data that it can hold. • They are also useful because you can iterate through each element in the array with a loop construct. • Some arrays have more than 1 dimension. BACS 287

6. Two-Dimension Arrays Dim Homework (1, 6) as Integer Columns (0,0) (0,1) (0,2) (0,3) (0,4) (0,5) (0,6) 98 63 88 97 100 81 68 Rows (1,0) (1,1) (1,2) (1,3) (1,4) (1,5) (1,6) 74 93 74 61 0 0 0 Homework (row, col ) x = Homework(1,3) Value of x is 61 BACS 287

7. Array Definition • One-dimension arrays are called vectors. When an array has 2 or more dimensions, it is called a matrix. • You can define matrixes with many dimensions in Visual Basic. It is unwise to define a matrix with more than 3 or 4 dimensions. • If you do not give your array a data type, it is type object by default. BACS 287

8. Array Definition • Array elements can hold any valid data type (including user-defined data structures, objects, and other arrays). • You can dynamically change the number of elements in an array with the REDIM statement. Dim intDynamicArray() as integer ReDim intDynamicArray(x + 3) BACS 287

9. Array Definition • You can declare arrays using all the valid scope modifiers (Public, Private, Dim, …). The following are valid. Dim strName (10) as string Public intCount(14) as integer Static datDates(12, 11)as date Dim intMatrix(3, 9, 20)as integer Public dblArray() as double BACS 287

10. Array Use • Once defined, arrays can be used in the same way as variables. intX = intArray(2) strX = strArray(3,4) strArray(1,2) = “A string” ...assume Row = 2 and Col = 5... intX = intArray(Row, Col) intY = intArray(Row + 1,Col + 2) BACS 287

11. Array Use • Arrays can be assigned a value at the same time that they are declared. • When you do this, you use a special syntax that does not give a number of elements and sets the array equal to a list in { } . Dim bytX () as byte = {1, 4, 50, 25} Dim intY (,) as integer = {{1,2}{2,4}{14,34}} Normal Parenthesis Curley Brackets BACS 287

12. Collections • A collection is a group of individual objects treated as one unit. • Each object in the collection has a unique index assigned to it (starting with 0). • While this is similar to an array, it is actually more flexible because the objects do not have to be of the same type (i.e., not strongly typed). • Collections automatically resize when they need more room. BACS 287

13. Collection Classes • There are several built in collection classes in Visual Basic. 3 commons ones are: • ArrayList – Similar to a standard array, but it can hold any object and automatically resizes. • Stack – Data structure where items are added and removed from the same side (LIFO). • Queue – Data structure where new items are added and old ones removed from different ends (FIFO). BACS 287

14. Collection Example Dim MyArrayList as New ArrayList MyArrayList.Add(“Fred”) MyArrayList.Add(5) MyArrayList.Add(#01/03/2007#) . . . For Each str as String in MyArrayList Console.WriteLine(str) Next str BACS 287

15. Generics • Generics are a way to store strongly typed collections. So, they are a special type of collection. • Remember, “strongly typed” means that when the variable is defined, it can only be assigned certain types of values (e.g., integer). • An advantage of a generic over an array is that it automatically resizes when it is full and needs more space. • The advantage of a generic over a collection is that it produces more efficient (and less error prone) code because it is strongly typed. BACS 287

16. Generics Example Dim MyStringList as New List(Of String) MyStringList.Add(“a”) MyStringList.Add(“b”) MyStringList.Add(5) Compiler error . . . For Each str as String in MyStringList Console.WriteLine(str) Next str BACS 287

17. Array Practice 1 • Create a 1 dimensional string array that holds 27 elements. • Create a 2 dimensional integer array with 5 rows and 4 columns. • Create a 1 dimensional dynamic array to hold double values BACS 287

18. Practice 1 Answers Dim strArray(26) as string Dim int2DArray(4,3) as integer Dim dblDynamic() as double BACS 287

19. Array Practice 2 • Create a 2 dimensional integer array with 3 rows and 4 columns. Use a FOR-NEXT loop to load it with the numbers 1 to 12. BACS 287

20. Practice 2 Answer Dim intArray(2,3) as integer Dim intCnt as integer = 1 For row as integer = 0 to 2 For col as integer = 0 to 3 intArray(row, col) = intCnt intCnt += 1 Next col Next row BACS 287

21. Array Practice 3 • Create a 2 dimensional integer array with 3 rows and 4 columns. Use a FOR-EACH-NEXT loop to load it with the numbers 1 to 12. BACS 287

22. Practice 3 Answer Dim intArray(2,3) as integer Dim intCnt as integer = 1 For each element as integer in intArray element = intCnt intCnt += 1 Next element BACS 287