1. 1. CHAPTER 3 : DESCRIPTION OF DATA
2. 2. Chapter Objectives At the completion of this chapter, you would have learnt: the different types of data; the different types of data structures.
3. 3. Introduction Data is any collection of numbers, characters or other symbols that has been coded into a format that can be input into a computer and processed. The main function of a computer program is to process data to produce information. That is, to accept data from the user as input, process the data and present the information to the user as output
4. 4. 3.1 Types of Data There are two fundamental types of data: Numeric and Non-numeric. Numeric(Qualitative) Numbers used for sums of money, ages, distances. For example 0.33, -0.1, 0.5, 2, 50.01, -100, etc. Non-Numeric(Quantitative) Non numbers such as strings of characters. For example John, AS1234G and 2@**556, etc.
5. 5. DATA DIAGRAM Data Numeric Non Numeric Char String Logical Pointer DatesInteger Real
6. 6. 3.1.1 Numeric Divided into two main groups: Integer and Real. Integer Whole numbers used to count things, example 1, 2, 3,... Count discrete objects such as number of bananas, orange or pencils. Precise values. The range of integers provided in a computer system depends on the hardware/language used.
7. 7. REAL May have a fractional part, example 1.0, 234.66, ... Can be positive or negative. To measure things like the length of a piece of string, or the cost of an item in dollars and cents. Generally NOT a precise value. Nearest value is according to the method of storage and type of arithmetic used (e.g. for rounding off method, 14.385 in this case will become 14.39 and 14.382 will become 14.38 whereas for truncation to two decimal places both 14.385 and 14.382 will become 14.38).
8. 8. 3.1.2 Non-Numeric 1. Character strings Character strings may consist of Numbers, Alphabets or Alphanumeric. To represent such character strings, quotes () are needed. For example the name JAMES will need to be quoted as JAMES.
9. 9. 2. String String characters are stored as codes using the American Standard Code for Information Interchange (ASCII) or Extended Binary Coded Decimal Interchange Code (EBCDIC) code system. String operation usually involves comparison, concatenation or selection to allow combinations of strings. Thus strings should have different values also. For example, A < Z, ABC > AAA Alpha < Alphabet and lowercase characters > Uppercase characters
10. 10. 3.Logical / Boolean data Consists of 2 values: True or False. 4.Pointers which store the address of variables. 5. Dates which allow arithmetic operation on calendar dates.
11. 11. 3.1.3 Identifiers All data items used in a program need to be declared before usage. This means to state down the name given to each data item storage cell known as identifier. An identifier may be numeric or non- numeric. The data type that can be contained in the storage cell will depend on the declaration of the cell.
12. 12. These storage cells can be a constant storage cell or variable storage cell. Constants It is a Read-only variable which contains fixed data values that cannot be changed during the program execution. Variables Their content changes according to the users input or the processing of the program during execution.
13. 13. 3.2 Types of Data Structure Data items grouped logically together to give information about an individual. For example, an address book is a data structure containing data items (names, addresses, phone numbers etc.). 3.2.1 Arrays and Tables 3.2.2 Structured Diagrams 3.2.3 Static vs. Dynamic Data Structure 3.2.4 Files, Records and Fields
14. 14. 3.2.1 Arrays and Tables An array is a sequence of consecutive storage locations having a common name. Refer to the contents of the storage cells of each array element by a subscript or index which is enclosed with a square bracket, e.g. [1], [5,3] or [counter]. Grouping of variables in an array of several elements. For example, a one-dimensional array that stored 6 test marks of a student. This array is called StudentFile. StudentFile 76 77 78 56 89 90 0 1 2 3 4 5
15. 15. The value in each of the box stores a test mark. Refer to the contents of the storage cell of each array element by a subscript or index. For example, to refer to the test mark in box number 3, what you will need to do is to call the array by its name, followed by the subscript or index shown below it. In this case, you can refer to the contents of box number 3 by calling StudentFile[3]. A one dimensional array can only hold a students test marks in this case, so if a data structure that is supposed to be created to store all the test marks of a class of students, a two dimensional array must be used. A two dimensional array is a table that can store more information. An example is shown on the next page.
16. 16. A two-dimensional array that stores 6 tests marks of 3 students. StudentFile In this case, you can refer to the content of the third box in the first row by calling StudentFile[1,3]. 1 76 77 78 56 89 90 2 76 89 87 47 85 58 3 56 34 78 67 98 76 1 2 3 4 5 6
17. 17. 3.2.2 Structured Diagrams To show the grouping and relationship of data items in a real case in pictorial form, we can use data structure diagrams. Characteristics of a Structured Diagram Shows how to sub-divide a data item into its parts using the three data structure constructs. Three Data Structure Constructs Simple elements such as address can be represented with a rectangle box. 1. Sequence Construction 2. Selection Construction 3. Repetition/Iteration Construction Address
18. 18. Three Data Structure Constructs Simple with lower level - To show its contents more clearly, it is sub-divided into different parts shown at the lower level in the following diagrams. Address Street Name Postal CodeHouse Number (Sequence construct) (Selection construct) CAR SALOON ESTATESPORTS O O O
19. 19. Note: If one element connected upwards is a selection, all elements connected up to the same point must be selections. Only one selection will be made regardless of the number of choices. Example: (Repetition / Iteration construct) Main Process EndIntro *
20. 20. Data Structure on an Address Book FILE RECORD TRAILERHEADER * ADDRESS TELEPHONENAME INITIALSSURNAME UNIT-CODEAREA CODE STREET TOWNHOUSE NAMENUMBER oo
21. 21. 3.2.3 Static vs. Dynamic Data Structure STATIC data structure - the size of the data structure needed to store information CANNOT change as the program runs. E.g. array. DYNAMIC data structure - the size of the data structure needed to store information CAN change as the program runs. E.g. linked list. Each member of the list essentially forms a record in the list. Each member record is linked to the NEXT LOGICAL RECORD by means of a special field within the record called a POINTER. The POINTER field contains the address of the next logical record in the list. Note: Not all languages have linked lists.
22. 22. Types of linked lists are Binary tree Ring Start point
23. 23. 3.2.4Files, Records and Fields File A file is defined as a group of related data records handled as a logical unit. For example, the stock master file contains data records of all stock items in the warehouse, the payroll master file contains data records of all employees currently on the payroll of the organisation.
24. 24. Record A record is defined as a group of related data items called fields, forming a logical unit. For example, the stock master record may contain data items like ITEM-CODE, DESCRIPTION, UNIT-PRICE, QUANTITY-ON- HAND and so on. These data items make up the stock master record, and they are called FIELDS.
25. 25. Field A field is a data item forming a subdivision of a record. Record no. 1 Student Number Student Name Date of Birth Sex Exam Mark 1289 Cindy Bridges 080471 F 76 1290 Alan Lim 060970 M 80 Record no. 1 Record no. 2 File Fields
26. 26. Record key Definition A record key is defined as a field within a record that is used as the basis for ordering the records in a sequential file and also as a means of direct access. Examples STOCK MASTER file arranged in ascending order of item code which is the record key that must be unique. SUPPLIER MASTER field arranged in order of vendor number, which is the record key.