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1. Ltd 41 validation L3 7540 Unit 033 Software design fundamentals Check a check digit modulus 11 When data input is a number containing a check digit the software does a calculation on the digits in the number as follows A weight is assigned to each digit in the number The rightmost digit in the number is assigned the weight 1 the digit at its left is assigned the weight 2 the next digit at the left the weight 3 and so on for however many digits are in the number Each digit is then multiplied by its weight Number 3 5 2 4 Weight 5 4 3 2 1 Weight multiplied by Number 15 20 6 8 The products are then added together 15 20 6 8 6 55 The result is then divided by the modulus 11 55 11 5 remainder 0 If the remainder is 0 then the number is a valid modulus 11 number If the number 35246 was keyed in incorrectly as 32546 and the modulus 11 check done as follows Number 3 2 5 Weight 5 4 3 Weight multiplied by Number 15 8 15 8 P N The products are then added together 15 8 15 8 6 52 The result is then divided by the modulus 11 52 11 4 remainder 8 The remainder is not 0 so the number is not a valid modulus 11 number The software should reject this number as invalid If software requires input of a number with a check digit then a tester must create test data that includes valid numbers and invalid numbers to make sure that the invalid2. manual is created the error messages can be listed in number order so that they are easier to locate When a program contains validation routines then error messages must be displayed if a user enters incorrect data The error message must be understandable by a user For instance if a validation routine checks that an account number is a valid modulus number and the user enters an incorrect number then the error message should display Incorrect account number entered and not Invalid check digit This is because a user will not understand what a check digit is and will not know how to respond to the error message Displaying Incorrect account number entered means a user does not need to look up what the error message means but will re enter the account number Example error messages Error code Error message 001 File does not exist 002 Incorrect member number entered 003 Member name must be entered 004 Value must be M or F Validation checks A designer must specify any validation checks that are to be made on the data input into a program For a customer record the validation checks could be as follows Field Validation Customer account number Check digit modulus 10 Must not be spaces Input terminated by 999999 Name Must not be spaces Balance Must be numeric The alphanumeric fields for Title Name Address and Postcode are not validated as any chara
3. Computing Ltd Version 1 7540 Unit 033 Software design fundamentals Contents Page Concepts of software design FORM aC le EE 1 Advantages of formal design 2 Programs specification EE 3 beleen Een EE EE 6 BOHOM UP desiGP ss ER ER GE EE EG eg 6 Patatmelets ese eg 6 MOGUICS ir Roli EE RO DE ORE DE OR RE EA 7 Data types Basic data types LEE Re ERE 8 Floating PO EE 8 A A 8 SNS aeie EE EE ER EE N 8 lore ats lg terre EE RE ille 8 bileial AR AD ERRATA 9 User defined data type EE EE n Ee EE eng 9 Program constructs EIERE EEN 10 ee E 10 Iteration repetition Pre condilion I00p s acciaieria 11 Post condition Nee EE 11 A RR Re EE ED 12 Indelinite BODE EE ee 12 Diagrams Procedural software High cohesion se ee se ee ER ve N ee 13 Low coupling EN Oe OE 13 Str ct re charte aeiia aini de Ee Ee ee ge ie Eg ene 13 Event driven software Customer example EE 17 Object oriented software Object model Re ER ed ee ie ee 20 Encapsulation EE 20 EE 21 e Wine EE 22 DECISION Ee EE 26 Et EE 28 PM UE aaa eee 30 QUESO E 32 Files Sequential files i 35 INnplttlE tus a E iaa 36 EH DEER 36 Leien BEE 36 Limitations of a sequential file ee 36 External filename 37 Tench Computing Ltd Version L3 1 7540 Unit 033 Software design fundamentals lniternaldilename se a ed aa 37 e E LEE 37 Random access EE 38 Error Gonalions asta a I RR Ri 39 Validation Types of validation IRC A RA 40 Pane elek RD DE ARIA 40 T
4. Each record is made up of fields that store data Below is an example of a random access file with 3 fields per record The first field is a 1 byte record number The second field is a 5 byte string that holds the name of a person The third field is a 2 byte string that holds the age of the person The length of each record in this file is 8 bytes 1 5 2 The first sequence of 8 bytes belongs to the first record the second sequence of 8 bytes to the second record and so on Each record stores data about a specific person i e the record number name and age Record 1 2 3 Data 1 J O H N 2 0 2 J A N E SI5SI3 RIO N 4 Field 1 2 3 1 2 3 1 2 3 Random access files allow any record in the file to be accessed without first having to read the records before the required record in the file So in the example shown above to read record 3 you do not need to read records 1 and 2 first but can access it directly The way that records are read from or written to a random access file is dependent on the programming language used In some languages you define a record by specifying its fields and field types and then access a record by giving its record number position in the file e g 3 In other programming languages a record is accessed by its address which is the number of bytes from the beginning of the file So for the record in position 3 in the above file the number of bytes would
5. Software design fundamentals Level 3 Notes for City amp Guilds 7540 Unit 033 Version 1 Tench Computing Ltd Pines Glendale Road Burgess Hill West Sussex RH15 OEJ Web address www tenchcomputing co uk Email address jtench globalnet co uk About the author Jackie Tench MSc ACIB Cert Ed Jackie started her working career in branch banking with the Midland Bank now HSBC and was transferred to their Computing Department after achieving 100 in their ability test for programmers She then worked for more than a decade in this department and was one of the first women to achieve a junior management grade at the age of 21 She attended a significant number of IBM programming training courses during her time there Jackie was the first woman to pass the ACIB Associate Chartered Institute of Bankers examinations in the Midland Bank HSBC and the youngest person at 21 years of age Jackie then left to raise a family but still found time to teach part time at a college in Sheffield and to obtain a MSc in Computing and a Cert Ed in teaching When her children were old enough Jackie returned to work full time and was a Senior Lecturer in Software Engineering and Computer Studies at a college in Brighton for nearly 10 years teaching all levels up to and including HND Therefore Jackie has considerable business knowledge and qualifications plus wide experience in practical computing and training covering areas such as
6. Specify which of the following values should be accepted or rejected a b d e f 232411 36789X 169430 661239 11789 529648 6 A data entry must be in the format of two alphabetic characters followed by 5 numeric digits Specify which of the following values should be accepted or rejected a b a 9 RG6398 PL34512 HBS8321 F7K75P 7 A company Computer Bytes sells computer equipment You have been asked to design software that will input data from the keyboard for each stock item and write the record to a sequential file The product number for the stock item is 5 digits and has a modulus 10 check digit The software must validate the input data so that invalid data is not accepted A printed list is required of all the stock items in the file with the total number of records at the end of the report The company stock over 500 items so the printed output must allow for multiple pages Produce the following documentation Structure chart File layout List of the validation checks on input List of the error messages Screen layout Print layout Tench Computing Ltd 47 validation L3 7540 Unit 033 Software design fundamentals This page is intentionally blank Tench Computing Ltd 48 validation L3
7. Version L3 1 7540 Unit 033 Software design fundamentals This page is intentionally blank Tench Computing Ltd Version L3 1 7540 Unit 033 Software design fundamentals Files Software uses files to store data that needs to be used at a later time possibly by another application There are two main types of file access that can be used in software sequential and random but the simplest to understand is a sequential file Files can be stored in various formats not all of which are readable e g binary Text files are readable Sequential files Sequential files can be stored and accessed on tape or disk A sequential text file normally consists of records and each record is made up of related fields For example a product record may be made up of the fields ProductCode Description Quantity Price Field name ProductCode Description Quantity Price Data ATO9 Flat Screen 19 109 265 99 PFO1 Monitor 17 20 50 95 Field size 4 30 6 10 maximum in bytes Record size 50 bytes The data in the file would be held as shown below ATO9Flat Screen 19 109 17 20 50 95EOREOF 265 99EORPF01Monitor Each record is a fixed length 50 bytes in the above example and one record follows another in the file Each record normally has a delimiter character at the end shown as EOR in the example There is also a delimiter character at the end of a file shown above as EOF Sequential files c
8. an have variable length records Before a file can be used by software it has to be opened The basic methods for accessing a sequential file are input output or append After use the software must close a file Files output by software must be checked to ensure that the correct data has been written to them Field size A field size must be as small as possible because a field is in every record in the file For instance if a field is required to represent a colour the colour could be represented by a code rather by text to make the field size smaller For instance the codes 1 2 and 3 OR R Y M could represent the colours Red Yellow and Magenta respectively This means that instead of the field having a maximum size of 7 Magenta it would have a size of 1 So a file with 3 000 records will save 3000 x 6 bytes which is 18000 bytes Tench Computing Ltd 35 validation L3 7540 Unit 033 Software design fundamentals Dates would also be held in a file using the smallest possible field size This means that extra characters such as or are not added to dates in a file Dates in a file would be held as ddmmyyyy e g 23042003 Extra characters are only added to make the dates more readable when they are displayed on screen or printed out Input file A sequential file can be opened as an input file so that the software reads data in the file When a file is opened as input data it is read into a memory storage area called a buffer read
9. apping the errors The check digit is calculated by using the existing digits in the number A widely used method for calculating check digits is modulus 11 which traps over 99 of errors Barcodes normally incorporate a check digit Some systems use modulus 10 check digits The calculation for a modulus 10 check digit is the same as for modulus 11 except that 10 is used in place of 11 for the calculation and the check digit can only be in the range 0 9 Create a check digit modulus 11 A check digit is created and then added as the last digit in a number To create the check digit using modulus 11 a weight is assigned to each digit in the number The rightmost digit in the number is assigned the weight 2 the digit at its left is assigned the weight 3 the next digit at the left the weight 4 and so on for however many digits are in the number Each digit is then multiplied by its weight Number 3 5 2 4 Weight 5 4 Weight multiplied by Number 15 20 6 8 LA N The products are then added together 15 20 6 8 49 The result is then divided by the modulus 11 49 11 4 remainder 5 The remainder from the division is then subtracted from the modulus 11 11 5 6 The result is the check digit and this is added as the last digit in the number to give 35246 Note the following exceptions for modulus 11 If the remainder is 0 the check digit is 0 If the remainder is 10 the check digit is X Tench Computing
10. ave a set number of categories Fiction Historical Information Technology The software must check that only these categories can be entered This can take the form of a list presented to a user so that they can only select from the displayed list Hash totals Hash totals are used as controls for data A hash total is a total of data that has no meaningful numerical value such as a total of customer account numbers This type of total can be used to check that records have not been incorrectly inserted or deleted from files or data structures Control totals that have numerical values would a total of the number of items input or the total of customer balances Tench Computing Ltd 43 validation L3 7540 Unit 033 Software design fundamentals Error conditions The following are examples of the type of error conditions that can occur in software The software must be designed so that it checks for these errors If possible the software should take action to prevent the error occurring If some action must be taken by the user then an error message should be displayed on the screen Division by zero This will always cause a program to crash It is a designer s responsibility to ensure that a division by zero does not occur by checking the data used for division File does not exist This will cause a program to crash It is a designers responsibility to ensure that a check is made that a file opened for input exists before a
11. be 16 as the first byte in the file is 0 The formula for working out the address of a record is given below record number 1 number of bytes in record Random access files can be opened for input to read records from the file or output to write records to the file Some programming languages allow files to be opened as input output which means that records can be read from and written to the file This allows a record to be read updated and then written back to the file Advantages Random access files provide fast access to a record in the file If record number 2000 needs to be accessed it can be moved to directly and read without having to read records 1 1999 Records can be read amended and then written back to the same position in the file Disadvantages Random access files must have fixed length records otherwise the address of a record cannot be worked out Random access files cannot be used on tape Unused records take up space in the file as shown below where some records have been deleted and are no longer accessible but the space they occupied is still held in the file Tench Computing Ltd 38 validation L3 7540 Unit 033 Software design fundamentals Record 1 Record 2 Record 3 Record 4 Record 5 Data Deleted Data Data Deleted Error conditions Whenever an access is made to a file e g open read write update close a status code is returned which indicates whether the access was succes
12. ccess is made to it If the file does not exist an error message should be displayed to the user Record does not exist Reading past the end of a file can cause this When reading a file the EOF marker should always be looked for Data mismatch This can be caused by a program trying to put the wrong type of data into a variable Input data should always be checked to ensure it is the correct type Calculation error This is caused by trying to do a calculation on non numeric data Input data should always be checked to ensure it is the correct type Array error This is caused by using a subscript for an array that is outside the bounds of the array This may not cause the program to crash but may make it produce incorrect results A run time error is an error that causes a program to crash A designer should design software so that it does not crash A logical error in software is an error that the programmer has made in the logic of the code A logical error may cause a run time error it or may just produce incorrect results as output Tench Computing Ltd 44 validation L3 7540 Unit 033 Software design fundamentals Screen error messages The error messages that are to appear on the screen when an error occurs in the software must be documented A designer must specify the error messages that are to be produced by a program Normally error messages are given an error code number so that when the user
13. cters can be entered into these fields The telephone number does not need to be validated because it is not used in a calculation and is entered as alphanumeric data Tench Computing Ltd 45 validation L3 7540 Unit 033 Software design fundamentals Questions 2 1 Design a file layout for a randomly accessed member file named member dat to be stored on the c drive in the club directory The file must contain the following member details member number name address telephone number date of birth membership type Full Social Gym Tennis or Squash date joined subscription due date and subscription amount 2 If the date format to be entered into software is dd mm yyyy specify whether the following dates should be accepted or rejected a 31 09 2002 b 29 02 2003 c 31 08 2002 d 30 1 1 2003 e 31 03 2003 f 29 02 2000 g 30 04 1998 h 31 06 2002 3 A data entry must be numeric in the range 25 to 100 inclusive Specify which of the following values should be accepted or rejected a 25 b 99 c 24 d 100 e 26 f 101 g 66 4 A data entry must be 6 characters and a valid modulus 11 number Specify which of the following values should be accepted or rejected Note that X counts as a 10 a 19723Y b 921343 c 11458 d 66135X e 125350 f 2960X g 326410 Tench Computing Ltd 46 validation L3 7540 Unit 033 Software design fundamentals 5 A data entry must be 6 digits and a valid modulus 10 number
14. n the disk e Attempt to input from a file opened for output this error occurs if an incorrect access is attempted for a file for instance a read from a file that is OUTPUT only e Attempt to open a file that is already open this error occurs if an open is done for a file which is already open e Attempt to access a file with incorrect permission this occurs when a file access is attempted which is not allowed e g trying to write data to a file that is READ only Some of these errors should be found when software is tested e g reading past the end of file but some will occur when the software is operational A programmer should always check the status codes returned after an access to a file and take appropriate action in the software e g display a message to the user or close a file Tench Computing Ltd 39 validation L3 7540 Unit 033 Software design fundamentals Validation It is important that software does not allow invalid data to be entered If invalid data is accepted by software then it may crash with a run time error If numeric data was required by software because it was to be used in a calculation and non numeric data was entered and accepted when the software tried to do the calculation it would crash because the computer cannot do calculations on non numeric data Software developers should write routines in the software that check the entered data to make sure it is valid A tester must use test data that i
15. nternal filename so that if the location of the file is changed only one amendment to the program is required the external filename because the internal filename will not change File layout When designing a program the file layout for any files must be specified The file layout specifies the access method the external name the internal name the record name the size of a record the type of the record the field names field sizes and field data types For example a file for products could have a file layout as follows Products file layout External filename D mycompany products fl Internal filename ProductFile Access method Sequential Size of record 50 Record description Record name Product Type of record Fixed length Field name Field size Data type ProductCode 4 String Description 30 String Quantity 6 Integer Price 10 Floating point includes 2 decimal places It is important to document the layout of a file because the file may be used in several programs A programmer needs to know the details about a file before designing software to access it It also means that consistent names are used for the internal filename record name and field names in any software that accesses the file Tench Computing Ltd 37 validation L3 7540 Unit 033 Software design fundamentals Random Access files Random Access files are made up of fixed length records and can only be used on disk
16. numbers are rejected by the software This means you have to know how to create numbers with check digits to create the test data Presence check Input data may be specified as required which means it cannot contain spaces Fields such as customer account number for a sales order or student enrolment number for a student record must be present and cannot be omitted The software must check that these fields do not contain spaces A tester must create test data that includes spaces for this data to ensure that the software will reject the spaces Tench Computing Ltd 42 validation L3 7540 Unit 033 Software design fundamentals Character count Input data such as a customer account number may be specified as fixed length for example 6 characters The software must check that data that has less or more characters is rejected A tester must create test data that contains the correct number of characters and data that contains less and more characters to ensure only the correct length data is accepted Format check Input data may be specified as being in a set format for example two letters followed by four digits AG2372 The software must check that the data contains the correct format A tester must create test data that contains the correct and incorrect format to ensure that data with the incorrect format is rejected Lookup Input data may be specified so that it can only contain certain specified values For example books may only h
17. r values being accepted The range for data input should be specified in the design specification A tester must decide what test data to use to prove that invalid values would not be accepted by the software For data input restricted to the values 100 500 the test data would be 99 100 101 499 500 and 501 This data tests the boundaries of the values The values 99 and 501 should be rejected and the values 100 101 499 and 500 should be accepted Tench Computing Ltd 40 validation L3 7540 Unit 033 Software design fundamentals Type checks Numeric Input data that has numeric calculations done on it must be checked to ensure that it is numeric If non numeric data is allowed as input the software will crash when the computer attempts to do the calculations The software must reject any data input that is non numeric A tester must create test data that includes alphanumeric data to ensure that the software rejects the non numeric data Alphabetic Input data that must be alphabetic can be checked to ensure that it is alphabetic Alphabetic data only includes the characters A Z and a z therefore data that includes spaces and special characters e g a hyphen cannot be alphabetic Check digits Bank account numbers customer numbers stock code numbers are all keyed into computer systems If a number is keyed in incorrectly then the wrong records will be accessed Check digits are added to these types of data input as a means of tr
18. s invalid to prove that the software rejects invalid data Types of validation Date checks Whenever a date is used as input the software should check that it is valid The type of check made depends on the format of the date entered If a date is entered in the format dd mm yyyy then full checks are required The days dd must be inside the boundaries for days in a month For most months this is between 1 and 31 but for the months April June September and November the days are between 1 and 30 For February the days are between 1 and 28 except for a leap year when the days are between 1 and 29 The month mm is checked to make sure it is between 1 and 12 The year yyyy cannot be checked properly but could be checked for the first two digits being 19 or 20 It depends on the application what the range of years can be If the software was for a museum which was holding details about historical artefacts then the range of years could not be checked because there would be a wide range of years allowable The format in which the date is entered should be specified in the design specification and it is up to a tester to decide what test data and tests should be done to make sure that the software will not accept invalid dates Range checks Some data input may be specified as restricted to a range For instance a value entered may be restricted to values between 100 500 A range check must be written in the software to prevent any othe
19. sful or not and sometimes an error code is supplied as well to indicate what type of error has occurred The status code should always be checked so that the software can take action if a file access was unsuccessful An unsuccessful file access which is not checked and action taken in the software can cause a program to crash resulting in corruption of the data in a file The types of error that can occur when accessing a file are files does not exist this error can occur when a file is being opened e record does not exist this error can occur when a record is being read read past end of file this error can occur when the software tries to read a record past the end of file and is normally because a read is done without checking for the end of file This type of error should be found when testing the software and not when it is operational e hardware fault this error can be found at any time when access is made to a file The software should display a message to the user and close down so that the data in the file does not become corrupted by further accesses e File not open this error occurs if access to a record is attempted before the file is opened e Attempt to output to a file opened for input this error occurs if an incorrect access is attempted for a file for instance a write to a file that is INPUT only e Disk full this error occurs if a write to a file is attempted and there is no more space available o
20. structured design analysis coding testing and implementing software applications plus training students to fulfil an important role in the computer industry Jackie has worked as a consultant for several blue chip companies and examination boards using her software engineering and educational training skills and is now one of the foremost experts in computing with an extensive knowledge of programming languages and applications Copyright 1999 Tench Computing Ltd Microsoft Windows Windows NT or other Microsoft products referenced herein are either the trademarks or registered trademarks of the Microsoft Corporation Other trademarks for products referenced herein are also acknowledged All rights are reserved and no part of this training manual may be reproduced stored in a retrieval system or transmitted in any form or by any means electronic mechanical photocopying recording or otherwise without the purchase of a licence This training manual is sold subject to licence and on condition that it shall not by way of trade or otherwise be lent re sold hired out or otherwise circulated without the prior consent of Tench Computing Ltd in any form of binding or cover other than that in which it is issued and without a similar condition being imposed on the subsequent purchaser Any program listings within this training manual may be entered stored and executed in a computer but they may not be reproduced for publication Tench
21. uential file Slow access A sequential file provides a slow method of access to data For example if a file has 5000 records and only record numbers 1000 2500 and 4580 need to be accessed all the records up and including record number 4580 will need to be read Sorting required Since records are written to a sequential file one after the other a sequential file normally has to be sorted into order after the initial records have been written For instance a file that contains product records would be sorted into product number order Amendments Records in a sequential file cannot be amended in place So if an amendment is required to a customer address for record number 50 records 1 49 have to be written to a new output file record number 50 amended and written to the output file and then all the remaining records written to the output file Tench Computing Ltd 36 validation L3 7540 Unit 033 Software design fundamentals External filename An external filename is the location and name of the file on a storage device e g c customer labels fl An external filename is only used once in a program to assign it to an internal filename Internal filename An internal filename is a variable name used for the file within a program The external filename is assigned to the internal filename when the file is opened and then the internal filename is always used within the program to refer to the file The external filename is assigned to an i
22. y for the software to access it The amount of data read into memory depends on the size of the buffer area provided A sequential file is read from the beginning one record at a time So if you need to access the 50 record you must first read the 49 records before it in the file The file must be closed after use A file opened as input must exist A programmer must include code to check that the file does exist before attempting to read from the file If the file does not exist and the software does not check then the software will crash with a run time error Output file A file can be opened as an output file and data will be written to the file by the software Data is stored in a temporary buffer in memory before being written to the file A file opened as output will overwrite any file that already exists with the same name in the selected directory Records are written to a sequential file one after the other It is important that a file opened as output is closed as the close writes any remaining data from the memory buffer to the file and adds the end of file marker Append file An append to a file is used to write data to a file but if the file already exists with the specified name the data will be written at the end of the existing file If the file does not already exist then a new file will be created The new records are written to the sequential file one after the other The file must be closed after use Limitations of a seq
23. ype EE 41 nee e 41 Presence Check ai da 42 Character count Adenauer deene edd e 43 Bal AER EE 43 ee ie EE RR aa a a eA ARE 43 eeh e 43 Error ONREG N ai 44 Screen error messages Validation ege 45 Questions SEE A EG be 46 Program design language PDL Pseudocode Programi boeke EE DE elisa 50 Procedure subroutine block 50 FUNCIONA 50 SCIECHO EE 51 Repetition RT de EE 52 RECKEN 52 Arguments parameters ee ee ee ee ee ee ee 54 FIO IIS EE 59 QUIN EE 62 Data structures Set 65 Static data StrUCtUreS iii 65 Dynamic data StrUCtures ee ee ee ee 65 Array Single dimension array ee ee ee en 66 Multi dimension array ss EE EED EE amen 66 Stack LIPO da 66 Quel PIP O builder 67 Linked list Insert an elemento 68 Delete E Een EE 69 Circular linked TC EE 70 UA 71 Table eet EE 74 Directiaccess able ee Ee Ed Ges 74 Hash table eege 74 Pre written rotlines EE 74 Tench Computing Ltd Version L3 1 7540 Unit 033 Software design fundamentals Search Search UIE Ee Re Ge GE EG ee Ee ed Se NE 75 Searc M length ER RE EE EE EE dectans 75 Linear or Sequential search Ae 75 Binary search AEN 76 Sort STT EE 80 CUISINE 82 QUES e Li 84 Validate the design specification OR d e EE 86 Logical IN ainia 86 AS ds ee AE EO ON OE i 87 RE e 87 DU EE 89 e 92 sample QUESTIONS s a eg ee ee ha lena iipolene 101 Sample assignment ssc ces es en veces in ee dd Ge ri va ene 106 Tench Computing Ltd
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