File Handling and Control Break Logic

Objectives In this chapter, you will learn about: • Computer files • Writing a program that reads from and/or writes to a file – Input file

– Output file

• The data hierarchy • Control break logic 2

Understanding Computer Files • Random access memory (RAM) – aka Primary Storage • Temporary storage • Volatile storage (requires constant refreshing)

– Example: store a value in a variable

• Permanent storage – aka Secondary Storage • Non-volatile storage

– Example: save a program to a USB drive

3

Understanding Computer Files • File Concepts – A file is a collection of related data stored on a storage device – Types • Computer Files – Text file

(Notepad file)

– Binary file

(created by a user-written program)

• Word document

• Excel Workbook • Access Database • Music (MP3) file / Video (MP4) file • PDF files

– Characteristics (attributes) of a file • Name • Time and date of creation and last modification • Size measured in bytes • Application used to open it – Windows looks at the file extension to determine this 4

Organizing Files •

Folders (aka directories) – Container for storing related files – My Music, My Pictures, My Documents, Java, RAPTOR, Assignments •

Microsoft DOS used the term directory –

•

Windows uses the term folder –

•

directory / subdirectory

folder / subfolder

Path – Combination of the disk drive letter plus the complete hierarchy of directories in which a file resides – F:\CIS 103\Assignments\Assignment_7\PayrollData.dat

– Put your program and any data files in the same folder

– Do NOT work directly with files in a compressed folder!

5

Understanding the Data Hierarchy • Data hierarchy – Describes the relationships between data components – Consists of: File

[collection of records] Record

[collection of fields] Field

[collection of characters] letters, digits, etc.

6

Performing File Operations (pseudocode) • Use data files in your programs – Declare a file • InputFile:

EmployeeData.txt

– RAPTOR Redirect_Input("EmployeeData.txt")

• OutputFile: EmployeeReport.txt – RAPTOR Redirect_Output("EmployeeReport.txt")

– Open a file - pseudocode • open "EmployeeData.txt" as inputFile • open "EmployeeReport.txt" as outputFile

– Read from a file (get) - pseudocode • get name [from inputFile] • get address [from inputFile] • get payRate [from inputFile] 7

(file handle) (file handle)

Performing File Operations (continued)

Figure 7-2 Reading three data items from a storage device into memory

8

Performing File Operations (pseudocode) – Write (put) • Put name, address, payRate to outputReport • single statement or multiple statements

– Close • close inputFile • close outputFile • RAPTOR closes files automatically

9

Performing File Operations in Java • Declare – public Scanner inputFile; – public PrintWriter outputFile;

• Open – inputFile = new Scanner( new File("EmployeeData.txt") ); – outputFile = new PrintWriter( “EmloyeeReport.txt" );

• Read – name

= inputFile.next(); //get a string delimited by whitespace

– name

= inputFile.nextLine(); //get rest of line as a string

– payRate = inputFile.nextDouble(); – hoursWorked = inputFile.nextDouble(); 10

Performing File Operations in Java • Write – outputFile.println(name);

– formatted output • outputFile.printf("%-15s %-40s %,10.2f%n", name, address, netPay);

• Close – inputFile.close();

– outputFile.close();

11

Priming read Don't usually need this in Java or RAPTOR!

Regular read

Figure 7-3 Flowchart for a program that uses files and a priming read

Programming Logic & Design, Sixth Edition

12

A priming read

Remove priming read

is not usually necessary in RAPTOR or Java Move regular read

Figure 7-3 Revised (no priming read) Flowchart for a program that uses a file

Programming Logic & Design, Sixth Edition

13

No priming read in housekeeping( )! Error in pseudocode…

Figure 7-3 Pseudocode for a program that uses a file

Programming Logic & Design, Sixth Edition

14

Understanding Control Break Logic • control break logic – Temporary “detour” in the logic of a program • We will use control break logic to print a summary line

– Create a report • summary report only • detail and summary report

– To print a summary line you need to first accumulate all the relevant data. 15

Understanding Control Break Logic (continued) • Control break program – Typically a change in the value of a variable initiates special actions or causes special processing to occur • Hold field does not match input field

– Single- or multi-level control breaks are possible – field / hold field

• Note: control break programs usually have a priming read; this is necessary to set up hold fields! 16

Understanding Control Break Logic (continued) state is the control break field

Control Break logic requires a file be sorted on one or more fields

Hold field could be called oldState Note: File is sorted by city within state – a double level control break is possible.

Figure 7-4 A control break report with client totals for each state 17

Clients by State Report

Understanding Control Break Logic (continued) • Examples of control break reports – employee report • sorted by department number, with a new page started for each department – Instead of a summary line a page break is generated when the department number changes.

– books for sale by category report • sorted by category (such as reference or self-help), with a count following each category of book

18

Priming read.

Set up the hold field (oldState). The first record had to be read to set up the hold field. Figure 7-5

Mainline logic and getReady()module

for a program that produces a

clients by state report

Programming Logic & Design, Sixth Edition

19

hold field declaration - oldState this variable will hold the state we are accumulating information for

priming read initialize hold variable Figure 7-5 Mainline logic and getReady()module for a program that produces a report of clients within state report

Programming Logic & Design, Sixth Edition

20

Figure 7-6 The produceReport( ) and controlBreak( ) modules for a program that produces a report of clients within state

normal processing for a detail and summary report

Programming Logic & Design, Sixth Edition

21

check for a control break

normal processing

Notice use of oldState here instead of state

Figure 7-6 The produceReport() and controlBreak( ) modules for a program that produces a report of clients within state

Programming Logic & Design, Sixth Edition

22

Figure 7-7 The finishUp() module for a program that produces a report of clients within state

23

Performing Multiple-Level Control Breaks New Example • Summary report: – group totals, not detail records • city totals • state totals • grand total

– contains summary information and optionally detail information

• Multiple-level control break: – breaks occur for more than one change in condition [multiple hold fields]

Summary lines: • City total • State total • Grand total

Book Sales by city within state state city

Report of book sales by city and state [summary information for city and state ] 24

major minor

Performing Multiple-Level Control Breaks

Check for a change in the state then for a change in the city

• In this example, a control break occurs when: – Value of city

variable changes

– Value of state variable changes

[ oldCity ] [ oldState ]

Check highest to lowest

• Input file must be sorted by city within state • When you detect a new city record print total for

city

• When you detect a new state record print totals for

city and state

• When you detect end-of-file print totals for

city, state, and grand total

• Use arrays to store book counts as well as control break fields IMHO: The use of arrays here is an additional unnecessary complication

25

Performing Multiple-Level Control Breaks • cityBreak( ) module performs standard tasks: – Performs processing for previous group – Rolls up the current-level totals to next higher level – Resets current level’s totals to 0 – Performs processing for new group – Updates the control break field

• stateBreak( ) module does the same, starting with processing cityBreak( ) module • you must check for a break in the state before checking for a break in the city! 26

Programming Logic and Design, Fifth Edition, Comprehensive

Performing Multiple-Level Control Breaks • Main program checks for change in city and state variables [ checks for state change first… ] • When city changes, city’s name and total are printed • When state changes, state’s name and total are printed • All city totals within a state print before state total for same state – Seems logical to check for change in city before state, but that would be incorrect logic!

• Must check for state change first 27

Programming Logic and Design, Fifth Edition, Comprehensive

Performing Multiple-Level Control Breaks • If two cities with the same name (in the same state) follow each other: – Program will not detect new city name

• Always check for a major-level control break first – if the records are sorted by city within state, a change in the state causes a major-level break

• If the records are sorted by city within state, then a change in the city causes a minor-level break • Change in state implies a change in city – Even if the cities have the same name

28

Performing Multiple-Level Control Breaks • Within each control break module, check if you need to: – Perform control break processing for the previous group(s) (if any) • if the state changed, then you must perform a control break on the city

– Perform any control break processing for current group – Roll up the current-level totals to next higher level – Reset the current-level totals to 0 – Update the control break field(s) 29

Figure 8-21 Sample portion of data for Book Sales report control break module would need to be called 3 times

30

Summary • Control break: – a change in a variable’s value causes special actions to occur

• Control break field: – holds data from a previous record to compare to the current record

• Control break data can be used in a heading or footer • Control break report prints summary lines and optionally detail lines 31

Summary (continued) • For multiple-level control breaks, test for a majorlevel break before a minor-level break • In a control break, check if lower-level breaks need to be processed

32