organizing filesweb.cerritos.edu/jwilson/sitepages/cis_103… · ppt file · web view ·...

File Handling and Control Break Logic

ObjectivesIn 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

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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

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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 docx

• Excel Workbook xlsx

• Access Database accdbx

• 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

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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

– directory / subdirectory

• Windows uses the term folder

– 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!

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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.

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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 (file handle)• open "EmployeeReport.txt" as outputFile (file handle)

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

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Performing File Operations (continued)

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

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

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

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Performing File Operations(pseudocode)

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();

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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();

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Programming Logic & Design, Sixth Edition 12

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

Priming read

Don't usually need this in Java or RAPTOR!

Regular read


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

Moveregular read

A

priming read

is not usually necessary in RAPTOR or

Java

Remove priming read


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

No priming read in housekeeping( )!

Error in pseudocode…

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.

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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!

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

Clients by State Report 17

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

state is the control break field

Hold field could be called oldState

Note: File is sorted by city within state – a double level control break is possible.


• 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

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Figure 7-5 Mainline logic and getReady()module

for a program that produces a clients by state report

Set up the hold field (oldState).

The first record had to be read to set up the hold field.

Priming read.


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

hold field declaration - oldState

this variable will hold the state we are accumulating information for

priming read

initialize hold variable


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


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

check for a control break

normal processing

Notice use of oldState here instead of state

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

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Performing Multiple-Level Control BreaksNew 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]

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

Book Sales by city within state

state major

city minor

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

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Performing Multiple-Level Control Breaks

• In this example, a control break occurs when:

– Value of city variable changes [ oldCity ]

– Value of state variable changes [ oldState ]

• 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 fieldsIMHO: The use of arrays here is an additional unnecessary complication

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

Check highest to lowest

Programming Logic and Design, Fifth Edition, Comprehensive 26


• 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!

Programming Logic and Design, Fifth Edition, Comprehensive 27


• 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

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• 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

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• 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)

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Figure 8-21 Sample portion of data for Book Sales report

control break module would need to be called 3 times

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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

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Summary (continued)

• For multiple-level control breaks, test for a major-level break before a minor-level break

• In a control break, check if lower-level breaks need to be processed