PreludeProgramming6ed_pp10.pdf

Chapter 10 Sequential Data Files

PRELUDE TO PROGRAMMING, 6TH EDITION BY ELIZABETH DRAKE

10.1 An Introduc/on to Files  A file is a collec=on of informa=on that has been assigned a name and stored separately from the program that created it.  A file may contain programs or data.  There are two main types of files: ◦ Text files ◦ Binary files


Classifica/on of Files

 By content: • Text, readable by humans ◦  ASCII characters only

• Binary, readable only by the computer ◦  ASCII data plus special codes

 By method of access: ◦  Sequen2al files contain records that must be read in the order in which they were created ◦  Similar to an audio or VCR tape

◦  Direct Access files contain records that can be accessed in any order. ◦  Similar to a CD or DVD


Text Files or Binary Files?

 Binary files ◦ Used by most opera=ng system files, program files, and data files produced by applica=ons

 Advantages of text files ◦ Easier to create by using a text editor ◦ Can be displayed on screen or printed without any special soTware ◦ Universal: virtually any computer system can interpret their contents without special soTware


Records and Fields

Ø One file may be broken up into groups of related data, called records

Ø Records contain data items and each data item in a record is called a field

Ø Example: A business might keep a file for customers. The data might be customer name, address, phone number. A record of one customer consists of the 3 fields: ◦  the customer’s name ◦  the customer’s address ◦  the customer’s phone number.



Records and Fields in a Data File

Records and Fields in a Data File Ø  The fields are separated from one another by commas

Ø  Each record is terminated by a special symbol: <CR>

Ø  The first two records of this file would look as follows:

”R. Abrams”,86,64,73,84<CR> ”J. Chavez”,94,87,83,90<CR>


Sequen/al and Direct-‐Access Files

Data files can be divided into two other categories:   Sequen2al files ◦  contain records that must be processed in the order in which they were created ◦  are accessed in linear fashion ◦  For example, to print the 50th record in a sequen=al file, must first read (or scan) the 49 records that precede it

  Direct-‐access files (or random-‐access files) ◦  each record can be accessed independently of the rest ◦  loca=ng a data item is like finding a certain track on a DVD


Crea/ng a Sequen/al File

Ø  Open the file. Specify: ◦  External name: the full name of the file on disk. ◦  Internal name: the name by which the file will be known in the program ◦  File mode: the purpose of the file: Input or Output

Ø  Write data to the file, or Ø  Read data from the file

Ø  Close the file ◦  Saves the file and puts an end-of-file marker (EOF) aTer the last record if the file was created in the program

◦  Closes the file if the file is an Input file


Example: Opening and Closing a File

Ø  If we issue the command: Open “grades” for Output As NewFile

◦  grades is the external name ◦  NewFile is the internal name ◦  The mode is Output

Ø  If we issue the command:

Close NewFile ◦  An EOF (end-‐of-‐file) marker is placed at the end of the file ◦  The file is closed ◦  The file is saved with the external name grades


Example: Reading a File

Ø  To open a file to be read: Open “grades“ for Input As GradeFile

Ø  Read the internal filename and the fields/variables:

Read GradeFile, Name, Score

Ø  Read records within a loop (EOF() is discussed later): While NOT EOF(GradeFile) Read GradeFile, Name, Score ... End While Close GradeFile


Example: Crea/ng a Sequen/al File

1 Declare Student As String

2 Declare Score As Integer

3 Open “grades” For Output As NewFile

4 Write “Enter student’s name and test score.”

5 Write “Enter 0 for both when done.”

6 Input Student, Score

7 While Student != “0”

8 Write NewFile, Student, Score

9 Write “Enter student’s name and test score.”

10 Write “Enter 0 for both when done.”

11 Input Student, Score

12 End While

13 Close NewFile


The EOF() Func/on

To terminate the input or output process of a loop that works with data in a data file, and to force an exit from the loop, most programming languages contain an end-‐of-‐file, EOF() func=on:

EOF(InternalName)

The EOF() func2on Ø may appear in the test condi=on of any loop or selec=on structure

Ø has the value true if the end of the file InternalName has been reached -‐-‐ i.e. if the file pointer is located at the end-‐of-‐file (EOF) marker

Ø otherwise the value is false

The use of the EOF() func=on is illustrated in the example in the next slide


Example: Using the EOF() Func/on

1 Declare Student As String 2 Declare Score As Integer 3 Open “grades” For Input As GradeFile 4 While NOT EOF(GradeFile) 5 Read GradeFile, Student, Score 6 Write Student + “ “ + Score 7 End While

8 Close GradeFile


10.2 Modifying a Sequen/al File

Ø  The most common opera=ons on sequen=al files are: ◦  dele2ng an exis=ng record ◦  changing an exis=ng record ◦  inser2ng or adding a new record

Ø  Read the file, one record at a =me, rewri=ng each record to a temporary or scratch file un=l reaching the one to be modified. Ø  The mode is Input for the original file and Output for the scratch file. Ø  If modifying an exis=ng record, make the change and write the new version to the scratch file.


Modifying a Sequen/al File (con/nued)

Ø  If dele=ng an exis=ng record, skip over the record to be deleted. Ø  If inser=ng a new record, read down to the proper loca=on, then write the new record. Ø  If the loca=on is to be the end of the file, read to the end and write the record. Ø  Close the Input and Output files. Ø  Copy the scratch file onto the original file.



Example: Modifying a Sequen/al file Dele/ng a record from a file that contains student names and one test score:

1 Declare Student As String

2 Declare DeleteName As String

3 Declare Score As Integer

4 Open “grades” For Input As GivenFile

5 Open “scratch” For Output As TempFile

6 Write “Enter name of student to be deleted:”

7 Input DeleteName

8 While NOT EOF(GivenFile)

9 Read GivenFile, Student, Score

10 If Student != DeleteName Then

11 Write TempFile, Student, Score

12 End If

13 End While

14 Close GivenFile, TempFile


Example: Modifying a Sequen/al file (continued) The updated file on the previous slide is now named TempFile. To restore ”grades” as the name of the updated file, the records from the ”scratch” file must be copied to ”grades” as follows:

Open “grades” for Output As TargetFile

Open “scratch” for Input As SourceFile

While NOT EOF(SourceFile)

Read SourceFile, Student, Score

Write TargetFile, Student, Score

End While

Close SourceFile, TargetFile

Note: the scratch file s=ll contains the informa=on but it doesn’t maber. The next =me you open the scratch file to modify another file, the old contents will be erased.


Example: Modifying One Field in One Record

1 Declare Name As String 2 Declare NewScore As Integer 3 Open “grades” For Input As GivenFile 4 Open “scratch” For Output As TempFile 5 Write “Enter the name of the student: “

6 Input Name 7 Write “Enter new test score: “

8 Input NewScore 9 While NOT EOF(GivenFile) 10 Read GivenFile, Student, Score 11 If Student == Name Then 12 Write TempFile, Student, NewScore 13 Else

14 Write TempFile, Student, Score 15 End If

16 End While

17 Close GivenFile, TempFile 18 Copy the file scratch onto the file grades


Example: Inser/ng a Record into a Sequen/al File 1 Declare NewName As String 2 Declare NewScore As Integer 3 Open “grades” For Input As GivenFile 4 Open “scratch” For Output As TempFile 5 Write “Enter name and score for new student:” 6 Input NewName, NewScore 7 Set Inserted = 0 8 While (NOT EOF(GivenFile)) AND (Inserted == 0) 9 Read GivenFile, Student, Score 10 If NewName < Student Then 11 Write TempFile, NewName, NewScore 12 Set Inserted = 1 13 End If 14 Write TempFile, Student, Score 15 End While 16 If Inserted == 0 Then 17 Write TempFile, NewName, NewScore 18 End If 19 While NOT EOF(GivenFile) 20 Read GivenFile, Student, Score 21 Write TempFile, Student, Score 22 End While 23 Close GivenFile, TempFile 24 Copy scratch onto grades

Using Arrays in File Maintenance

Ø  Some=mes it is beber to load a file into arrays in the computer’s internal memory. ◦  Works if the file is small enough to fit into available memory ◦  Also good if there are many changes to be made to the file ◦  Internal memory works fast and can be more efficient than using a scratch file

 General procedure: 1. Open the given file for Input (to be read from) 2. Read file records into parallel arrays, one array for each field 3. Close the file (so that it can later be opened for Output) 4. Make the desired modifica=ons to the arrays 5. Open the file for Output (which erases all the original data in this file) 6. Write the contents of the arrays (the modified data) to the given file 7. Close this file.



Pseudocode for Using Arrays for File Maintenance

1 Declare Student[100] As String 2 Declare Test1[100] As Integer 3 Declare Test2[100] As Integer 4 Declare Count As Integer 5 Open “grades” For Input As DataFile 6 Set Count = 0 7 While NOT EOF(DataFile) 8 Read DataFile, Student[Count], Test1[Count] 9 Set Count = Count + 1 10 End While 11 Close DataFile 12 Open “grades” For Output As DataFile 13 For (K = 0; K < Count; K++) 14 Write “Enter Test 2 score for “ + Student[K] 15 Input Test2[K] 16 Write DataFile, Student[K], Test1[K], Test2[K] 17 End For 18 Close DataFile

10.3 Merging Sequen/al Files

Ø  Open the two given files, File1 and File2, for Input. Ø  Open the file that will hold the merged records, File3 for Output

Ø  Successively Read records from File1 and File2

Ø  If the current record for File1 precedes that of File2, then write the File1 record to File3; otherwise, write the File2 record to File3

Ø  Close the three files


General Pseudocode for Merging Files

Read the first record from each file While (NOT EOF(File1)) AND (NOT EOF(File2))

Compare the current records for File1 and File2 If the File1 record precedes the File2 record Then Write the File1 record to File3 Read another record from File1 Else Write the File2 record to File3 Read another record from File2 End If

End While Read the remaining records, if any, in File1 and write them to File3 Read the remaining records, if any, in File2 and write them to File3


Example: The Big Merger

A company wants to merge two payroll files (payroll1 and payroll2) into a single file. Suppose that each record in these files has the following form: Employee_number (Integer) employee_name (String) rate_of_pay (Float)

We will assume that the records are ordered (in increasing order) by employee number and that the last record in each file is 0, “0”, 0.0. We will merge these two files into a new file called payroll. The variables are declared below and the rest of the program is on the next slides

1 Declare Number1, Number2 As Integer 2 Declare Name1, Name2 As String 3 Declare Rate1, Rate2 As Float

Con=nued on next slide ò



The Big Merger (con/nued) 4 Open “payroll1” For Input As File1 5 Open “payroll2” For Input As File2 6 Open “payroll” For Output As File3 7 Read File1, Number1, Name1, Rate1 8 Read File2, Number2, Name2, Rate2 9 While (Number1 != 0) AND (Number2 != 0) 10 If Number1 < Number2 Then 11 Write File3, Number1, Name1, Rate1 12 Read File1, Number1, Name1, Rate1 13 Else 14 Write File3, Number2, Name2, Rate2 15 Read File2, Number2, Name2, Rate2 16 End If 17 End While 18 While Number1 != 0 19 Write File3, Number1, Name1, Rate1 20 Read File1, Number1, Name1, Rate1 21 End While 22 While Number2 != 0 23 Write File3, Number2, Name2, Rate2 24 Read File2, Number2, Name2, Rate2 25 End While 26 Write File3, 0, “0”, 0.0 27 Close File1, File2, File3 Con=nued on next slide ò

The Big Merger (con/nued)

To understand this program beber, walk through the pseudocode using files with the indicated records: payroll1: payroll2: 115, “Art”, 11.50 120, “Dan”, 14.00

130, “Ben”, 12.25 125, “Eva”, 15.50 135, “Cal”, 13.75 0, “0”, 0.0

0, “0”, 0.0

Then, aTer the merge, payroll should contain the following records: 115, “Art”, 11.50 120, “Dan”, 14.00 125, “Eva”, 15.50

130, “Ben”, 12.25 135, “Cal”, 13.75

0, “0”, 0.0


10.4 Control Break Processing

  When we need subtotals (perhaps for a report), we use a technique called Control Break Processing.

 This technique will do something (for example, calculate a subtotal) depending on the value of a control variable. When a change occurs in the value of this variable, a break occurs that results in something happening in the program:

 For example: ◦  A program accepts monthly sales amounts from the user and computes monthly subtotals ◦  When month changes from January to February, January’s sales are subtotaled and printed.

◦  Then the program starts adding up February’s sales.

◦  The cycle repeats when February changes to March and so on…


PreludeProgramming6ed_pp10.pdf

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