Transcript Chapter 6

Chapter 6:
Sequential Data Files
Extended Prelude to Programming
Concepts & Design, 3/e
Stewart Venit and Elizabeth Drake
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
6.1 An Introduction to Files
• A file is a collection of information that has
been assigned a name and stored separately
from the program that created it
• A file may contain programs or data.
• Two types of files:
– Text files
– Binary files
Classification 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:
– Sequential 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
• Similar to a CD or DVD
More on Data Files
• One file may be broken up into groups of
related data, called records
• Records contain data items and each data item
in a record is called a field
• 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: that customer’s name,
address, and phone number.
Creating a Sequential File
• 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
• Write data to the file, or read data from, the file
• Close the file
– Saves the file and puts an end-of-file marker (EOF) after
the last record if the file was created in the program
– Closes the file if the file is an input file
• 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
• 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
Reading a File
• To open a file to be read:
Open “grades“ for Input As GradeFile
• Read the internal filename and the fields/variables:
Read GradeFile, Name, Score
• Read records within a loop:
While NOT EOF(GradeFile)
Read GradeFile, Name, Score
End While
Close GradeFile
6.2 Modifying A Sequential File
• The most common operations on sequential files are:
– deleting an existing record
– changing an existing record
– inserting or adding a new record
• Read the file, one record at a time, rewriting each
record to a temporary or scratch file until reaching the
one to be modified.
• The mode is Input for the original file and Output
for the scratch file.
• If modifying an existing record, make the change and
write the new version to the scratch file.
Modifying a Sequential File
• If deleting an existing record, skip over the
record to be deleted.
• If inserting a new record, read down to the
proper location, then write the new record.
• If the location is to be the end of the file, read to
the end and write the record.
• Close the Input and Output files.
• Copy the scratch file onto the original file.
Example: Modifying a Sequential File
Deleting a record from a file that contains student names and
scores on one test:
Open “grades” for Input As GivenFile
Open “scratch” for Output As TempFile
Write “Enter name to be deleted: “
Input DeleteName
While NOT EOF(GivenFile)
Read GivenFile, Student, Score
If Student <> DeleteName Then
Write TempFile, Student, Score
End If
End While
Close GivenFile, TempFile
Example: Modifying a Sequential File
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 that the scratch file still contains the information but it doesn’t matter. The
next time you open the scratch file to modify another file, the old contents
will be erased.
6.3 Merging Sequential Files
• Open the two given files, File1 and File2, for
Input. Open the file that will hold the merged
records, File3 for Output
• Successively Read records from File1 and File2
• If the current record for File1 precedes that of
File2, then write the File1 record to File3;
otherwise, write the File2 record to File3
• Close the three files
Problem Solving
• When we need subtotals 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…
• Identify the input and output variables
• Study the report that must be produced to discern the
calculations that must be done, and where breaks must be set
– In the problem given in the text, breaks will occur when the store number
Divide the tasks into modules
Create the hierarchy chart
Code the modules
Test the modules
Pseudocode Language (Ch 6)
File I/O:
Open “file” For Input/Output As Name
Close Name
Read Name, Variable
Write Name, Variable