Computer ProgrammingTCP1224

Chapter 13Sequential File Access

Objectives

• File types• Open a sequential access file• Determine whether a file was opened

successfully• Write data to a sequential access file• Read data from a sequential access file• Test for the end of a sequential access file• Close a sequential access file

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File Types

• A program can “read” from or “write” to a file▫Files to which information is written are

output files▫Files that are read by the computer are input

files

• Types of files in C++▫Sequential

Information is accessed in consecutive order

▫Random Can be accessed in consecutive or in random order

▫Binary Information can be accessed by its byte location

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Using Sequential Access Files

•A sequential access file is often called a text file

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Using Sequential Access Files (continued)

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Creating and Opening a Sequential Access File• You must create the input and output file objects

used in a program

▫#include <fstream> For ifstream and ofstream classes (see next slide)

▫using std::ifstream; and using std::ios;

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Creating and Opening a Sequential Access File (continued)

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Creating and Opening a Sequential Access File (continued)

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Default

Conceptually similar to the >> and >

Creating and Opening a Sequential Access File (continued)

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Determining whether a File was Opened Successfully• open() may fail when attempting to open a file

▫E.g., it will not be able to create an output file when the path in fileName does not exist, or when the disk is full

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Determining whether a File was Opened Successfully (continued)

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! is the Not logical operator

Closing a Sequential Access File

• In most programming sequence, after a “handle” is open, it needs to be close. Handle can be file (in this case), network or other devices.

• To prevent the loss of data, close a sequential access file as soon as program finishes using it

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Skeleton Code for File Access#include <iostream>

#include <fstream>

using namespace std;

ifstream inputFile;

ofstream outputFile;

int main()

{

inputFile.open("myinput.txt");

// This is the same as inputFile.open("myinput.txt", ios::in);

if (inputFile.is_open())

{

cout << "Input file successfully open" << endl;

inputFile.close();

}

else

cout << "Input file cannot be opened" << endl;

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Skeleton Code for File Access (Con’t)

outputFile.open("myoutput.txt", ios::out);

if (outputFile.is_open())

{

cout << "Output file successfully open" << endl;

outputFile.close();

}

else

cout << "Output file cannot be opened" << endl;

}

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Writing Information to a Sequential Access File• Field: single item of information about a person,

place, or thing▫E.g., a name, a salary, a SSN, or a price

• Record: a collection of one or more related fields▫Contains data about a specific person, place, or

thing▫The college you are attending keeps student

records Examples of fields include your SSN, name, address,

phone number, credits earned, and grades earned

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Writing Information to a Sequential Access File (continued)

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Writing Information to a Sequential Access File (continued)• To verify if information was written correctly,

open the (sequential access) file in a text editor▫E.g., the text editor in Visual C++ or Notepad

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Reading Information from a Sequential Access File• Use >> to read char and numeric data from a

file• Use getline() to read string data from a

sequential access file▫The default delimiter character is the newline

character (‘\n’)

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Reading Information from a Sequential Access File (continued)

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Testing for the End of a Sequential Access File• We need to know when we have reached the end

of the file.• A file pointer keeps track of the next character

either to read from or write to a file▫When a sequential access file is opened for

input, the file pointer is positioned before the first character

▫As characters are read, the pointer is moved forward

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Testing for the End of a Sequential Access File (continued)

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Summary

• Sequential access files can be input or output files

• To use a text file, program must contain:▫include <fstream> directive▫using std::ios; statement

• Use the ifstream and ofstream classes to create input and output file objects, respectively

• Use is_open() to determine whether a text file was opened successfully

• Use close() to close a file▫Failing to close an open file can result in loss of

data

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Summary (continued)

• Records in a text file are usually written on a separate line in the file▫Use endl

• eof() determines if file pointer is at end of the file

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