csc1100 lecture15 ch09

A First Book of C++A First Book of C++Chapter 9Chapter 9

I/O Streams and Data FilesI/O Streams and Data Files

In this chapter, you will learn about: I/O File Stream Objects and Methods Reading and Writing Text Files Random File Access File Streams as Function Arguments Common Programming Errors The iostream Class Library

A First Book of C++ 4th Edition 2

Objectives

To store and retrieve data outside a C++ program, you need two things: A file A file stream object


I/O File Stream Objects and Methods

File: collection of data stored together under common name, usually on disk, USB drive, or CD/DVD C++ programs stored on disk are examples of files Stored data in program file is the code that

becomes input data to C++ compiler A C++ program is not usually considered data

file Data file typically refers only to files containing

the data used in C++ programA First Book of C++ 4th Edition 4

Files

External name: unique filename for file External name is how operating system knows

file Contents of directory or folder are listed by

external names Each computer operating system has its own

specifications for external filename size Table 9.1 lists specifications for more commonly

used operating systemsA First Book of C++ 4th Edition 5

Files (cont'd.)


Files (cont'd.)

Use descriptive names Avoid long filenames

They take more time to type and can result in typing errors

Manageable length for filename is 12 to 14 characters, with maximum of 25 characters

Choose filenames that indicate type of data in file and application for which it is used Frequently, first eight characters describe data,

and an extension describes applicationA First Book of C++ 4th Edition 7

Files (cont'd.)

Using DOS convention, the following are all valid computer data filenames:

prices.dat records info.txtexper1.dat scores.dat math.mem


Files (cont'd.)

Two basic types of files: both store data using binary code Text (character-based) files: store each

character using individual character code (typically ASCII or Unicode) Advantage: allows files to be displayed by word-

processing program or text editor Binary-based files: store numbers in binary form

and strings in ASCII or Unicode form Advantage: provides compactness


Files (cont'd.)

File stream: one-way transmission path used to connect a file to a program

Mode (of file stream): determines whether path will move data from file into program or from program to file Input file stream: used to transfer data from

a file to a program Output file stream: sends data from a

program to a fileA First Book of C++ 4th Edition 10

File Stream Objects

Direction (mode) of file stream is defined in relation to program and not file: Data that goes into program are considered

input data Data sent out from program are considered

output data Figure 9.1 illustrates data flow from and to

file using input and output file streams


File Stream Objects (cont'd.)

Distinct file stream object must be created for each file used, regardless of file’s type

For program to both read and write to file, both an input and output file stream object are required Input file stream objects are declared to be of

type ifstream Output file streams are declared to be of type ofstream



Each file stream object has access to methods defined for its respective ifstream or ofstream class, including: Opening file: connecting stream object name to external

filename Determining whether a successful connection has been

made Closing file: closing connection Getting next data item into program from input stream Putting new data item from program onto output stream Detecting when end of file has been reached


File Stream Methods

open() method: Establishes physical connecting link between

program and file Operating system function that is transparent to

programmer Connects file’s external computer name to stream

object name used internally by program Provided by the ifstream and ofstream classes

File opened for input is said to be in read mode


File Stream Methods (cont'd.)

Example: inFile.open("prices.dat"); Connects external text file named prices.dat

to internal program file stream object named inFile

Accesses file using internal object name inFile Computer saves file under the external name prices.dat

Calling the open() method uses the standard object notation: objectName.open()



user-defined variable

fail() method: returns true value if file is unsuccessfully opened, false if open succeeded Good programming practice is to check that

connection is established before using file In addition to fail() method, C++ provides

three other methods, listed in Table 9.2, that can be used to detect file’s status

Program 9.1 illustrates statements required to open file for input, including error-checking routine to ensure that successful open was obtained



Example of use of fail() method:

//ifstream = ”input file” stream ifstream inFile; // any object name can be used here inFile.open("prices.dat"); // open the file // check that the connection was opened successfully if (inFile.fail()) { cout << "\nThe file was not successfully opened"

<< "\n Please check that the file currently exists." << endl;

exit(1); }



Different checking required for output files If file exists having same name as file to be

opened in output mode, existing file is erased and all data lost

To avoid this situation, file is first opened in input mode to see if it exists If it does, user is given choice of explicitly

permitting it to be overwritten (when it is later opened in output mode)

Code used to accomplish this is highlighted in Program 9.2 (refer textbook)



Embedding a filename in program causes problems No provision for user to enter desired filename

during program execution Any changes require modification of open()

method and recompile These problems can be solved by assigning

filename to string variable, as shown in Programs 9.3a and 9.3b




// ask user to enter filename

// declare inFile of type ifstream (input stream)

close() method: breaks connection between file’s external name and file stream object Object can then be used for another file Good programming practice is to close files no

longer needed Operating system automatically closes any

open files at end of normal program execution Example: inFile.close(); closes inFile

stream’s connection to its current file close() method takes no argument



Operations similar to reading input from keyboard and writing data to display screen For writing to file, the cout object is replaced by ofstream object name declared in program

Example: if outFile is declared as object of type ofstream, the following output statement is valid:

outFile << descrip << ' ' << price; The filename directs output stream to file instead of

standard display device Example: Program 9.4


Reading and Writing Text Files


Program 9.4


Program 9.4(cont…)

Program 9.4 output: File named prices.dat is created and saved

by computer as text file (the default file type) prices.dat is sequential file consisting of the

following data:Mats 39.95Bulbs 3.22Fuses 1.08

Actual storage of characters in file depends on character codes used by computer

Output file contains 36 characters (Figure 9.2)


Reading and Writing Text Files (cont'd.)


Reading and Writing Text Files (cont'd.)

Almost identical to reading data from standard keyboard cin object replaced by ifstream object declared

in program Example: the input statement:

inFile >> descrip >> price;

reads next two items in file and stores them in variables descrip and price File stream name directs input to come from file

stream rather than the keyboardA First Book of C++ 4th Edition 30

Reading from a Text File

Program 9.5 illustrates how the prices.dat file created in Program 9.4 can be read Also illustrates method of detecting end-of-file

(EOF) marker using good() function (see Table 9.2)

Other methods that can be used for stream input are listed in Table 9.3 Each method must be preceded by stream

object nameA First Book of C++ 4th Edition 31

Reading from a Text File (cont'd.)


Program 9.5#include <iostream> #include <fstream> // file stream#include <cstdlib> // needed for exit#include <string>using namespace std;

int main(){

string filename = "prices.dat"; // put the filename up front string descrip; double price; ifstream inFile; inFile.open(filename.c_str());

if(inFile.fail()) { // check for successful opencout << "The file is not successfully opened"

<< "\n Please check that the file currently exists." << endl; exit(1);}


Program 9.5 (cont…)

//read and display file’s contents inFile >> descrip >> price;

while (inFile.good()) // check next character { cout << descrip << ' ' << price << endl; inFile >> descrip >> price; }

inFile.close();

return 0;}


Reading from a Text File (cont'd.)

C++ supports logical and physical file objects Logical file object: stream that connects file of

logically related data (data file) to a program Physical file object: stream that connects to

hardware device such as keyboard or printer Standard input file: physical device

assigned to program for data entry Standard output file: physical device on

which output is automatically displayed


Standard Device Files

The keyboard, display, error, and log streams are automatically connected to the stream objects named cin, cout, cerr, clog Requires iostream header file

Other devices can be used if the name assigned by system is known Example: most personal computers assign name prn to printer connected to computer

Statement outFile.open("prn") connects printer to ofstream object named outFile


Other Devices

File access: retrieving data from file File organization: the way data is stored in a

file Sequential organization: characters in file

are stored in sequential manner, one after another

Random access: any character in an open file can be read directly without having to read characters ahead of it


Random File Access

File position marker: long integer that represents an offset from the beginning of each file Keeps track of where next character is to be read

from or written to Allows for random access of any individual character

Table 9.4 shows functions used to access and change the file position marker

Program 9.7 illustrates use of seekg() and tellg() to read and display file in reverse order (refer textbook, Pg 418)


Random File Access (cont'd.)


Random File Access (cont'd.)

A file stream object can be used as function argument Function’s formal parameter must be a reference

(see Section 6.3) to correct stream, either as ifstream& or ofstream&

Example: Program 9.8 ofstream object named outfile is opened in main()

Stream object is passed to the function inOut() inOut() is used to write five lines of user-entered

text to fileA First Book of C++ 4th Edition 41

File Streams as Function Arguments


Program 9.8


Program 9.8 (cont…)

Forgetting to open a file before attempting to read from it or write to it

Using file’s external name in place of internal file stream object name when accessing file

Opening file for output without first checking that file with given name already exists Not checking for preexisting file ensures that file will

be overwritten Not understanding that end of a file is detected only

after EOF sentinel has either been read or passed over


Common Programming Errors

Attempting to detect end of file using character variables for EOF marker Any variable used to accept EOF must be

declared as an integer variable Using integer argument with the seekg() and seekp() functions Offset must be a long integer constant or

variable Any other value passed to these functions can

result in an unpredictable resultA First Book of C++ 4th Edition 45

Common Programming Errors (cont'd.)

A data file is any collection of data stored in an external storage medium under a common name

A data file is connected to file stream using fstream’s open() method This function connects file’s external name with

internal object name A file can be opened in input or output mode

An opened output file stream either creates a new data file or erases data in an existing opened file


Summary

All file streams must be declared as objects of either the ifstream or ofstream classes

In addition to any files opened within a function, the standard stream objects cin, cout, and cerr are automatically declared and opened when a program is run


Summary (cont'd.)

Data files can be accessed randomly using the seekg(), seekp(), tellg(), and tellp() methods The “g” versions of these functions are used to alter

and query file position marker for input file streams The “p” versions do the same for output file

streams Table 9.5 lists the methods supplied by the fstream class for file manipulation


Summary (cont'd.)

iostream class library accesses files by using entities called streams

For most systems, the data bytes transferred on a stream represent ASCII characters or binary numbers

Mechanism for reading a byte stream from a file or writing a byte stream to a file is hidden when using a high-level language, such as C++


Chapter Supplement: The iostream Class Library


File Stream Transfer Mechanism


Components of the iostream Class Library

Class strstream is derived from the ios class Uses the strstreambuf class shown in Figure 9.5 Requires the strstream header file Provides capabilities for writing and reading strings

to and from in-memory defined streams In-memory streams are typically used to

“assemble” a string from smaller pieces Until a complete line of characters is ready to be

written to cout or to a file Attaching a strstream object to a buffer is similar

to attaching an fstream object to an output fileA First Book of C++ 4th Edition 52

In-Memory Formatting

csc1100 lecture15 ch09

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