File I/OFile I/O

TopicsTopics

I/O StreamsFile I/OFormatting Text FilesHandling Stream ErrorsFile Pointers

ObjectivesObjectives

After completing this topic, students should be able to:

Write programs that correctly read text data from a file,handling file errors and end of file conditions appropriately.

Write programs that correctly format and write text datato a file.

Correctly use the C++ stream manipulation functionsin a program

Input/Output StreamsInput/Output Streams

an ordered sequence of bytes

source

sink

We put data into an output stream

We take data out of an input stream

Standard I/OStandard I/O

cin – the standard input stream cout – the standard output stream cerr – the standard error stream

These streams are automatically created for you whenyour program executes. To use them you only need to#include <iostream> and the appropriate using directives.

File I/OFile I/O

When a program takes input from a file, we say thatit reads from the file.

When a program puts data into a file, we say that it writes to the file.

To read or write to a file, we create a stream object, and connect it to the file.

Text FilesText Files

Data in a file can either be text or binary.

Everything in a text file appears as readablecharacters.

Text files are also referred to as Formatted orSequential files.

The fstream classesThe fstream classes

We use objects of the ifstream class to read from a file, and objects of the ofstream class to write toa file.

These classes are defined in <fstream>. To use themwe must write

#include <fstream>using std::ifstream;using std::ofstream;

ifstream Functionsifstream Functions>> overloaded stream extraction operatorget (ch) extract one character into chget (str) extract characters into array str, until ‘\n’get (str, MAX) extract up to MAX characters into array strget (str, DELIM) extract characters into array str until

DELIM character is encountered. Leave DELIM character in buffer

get (str, MAX, DELIM)

these ought to look familiar, they are the samefunctions we use on standard In and standard Out.

Declaring StreamsDeclaring Streams

To use a stream object, it must first be declared

ifstream inStream;ofstream outStream;

Stream variablesStream variables

A stream object is like any other variable, butthere are some important exceptions:

You cannot assign a value to a stream variable

If you pass a stream variable as a parameter, youmust pass it by reference.

Connecting a Stream to a FileConnecting a Stream to a File

ifstream inputStream;inputStream.open (“theData.txt”);

Widget 123V89001 12.95theData.txt

inputStream

to program

you can declare and connect to a file at the same time:

ifstream inputStream (“theData.txt”);

PathsPathsifstream inputStream;inputStream.open (“theData.txt”);

if no path is specified, the file is assumedto be in the same directory as the programthat is executing.

When you code a \ in a pathname, you mustwrite \\. Why?

inputStream.open (“c:\\theData.txt”);

Reading From a Text FileReading From a Text FileTo read from a text file, use the streamextraction operator, just as you would to read from the keyboard.

string description;string partNumber;double price;

ifstream inputStream;inputStream.open (“theData.txt”);…inputStream >> description >> partNumber >> price;…

Type Safe I/OType Safe I/O

In the C programming language, input and outputare done with very different libraries than thoseused in C++ (Although you can still use the C I/Olibraries in C++ if you want to).

One of the advantages to using the new C++ librariesis that C++ I/O is typesafe. The << and >> operators areoverloaded to do the correct thing for all of the standarddata types.

These operators can also be overloaded for user defineddata types.

Widget 123V89001 12.95theData.txt

inputStream

Data is read from the file and converted intothe appropriate data type.

double price;inputStream >> price;

in the file, this ischaracter data.

price

convert fromcharacter to double

12.95

…111000011010…

Records and FieldsRecords and FieldsData in a text file is often organized into field and records.

Widget 123V89001 12.95 \nSprocket 456Y79321 21.50 \nZagget 678H0032 32.25 \nSpoolbot 345W2311 3.23 \n……

fields are usually separated by spaces,but may be separated by other delimiters,such as a comma or tab

records are usually separated bynewline characters, but may beseparated by other delimiters, suchas semicolons.

it is important to know how data in a fileis organized, or it cannot be read.

Keep in mind when the stream extraction operator stops reading!

Writing to a Text FileWriting to a Text File

Use the stream insertion operator to write to a text file,just as if you were writing to the display.

ofstream outputStream;outputStream.open (“theData.txt”);

…

outputStream << price;

Opening an Output fileOpening an Output file

If the named file does not exist, the file is created.

If the named file already exists, it is opened, andthe contents of the file are discarded, by default.

Formatting the OutputFormatting the Output

Most of the time, when we write data to a file, it iswith the idea in mind that the data will be read infrom this or some other program.

It is up to the programmer to format the data in theoutput file, so that it can later be read in a meaningfulway.

ExampleExample

int a = 5;int b = 15;int c = 239;

ofstream myOutputStream (“mydata.txt”);myOutputStream << a << b << c;

515239

What happens when you try to read this file?

int a = 5;int b = 15;int c = 239;

ofstream myOutputStream (“mydata.txt”);myOutputStream << a << “ “ << b << “ “ << c;

add white space toseparate the fields!

Closing the FileClosing the File

It is good programming practice to close a fileas soon as you are done using it.

myOutputStream.close ( );

Although a file is closed automatically when aprogram ends normally, it is not closed if an erroroccurs and the program terminates abnormally.If the file is not closed, all of the data written tothe file is lost!

Mode BitsMode Bits

When a file is opened, you can supply bits that furtherdefine the file. These mode bits are defined in the iosclass. The ios class contains a number of importantconstants we use in file I/O.

ios::in open the file for reading, the default for ifstreamios::out open the file for writing, the default for ofstreamios::app open the file for appending. All data is written at the end of the fileios::trunc open the file and discard contents, the default for ofstream

ios::binary open the file for binary content, Note there is no ios::text which is the default

ExampleExample

#include <fstream>using std::ofstream;using std::ios;

ofstream myOutputStream (“TheData.txt”, ios::app);

…

Stream StatesStream States

Objects of all of the stream classes have a state thatexpresses the condition of the stream..

The stream classes provide functions to query thestream’s state.

good( ) Everything’s fineeof( ) An input operation tried to read beyond the end of the filefail( ) An input operation failed to read the expected character, or an output operation failed to generate the desired charactersbad( ) Indicates the loss of integrity of the underlying input or output sequence

Checking that a File OpenedChecking that a File Opened

In C++, errors are not reported unless the programmerexplicitly asks. For example, your program could callfor a file to be opened and then read data from the file.If the file does not exist, no error is reported and youthink that everything worked fine!

ifstream myInputStream;myInputStream.open (“someData.txt”);

if (myInputStream.fail( )){ cout << “Could not open file”;}else{ …

check the state of thestream here…

We could also write

if (!myInputStream) {

Because I/O is subject to so many errors,it is good programming practice to check

the state of the stream after every I/O operation!

Checking for End of FileChecking for End of Fileint theData;istream myFile;

myFile.open (“someData.txt”);

while (!myFile.eof( )){ myFile >> theData; …}

when attempting to readpast the end of the file, the eof( ) function returns true.But note that the condition isnot checked until you go backto the while statement!

Could also have written while (myFile >> theData){ …}

File Names as InputFile Names as Input

char fileName[16];ifstream myData;cout << “Enter in a file name: “;cin >> fileName;myData.open (fileName);…

the filename must be storedin an array of characters. Wewill talk about char arraysin a later section…

PrecisionPrecision

Precision refers to the number of digits afterthe decimal point.

Every output stream has a member functionnamed precision, that sets the precision ofthe data that goes into the file.

outPutStream.precision ( 3 );

Precision stays as set until changed by anothercall to the precision function.

Formatting FlagsFormatting FlagsFormatting flags are defined in the ios class.

Stream classes have a member function,setf( flag ) that sets these formatting flags.

left left align output not setright right align output setdec output as decimal sethex output as hexadecimal not setoct output as octal (base 8) not setshowpoint show decimal point on output not setscientific output in exponential format not setfixed output in fixed format (not scientific) not set

You can combine flags with the | operator

myStream.setf (ios::fixed | ios::showpoint);

A flag remains set until it is unset.

myStream.unsetf (ios::fixed);

Stream ManipulatorsStream Manipulators

Stream manipulators go directly into the stream.They are inserted using the stream insertion operator

dechexoctendlsetw (w)setprecision (n);setiosflags (flags);setfill (fillChar);

myStream << setw (5) << theData;

Side EffectsSide Effects

You don’t want a function to have an unwanted side effect.One example would be setting I/O flags in a function andleaving them that way when you exit the function.

The function flags returns a long data type that contains the settings of all of the I/O flags.

long myFlags = myStream.flags ( );…

myStream.flags (myFlags);

without an argument the function returns the flags.

with an argument, the flags are restored to those set in the parameter.

File PointersFile Pointers

Streams have pointers associated with them get pointer points to the place next character will be read from put pointer points to where the next character will be written

tellg( ) returns current get pointertellp ( ) returns current put pointer

seekg ( ) positions get pointerseekp ( ) positions put pointer

seekp (15); move 15 bytes from start of file

seekp (-10, ios::end);move -10 bytes from end of file

seekg (6, ios::cur); move 6 bytes from current position

Updating Sequential FilesUpdating Sequential Files

When updating sequential files, you cannot dependon the size of a record always being the same. So, you must locate the position where you want to startwriting, and then rewrite the remainder of the file.

Binary I/OBinary I/O

Data is written to the output device exactly as itIs stored in memory.

Binary I/O is done with the functions * read( ) * write( )

The parameters are the address of the data bufferand the number of bytes to read or write.

The address must be cast to a char*

ExampleExample

// buff is the address of an array of MAX integers…

ofstream os(“myData.dat”, ios::binary);os.write(reinterpret_cast<char*>(buff), MAX * sizeof(int) );os.close( );

reinterpret cast tells the compilerthat you want to do this cast, eventhough it might not make sense tothe compiler.

…

buff

100110… …10100

buff + MAX

#include <iostream>using namespace std;int main( ){ int anInteger; // temp holding place for input do { cout << "\nType an integer ( 0 to quit ):"; cin >> anInteger; cout << "You typed " << anInteger;

} while ( anInteger != 0 );

return 0;}

Handling I/O Errors – An ExampleHandling I/O Errors – An Exampleint main( ){ int anInteger; // temp holding place for input do { cout << "\nType an integer ( 0 to quit ):"; if ( cin >> anInteger ) { cout << "You typed " << anInteger; }

else { cout << "\nBad input ... Integer expected!"; cin.clear ( ); cin.sync ( ); } } while ( anInteger != 0 );

return 0;}