7. Stream I/O Input and Outputlnx01.ee.polyu.edu.hk/~eewlchan/ENG2002/Ch7.pdf · Input in TWO lines...

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Input and Output • C++ does not, as a part of the language, define

how data are sent out and read into the program

• The input and output (I/O) are handled by the standard C++ library (such as iostream)

• A library is a collection of object code to be linked to the C++ program to provide additional functionality

• For different platforms, different libraries (that may use the same name) can be used for I/O functions

• This allows the C++ program to be less platform dependent

• It is because I/O is usually quite different from computers to computers.

Computer Programming7. Stream I/O

I/O implementation is hardware dependent

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Standard I/O Objects • When a C++ program that includes the iostream

classes starts, four objects are created and initialized• cin - handle input from the standard input, i.e.

keyboard

• cout - handle output to the standard output, i.e. display

• cerr - handle unbuffered output to the standard error device, i.e. display in a PC

• clog - handle buffered error messages that are output to the standard error device, i.e. display in a PC.


Buffered: Display only when a flush command is received, e.g. endl statement

Keyboard and Screen I/O

#include <iostream>

cin

(of type istream)

cout

(of type ostream)

Keyboard Screenexecutingprogram

input data output data


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Buffering • iostream classes view the flow of data as being

a stream of data, one byte following another

• Buffer is provided in some cases to the stream • The stream of data does not send out if the buffer is

not full or no flush request is received

• It is useful in some situations such as written data to disk since the overhead is very large. It is better to do it in single lots rather than byte by byte.

BufferData Stream

Hard disk


Temporary Memory I/O

Output: Buffer to output deviceInput: Buffer to computer program

Program

>> is a binary operator>> is called the input or extraction operator

>> is left associative

EXPRESSION RETURNS VALUE

cin >> age cin

STATEMENT

cin >> age >> weight ;


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Buffer:Memory in your keyboard

char xyz[10];cin >> xyz;

Chan

Chan xyz = "Chan"

Program

Keyboard

a b c

a b c

a b c

a b c

957 34

957 34 128

957 34

NOTE: shows the location of the reading marker STATEMENTS CONTENTS MARKER

POSITION

int a ; 957 34 1235\nint b ; 128 96\nint c ;cin >> a >> b ; 957 34 1235\n

128 96\n

cin.ignore(100, ‘\n’) ; 957 34 1235\n128 96\n

cin >> c ; 957 34 1235\n128 96\n


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Input using cin• cin has been generally used for input data from

keyboard

• It is a global object - doesn’t need to define in our own code

• The operator >> is overloaded such that different kind of data can be read into the buffer of cin and then to someVariable

int someVariable;cin >> someVariable;

float someVariable1;cin >> someVariable1;

double someVariable2;cin >> someVariable2;

char someVariable3[100];cin >> someVariable3;


Member functions of cinInput Stream Methods


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Member functions, e.g. cin.get()• As cin is an object, it has its own member

functions• Helps to obtain the input data in a more flexible way.

#include <iostream>using namespace std;int main(){

char ch;while ((ch = cin.get()) != EOF){

cout << "ch: " << ch << endl;}cout << "\nDone!\n";return 0;

}

cin.get() returns a character from standard input

cin.get() returns a character from standard input

The loop will stop if end-of-file (crtl-z) is input

The loop will stop if end-of-file (crtl-z) is input


EOF: 0xFF is a conditionEOF: 0xFF is a condition

Member functions of cinStatus methods:


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This is the "Enter" key (new-line character)

If we press crtl-z , it becomes End-of-file (EOF) condition.

//End‐of‐file at keyboard

total = 0;

cout << “Enter blood pressure (Ctrl‐Z to stop)”;

cin >> thisBP; // priming read

while (cin) // while last read successful; or while (!cin.eof())

{

total = total + thisBP;

cout << “Enter blood pressure”;

cin >> thisBP; // read another

}

cout << total;

Ctrl-D in UNIX


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Member functions, e.g. cin.getline()• cin.getline() allows the whole line of data to be

input to a string

cin.getline(buffer, MAXSIZE);

• The maximum length of data to be read is defined by MAXSIZE

• Unlike cin, getline() will read in the terminating newline character and throw it away

• With cin, the terminating newline is not thrown away, but left in the input buffer.


Include NULL, i.e. '\0'at the end

cin will neglect ,though, any preceding space, tab and newline

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• cin.getline() is different in that getline() can only read in string, but not other data types; however, it will not stop when encountering spaces or tabs.

Why do we use cin.getline()?• cin >> abc has been used quite often since

the redirection operator >> is overloaded, i.e.• such statement can be used no matter abc is a

string, an integer or a floating-point number

• However if abc is a string, cin >> abc allows only string with continuous characters input• cin >> abc will stop to read in data if it sees, e.g. a

space

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#include <iostream>using namespace std;int main(){ char stringOne[256];

char stringTwo[256];char stringThree[256];cout << "Enter string one: ";cin.getline(stringOne,256);cout << "stringOne: " << stringOne << endl;

cout << "Enter string two: ";cin >> stringTwo;cout << "stringTwo: "

<< stringTwo << endl;

cout << "Enter string three: ";cin.getline(stringThree,256);cout << "stringThree: " << stringThree << endl;return 0;

}

The >> operator reads until a new-line or a space character is seen. That’s why five six are read to stringThree[]

The >> operator reads until a new-line or a space character is seen. That’s why five six are read to stringThree[]


Notice the space

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Buffer : Memory in your keyboard

char abc[10], xyz[10];cin >> abc; cout << "abc: " << abc << endl;

cin.getline(xyz,10);cout << "xyz: " << xyz << endl;

Chan

Chan abc = "Chan\0"

ProgramKeyboard

xyz ='\0'and then buffer clear

The "Enter" code is still in the buffer

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Buffer : Memory in your keyboard

char abc[10], xyz[10];cin.getline(abc,10);cout << "abc: " << abc << endl;

cin.getline(xyz,10);cout << "xyz: " << xyz << endl;

Chan

Chan abc = "Chan\0"

ProgramKeyboard

The "Enter" key has been cleared

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Output with cout• Just as cin, cout is also an object created when

the iostream class starts• cout represents the standard output, i.e. display

• The operator << is overloaded such that different kind of data can be sent out

• Similar to cin, cout also has a number of member functionscout.put(char a)

• Put the value of character a to output, and return cout

cout.write(char *buf, int length)

• Write length characters in buf to output device , andreturn cout.


You may write cout.put(a)<<endl;

Not including NULL

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#include <iostream>#include <string>using namespace std;int main(){

char One[] = "One if by land";int fullLength = strlen(One);int tooShort = fullLength - 4;int tooLong = fullLength + 6;

cout.write(One,fullLength) << "\n";// fullLength=14,return cout after writecout.write(One,tooShort) << "\n";cout.write(One,tooLong) << "\n";return 0;

}

write() asks the system to write tooLong characters. Hence something unexpected is displayed

write() asks the system to write tooLong characters. Hence something unexpected is displayed


cout.width(int a)

• Set the width of the next output field to a's value

cout.fill(char a)

• Fill the empty field of the output by a's value

• return cout.

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Other Member Functions

#include <iostream>using namespace std;int main(){ cout << "Start >";

cout.width(6);cout << 123 << "<End\n";

cout << "Start >";cout.width(6);cout.fill('*');cout << 123 << "<End\n";return 0;

}

width(6) defines the total width of the field to be 6 char.

fill('*') fills the empty field with *

width(6) defines the total width of the field to be 6 char.

fill('*') fills the empty field with *


Formatting numbers for program outputostream Class Methods


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Administrator

Typewritten Text

XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

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Exercise 7.1Write a program that asks the user to

1. Input his/her surname

2. Input his/her first name

If the user puts a space between the two words of his first name, add a hyphen to replace the space. Skip all other spaces in front of or after the first name or surname.

For example: Chan

Tai Ming ⇒ Chan Tai-Ming

Show the whole name after you read the user inputs.

Hint: A string is a character array. You can use a loop to check the content of every character of a string one by one, and to display the characters out one by one.


Input in TWO lines

First name can be 1 word or 2 words

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File Input and Output• C++ provides library functions to help us deal with

file accesses

• File access is usually performed in the following way:Open fileOpen file

Close fileClose file

• To claim from the system the access of a particular file

• To indicate to the system the file access is finished (closed)

It is not guaranteed that data can be written to file if not closing.

• To perform the read or write instructions


Read data from file

Read data from file

Write data to file

Write data to file

Text file

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• Streams provide a uniform way of dealing with data sending to or reading from files• The objects that are used to deal with files are called

ofstream and ifstream objects

• To create an ofstream or ifstream object, we need to include fstream (header file) in our program

• To open a file for write or read, first create an ofstream or ifstream object with the filename as the input parameter.

File I/O Using Streams

ofstream fout("myfile.cpp");ifstream fin("myfile.cpp");


Classes

fout and fin are objects but NOT standard objects, unlike cout and cin. Any other names are O.K.

Unlike cin, we can have more than one file being processed in a program.

Diskette Files for I/O

your variable

(of type ifstream)

your variable

(of type ofstream)

disk file“C:\myInfile.dat”

disk file“C:\myOut.dat”

executingprogram

input data output data

#include <fstream>


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Statements for using Disk I/O

#include <fstream>

ifstream myInfile; // declarations

ofstream myOutfile;

myInfile.open(“C:\\myIn.dat”); // open files

myOutfile.open(“C:\\myOut.dat”);

myInfile.close( ); // close files

myOutfile.close( );


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Opening a file

associates the C++ identifier for your file with the physical (disk) name for the file

if the input file does not exist on disk, open is not successful

if the output file does not exist on disk, a new file with that name is created

if the output file already exists, it is erased

places a file reading marker at the very beginning of the file, pointing to the first character in it


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• After opening files, reading or writing files can be done in a similar way as cout or cin

• After writing/reading, the file should be closed by the member function close() of the ofstream/ifstreamobjects.

Read or Write Files

#include <iostream>using namespace std;#include <fstream>int main() {ofstream fout("myfile.cpp");fout << "This line is written to file.\n";fout.close();ifstream fin("myfile.cpp");char ch;while (fin.get(ch)) //NULL if end of file

cout << ch;fin.close(); return 0;}


If for any reason myfile.cpp cannot be opened, fout & fin will still be created, but fout.fail() return true.

If for any reason myfile.cpp cannot be opened, fout & fin will still be created, but fout.fail() return true.

To see the output, look at the content of the file myfile.cpp

Similar to cout and cin.get(ch).Similar to cout and cin.get(ch).

Reading Character-Based Files

Use an ifstream object to read data from a text file

Syntax:

inputFileStream >> variableName

The eof()method can be used to test if the end of file has been reached

Example:

//while not at end of file

while(!inFile.eof())


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Reading 20 Books from file

“C:\\book.dat”

3.98 P <eoln>7.41 H <eoln>8.79 P <eoln>

.

.

.

Price of bookHardback orPaperback?

WRITE A PROGRAM TO FIND TOTAL VALUE OF ALL BOOKS


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#include <iostream> // for cout#include <fstream> // for file I/Ousing namespace std;

int main (){

float price, total = 0.0 ; // declarationschar kind ;int count = 1;ifstream myInfile (“C:\\book.dat”); // myInfile.open(“C:\\book.dat”) ;

while ( count < 21 ) {

myInfile >> price >> kind ;total = total + price ; count ++ ;

}cout << “Total is: “ << total << endl ;myInfile.close( ) ;return 0 ;

}


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// Program counts frequency of each alphabetic character in a text file.#include < fstream >#include < iostream >using namespace std;

int main ( ) {ifstream dataFile ;int freqCount [26]={0} ; // zero out the arraychar ch, index;

dataFile.open ( “C:\\my.dat” ) ; // open and verify successif ( ! dataFile ) {

cout << “ CAN’T OPEN INPUT FILE ! “ << endl;return 1;

}

dataFile.get ( ch ) ; // read file one character at a timewhile ( dataFile ) { // while last read was successful

if (ch >= ‘A’ && ch <= ‘Z’ ) ) ++freqCount [ ch ‐ ’A’ ];dataFile. get ( ch ) ; // get next character

}

return 0;}

Counting Frequency of Alphabetic Characters

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Changing Default Behavior• Default behavior of opening a file (by ofstream) is

to create it if it doesn’t yet exist• If exists, it deletes all its contents and overwrite on it

(truncate)

• This default behavior can be changed by providing a second parameter to the constructor

ios::app – File will be opened for appending data to the end of the existing file

ios::ate – Place you at the end of the file (either input or output), but write data at the current position, like trunc

ios::trunc – Default for ofstream; truncates if it exists

ios::in – The file (if it exists) will NOT be truncated, the read/write cursor is positioned at the start of the file.


ofstream fout2("myfile.cpp",ios::app);

Open modes of class ios

ios::ate|ios::in is similar to ios::app

Still truncate for output

Over-write

No effect to input

Writing to Files

General form

object_name << variable_name;

Use ofstream object to write to file like coutwas used

ofstream outfile(“C:\\salary.out”, ios::app);

outfile << “Salary for week was ” << money;

Additional writing appended to file


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Appending to the End of File#include <fstream>#include <iostream>using namespace std;int main(){ofstream fout("myfile.cpp"); //Default truncfout << "This line is written to file.\n";fout.close();char fname[]="myfile.cpp";ofstream fout2(fname,ios::app);fout2 << "This line is appended to the end of the previous line\n";fout2.close();ifstream fin("myfile.cpp");char ch;while (fin.get(ch))cout << ch;

fin.close();return 0;}


The default behavior is changedThe default behavior is changed

To avoid redefining variable, a different name should be provided even for opening the same file

To avoid redefining variable, a different name should be provided even for opening the same file

File name can be supplied by a variableFile name can be supplied by a variable

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M//Assume myfile.cpp contains "This line is written to file.\n"ofstream fout2("myfile.cpp", ios::in);fout2 << "Beginning of file";fout2.close();

ifstream fin("myfile.cpp"); if (!fin) {cout << "File opening error!\n"; return 0;} //The operator ! is defined for stream object, if the stream is bad, return false (NULL)char ch;while (fin.get(ch))

cout << ch;

fin.close();return 0;M

Default behavior is changed to non-truncate. The line will replace the beginning part of the text in the file; but the rest words, if any, will be left in the file.

Default behavior is changed to non-truncate. The line will replace the beginning part of the text in the file; but the rest words, if any, will be left in the file.

It is a good practice to check if anything goes wrong when opening a file

It is a good practice to check if anything goes wrong when opening a file

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Editing File• It is often that we need to modify a particular

part of a file - editing a file• ofstream and ifstream offers 4 member functions

that set the current position of an opened file

seekp(long pos);tellp();

For ofstream

seekg(long pos);tellg();

For ifstream

• seekp sets the current position. pos is a byte offset from the file beginning

• tellp tells the current position. Return a byte offset from the file beginning

• seekp sets the current position. pos is a byte offset from the file beginning

• tellp tells the current position. Return a byte offset from the file beginning

• seekg and tellg are similar to seekpand tellp except they are for ifstream

• seekg and tellg are similar to seekpand tellp except they are for ifstream


The first position is position 0.

seekp has NO effect to ios::app

#include <fstream>#include <iostream>using namespace std;int main(){ ofstream test1file("test1");

cout << "Text written: \n";cout << "This text is written to file!\n"; //write to screentest1file << "This text is written to file!\n";test1file.close();ifstream tin("test1");tin.seekg(10);cout << "\nCurrent position: " << tin.tellg() << "\n";cout << "\nContent of the file after an offset of 10:\n";char ch;while (tin.get(ch)){ cout << ch;} //display filetin.close();return 0;

}

Change the current position to 10

Change the current position to 10

Tell the current positionTell the current position


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Two types of file access:

Sequential access: characters are stored and retrieved in a sequential manner

Random access: characters can be read or written directly at any position

For random access, the ifstream object creates a long integer file position marker representing the offset from the beginning of the file

File Position Marker Functions


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Random File Accessseekg()/seekp() functions allow you to move to any position in the file

Character position in a data file is zero‐relative

Arguments to seekg()/seekp() functions are the offset into the file and the mode (where the offset is to be calculated from)

Mode alternatives:

ios::beg: start from beginning of file

ios::cur: start from current position

ios::end: start from end of file


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Exercise 7.21. Build the program in the last page

2. Run the program in Command Prompt

3. Use the DOS command "dir" to find the file "test1" and display it by using the DOS command "type test1". Can you find the statement as shown in the program?

4. Use seekp such that the word "written" in the file is replaced by "*******".

5. Verify the result by using the DOS commands "dir" and "type test1" again.


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7. Stream I/O Input and Outputlnx01.ee.polyu.edu.hk/~eewlchan/ENG2002/Ch7.pdf · Input in TWO lines...

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