CSC 120 Lecture11 file

CSC-120

Programming

Fundamentals

Using Input/Output Files

 A computer file

• _{is stored on a secondary storage device (e.g., disk);}

• _{is permanent;}

• _{can be used to}

• _{provide input data to a program}

• or receive output data from a program

Streams

File Streams

General File I/O Steps

1. Include the header file fstream in the program.

2. Declare file stream variables.

3. Associate the file stream variables with the input/output sources.

4. Open the file

Header file

• _{stream - a sequence of characters}

• _{interactive (iostream)}

cin - input stream associated with keyboard.

cout - output stream associated with display

• _{file (fstream)}

ifstream - defines new input stream (normally associated with a file).

ofstream - defines new output stream (normally associated with a file).

File Streams

• ifstream (for reading from file) ofstream (for writing to file) fstream (read and write)

• Creating file streams

1. First define stream objects

ifstream [stream variable name]; ofstream [stream variable name]; fstream [stream variable name];

Declaring file stream

variables

ifstream

fsIn;

//input

ofstream

fsOut;

// output

Associate the file stream

variables with the I/O

sources

ifstream

fIn(“file1.txt”);

// input

ofstream

fOut(“file2.txt”);

// output

Opening File

• _{Opening a file associates a file stream variable declared in the}

program with a physical file at the source, such as a disk.

• _{In the case of an input file:}

• _{the file must exist before the open statement executes.}

• _{If the file does not exist, the open statement fails and the input}

stream enters the fail state

• _{In the case of an output file:}

• _{An output file does not have to exist before it is opened;}

• _{if the output file does not exist, the computer prepares an}

empty file for output.

• _{If the designated output file already exists, by default, the old}

More Input File-Related Functions

• _{ifstream fsin;}

• _{fsIn.open(const char[] fname)}

• _{connects stream fsIn to the external file fname.}

• _{fsIn.get(char& character)}

• _{extracts next character from the input stream fsIn and}

places it in the character variable character.

• _fsIn.eof()

More Output File-Related

Functions

• _{ofstream fsOut;}

• _{fsOut.open(const char[] fname)}

• _{connects stream fsOut to the external file fname.}

• _{fsOut.put(char character)}

• _{inserts character character to the output stream fsOut.}

• _fsOut.eof()

ifstream Operations

in >> stream extraction operator obj.get ( ) extract one character

Read char by char

Read line by line

Validate the file before

trying to access

First method _{Second method}

By checking the stream

variable;

If ( ! both)

{

cout << “Cannot open file”;

}

By using

bool

is_open()

function.

If ( !both.is_open()) {

cout << “File is not open.”;

Stream

Manipulators

Stream manipulators go directly into the stream.

They are inserted using the stream insertion operator

File Open Mode

Name Description ios::in Open file to read

ios::out Open file to write

ios::app All the data you write, is put at the end of the file. It calls ios::out

ios::ate All the data you write, is put at the end of the file. It does not call ios::out

ios::trunc Deletes all previous content in the file. (empties the file)

ios::nocreate If the file does not exists, opening it with the open() function gets impossible.

ios::noreplace If FileName is a new file, then it gets created fine.If the file exists, trying to open it with the open() function, returns an error.

File Open Mode

#include <fstream> int main(void)

{

ofstream outFile("file1.txt", ios::out); outFile << "That's new!\n";

outFile.close(); Return 0;

}

Example writing to file

#include <iostream>

#include <fstream>

using namespace std; int main()

{

ofstream outFile;

// open an exist file fout.txt

outFile.open("number.txt",ios::app);

if (!outFile.is_open())

{ cout << " problem with opening the file ";}

else {

outFile <<200 <<endl ;

cout << "done writing" <<endl;}

Dealing with Binary files

•

_{Functions for binary file handling}

get():

read a byte and point to the next byte to read

put():

write a byte and point to the next location for write

obj.read(char[],size):

block reading

obj.write(char[],size):

block writing

Dealing with Binary files

•

_{Some useful functions}

obj.seekg():

Go to a specific position when reading

obj.seekp():

Go to a specific position when writing

tellg()

: Retunrs an int type, that shows the current position of the

inside-pointer. This one works only when you read a file.

tellp():

The same as tellg() but used when we write in a file.

String Streams

•

_{A stringstream reads/writes from/to a string rather}

than a file or a keyboard/screen

String Streams

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

{

string str1;

stringstream ss; getline(cin,str1);

ss<< str1 << " number" ; int a;

ss>>a; a++;

cout<<ss.str() ;}

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

{

stringstream ss;

ifstream infile(“file.txt”); string line;

int total=0;