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Input/Output Stream
Lesson #10
Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek
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Streams
A stream is a sequence of bytes. Bytes are moved from memory to
devices, and vice versa. The stream I/O facilities convert typed
objects into sequences of characters, and vice versa.
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iostream Library Header Files
<iostream.h>basic stream input and output operations
<iomanip.h>formatted I/O with parametrized stream
manipulators <fstream.h>
file processing <strstream.h>//<strstrea.h> in VC++
formatting with character arrays
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I/O Class Hierarchy
ios
istream ostream
iostream
fstream
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Output of Built-in Typesclass ostream : public virtual ios {...
public:ostream& operator<<(const char*);ostream& operator<<(char);ostream& operator<<(short);ostream& operator<<(int);ostream& operator<<(long);ostream& operator<<(double);...
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Output of User-Defined Types
#include <iostream.h>class rational {
int num, den; public:
rational(int n=0, int d=1) { num=n; den = d; }int get_num() { return num; }int get_den() { return den; }
};
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Output of User-Defined Types
ostream& operator<<(ostream& os, rational r){ os << '(' << r.get_num() <<','
<< r.get_den() <<')';return os;
}void main(){ rational r(3, 10);
cout << "r = " << r << '\n' ;
}//ex10-1.cpp, output: r = (3,10)
Overloading the operator<<!!!
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The put Member Function
#include <iostream.h>void main(){ int n = 26;
float f = 5.0 / 2.0;cout << n <<"; "<<f<<endl;char c ='X' ;cout.put(c);cout.put(c).put('Y').put('\n') ;
}//ex10-2.cpp
26; 2.5XXY
Result:?
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The Input Stream cin
#include <iostream.h>void main(){ int a, b; float r; char ch;
cout <<"input two integers, one float and a character: \n";cin >> a >> b >> r >> ch;cout <<"a= " << a << ", b= " << b << '\n' ;cout <<"r= " << r << ", ch= " << ch << '\n' ;
}//ex10-3.cpp
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Input of Built-in Types
class istream : public virtual ios {// ...
public:istream& operator>>(char *);istream& operator>>(char&);istream& operator>>(short&);istream& operator>>(int&);istream& operator>>(long&);istream& operator>>(float&);...
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The istream::get functions
class istream : public virtual ios {
...
int get();
istream& get(char& c);
istream& get(char* p, int n, char=‘\n’);
...
};
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returns the next input character reads white space returns EOF when end of stream is
reached
get() with no arguments
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get() with no arguments
#include <iostream.h> int main() {char name[80]; char ch = '\0'; int i = 0; cout << "Enter your name: "; while(1) { ch = cin.get(); if( ch == '\n') break; cout.put(ch); name[i++] = ch; } name[i] = '\0'; cout <<'\n'<< name<<'\n'; return 0; }//ex10-4.cpp
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get(char&) with one argument
reads the next input character into the character argument
reads white space returns false when end of stream is
reached returns an istream reference
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Using get(char&) to Copy
#include <iostream.h>void main(){ char ch;
while( cin.get(ch) && (ch != '\n'))cout.put(ch);
}//ex10-5.cpp
It is a test.It is a test.
Result:
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The 3-argument istream::get()
istream & get (char * buf, int len, char delim = ‘\n’) reads at most (len-1) characters stores input into a vector of characters(buf) stores the null character at the end stops reading when delimiter is encountered delimiter will be the next character to read
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#include <iostream.h>main(){ char name[30]; while(1) {cout << “Enter a name:”;
cin.get(name, 30, ‘\n’); //cin.get(name, 30);if (cin.get(ch) && ch == ‘\n’) break;else{ cout << “excess input!”<<endl;
cin.ignore(1000, ‘\n’); } cout <<name<<endl;}}//ex10-6.cpp
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ignoreistream istream::ignore(int n=1, int delim=EOF);
skips the specified number of character the default is 1, or until the delimiter is reached, the default is EOF.
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getline()#include <iostream.h>void main(){ char name[30], ch;
while(1){ cout << "Enter a name:";
cin.getline(name, 30);if (cin.get(ch) && ch == '\n') break;else{ cout << "excess input!"<<endl;
cin.ignore(100, '\n');}
}cout << name<<endl;
}//ex10-6-2.cpp
Enter a name: John Smith
John Smith
Result:
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Error Detection
#include <iostream.h> void main() {int i; float r; long n; cin >> i >> r>>n; cout<<"i = "<<i<<'\n' <<"r = "<<r<<'\n' <<"n = "<<n<<'\n' <<"cin.good()= "<<cin.good()<<'\n'; }//ex10-6-3.cpp
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The io-state bitsclass ios {
//...public:
enum io_state {goodbit = 0,eofbit = 1,failbit = 2,badbit = 4
}; ...};
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The rdstate() Member Function
returns the state of a stream as a set of io-state bits:
int s = cin.rdstate();
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Examining the State if( s & ios::goodbit )
// last operation on cin succeededif( s & ios::badbit )
// characters from cin possibly lost if( s & ios::failbit )
// some formatting error if( s & ios::eofbit )
// reached the end of file
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Input of User-Defined Typesistream& operator>>(istream& s, rational & r){ /* allow only input of the form: (n) or (n,d)*/
int n=0, d=1;char c;s >> c;if ( c == '(' ) { s >> n >> c;
if (c == ',') s >> d >> c;if ( c == ')') r =rational(n,d);
} else s.putback(c);return s;
}//add this to ex10-1.cpp => ex10-6-4.cpp
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Input of User-Defined Types
void main()
{ rational r;
cin >> r;
cout << "r = " << r << '\n' ;
} //ex10-6-4.cpp
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Unformatted I/O functions read
istream& istream::read(char* ptr, int n); reads into a character array the number of characters
specified by the second argument n. write
ostream& ostream::write(const char*, int ); outputs from the character array the number of bytes
specified by its second argument. gcount
int istream::gcount(); returns the number of characters read by the last
input operation
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#include <iostream.h>
void main()
{ char v[20];
int n;
cin.read( v, 10);
n = cin.gcount() ;
cout <<n << endl;
cout.write( v, n);
}//ex10-6-1.cpp
1234567890101234567890
Result:
Example
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The Class iosclass ios {
...
public:
ostream* tie(ostream* s);//set the stream tied
ostream* tie();//get the stream tied
long flags(long f);//set the new, return the prev.
long flags() const;//return the current flag
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long setf(long setbits, long field);//set flags in a //particular field and return the old
long setf(long);//set one or more flags, return the old
long unsetf(long);//clear one or more flags, return old
int rdstate() const;//get the state of the stream
int eof() const;//check reaching the end of the stream
int fail() const;
int bad() const;
int good() const;
void clear()(int i=0);
...
};
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The fill Member function
Allows us to specify the fill character
cout.fill(‘#’);
(remains in effect until changed)
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Manipulators
allows formatting operations to be inserted directly in the list of input or output operations.
#include <iostream.h>
#include <iostream.h>
void main()
{ int n, m;
cin >>dec >>n;
cin >>hex >>m;
cout<<dec << n << "; "<<m<< endl;
}//ex10-7.cpp
12 1212; 18
Result:
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Standard Manipulatorsios& oct(ios&);//output in octal
ios& dec(ios&);//output in decimal
ios& hex(ios&);//outpur in hexadecimal
ostream& endl(ostream&); //add a ’\n’ and flush
ostream& ends(ostream&); //add a ’ ’ and flush
ostream& flush(ostream&);// output the buffer
istream& ws(istream&);// ignore the space
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Example#include <iostream.h>
#include <iomanip.h>
void main()
{ int j = 200;
cout << ".........." << '\n'
<<setw(5)<<1<< '\n'
<<2 << '\n'
<<setw(6)<<3<< '\n'
<< setfill('*')
<< setw(10) << j << '\n'
<< setw(6) << j +1 << endl;
} //ex10-8.cpp
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User-Defined Manipulators
manipulator to produce a tabostream& tab(ostream& os)
{ return os << ‘\t’ ; } produce an \n and a flush
ostream& ENDL(ostream& s)
{ return s << ‘\n’ << flush ; }
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Example#include <iostream.h>#include <iomanip.h>ostream& tab(ostream& os) {return os << '\t';};ostream& ENDL(ostream& s){return s << ‘\n’ << flush ; }void main(){ int A = 65, B = 42; char x[20];
cout <<A << ',' <<oct <<A << ',‘ <<hex <<A << ',' <<B << ',‘ <<dec <<B << ','<<endl;
cin >> ws >> x;cout << tab <<"x = "<< tab << x << ENDL;
}//ex10-9.cpp
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How Does it Work?class ostream : public virtual ios {// ...public:ostream& operator<<(ostream& (*f)(ostream &)){ return (*f)(*this); }
//...};ostream& tab(ostream& os) {return os << '\t';};ostream& ENDL(ostream& s){return s << ‘\n’ << flush ; } cout <<tab <<ENDL; //… cout.operator<<(tab).operator <<(ENDL)//tab(cout) ENDL(cout)
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Example#include <iostream.h>#include <iomanip.h>void main(){ float x = 123.45;cout.width(8);cout.fill('*');cout.setf(ios::left, ios::adjustfield);cout <<"x = " << x <<endl;cout.setf(ios::scientific, ios::floatfield);cout << 1234.56789<<endl;cout.precision(7);cout << 1234.56789 <<'\n';} //ex10-10.cpp
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Other Manipulators
smanip<int> setfill(int);//set the filling ‘c’
smanip<int> setprecision(int);
//set the spaces to display
smanip<int> setw(int);
//set the spaces to display the following
...
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ios Manipulator (with argument)
#include <iostream.h>#include <iomanip.h>void main(){ int x = 23;
float f = 12.345;cout <<setw(6)<<setfill('#')<< hex << x << '\n';//no dotcout << oct << x << '\n';cin >> oct >> x;cout << dec<<x<<ends;cout <<setprecision(6) << f << '\n';
//effective digits, round up}//ex10-11.cpp
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Format State
class ios {
public:
enum {
skipws=01, //skip white space on input
left=02, // padding after value
right=04, // padding before value
internal=010, //padding between sign/value
dec=020, // decimal
oct=040, // octal
hex=0100, //hexadecimal
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showbase=0200, // show integer baseshowpoint=0400, // print trailing zerosuppercase=01000, // ‘E’ ‘X’, not ‘e’ ‘x’showpos=02000, // explicit ‘+’ for integersscientific=04000, // .dddddd Eddfixed=010000, // dddd.ddunitbuf=020000, //flush after each operat.stdio=040000 //flush after each char.
}; // ...};
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Flags A set of formatting instructions using the
function flags().int options= ios::right | ios::hex | ios::fixed;cout.flags(options);// saving options and setting a new one
int old_opt = cout.flags(new_opt);// changing one option without affecting others
cout.flags(cout.flags() | ios::showpos );// restoring old options
cout.flags(old_opt);
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setf(long) and unsetf(long)
setf(long) sets the options specified by its argument, without changing any of the other options.
unsetf(long) unsets the options specified by its argument.
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The setf(long, long) Function
cout.setf(ios::oct, ios::basefield);cout.setf(ios::dec, ios::basefield);cout.setf(ios::hex, ios::basefield);cout.setf(ios::left, ios::adjustfield);cout.setf(ios::right, ios::adjustfield);cout.setf(ios::internal, ios::adjustfield);cout.setf(ios::scientific, ios::floatfield);cout.setf(ios::fixed, ios::floatfield);cout.setf( 0, ios::floatfield); // default
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The precision() Member Function
specifies the floating point precision the default is 6 remains set until next call to precision
cout.precision(8);
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Formatting Functions
class ios {//...
public:int width(int w);//set the width return the prev. oneint width() const;//return the current widthint precision(int);//set the new, return the old,int precision() const;//return the currentchar fill(char); //set the new char and returns the prevchar fill() const; //return the current fill character
…};
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The width Member Function
Allows us to specify the minimum number of characters to be used for the next numeric or string output operations.
cout.width(5); affects only the immediate numeric or
string output. larger values will not be truncated. smaller values will be padded with the fill
characters.
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Example 1#include <iostream.h> #include <iomanip.h>void main(){ float g = 1.2345678;
cout <<g <<'\n';char oldfill = cout.fill('0');int oldprecis = cout.precision(4);//four effective digitscout.width(8) cout << g << '\n';cout.fill(oldfill);cout.precision(oldprecis);cout.width(8);cout<< g << '\n';
}//ex11-12.cpp
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Example 2#include <iostream.h> #include <iomanip.h>void main(){ double e = 1.234;
float f = 2.1, g = 1212.25;char ch = 'a';cout.setf(ios::fixed|ios::showpoint);cout <<setfill('*')<<setprecision(4)<<setw(10)<<f<<'\n';int options= ios::right | ios::hex | ios::fixed, i = 22;cout.flags(options);cout <<setw(6)<<i<<endl;
}//ex11-13.cpp
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Files and Streamsclass ostream : public virtual ios { //...public:
ostream& flush();ostream& seekp(streampos);ostream& seekp(streamoff, seek_dir);streampos tellp();// ...
};
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Seek Direction
class ios {//...
public:enum seek_dir {
beg=0,cur=1,end=2
};//...
};
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Opening an Output File
ofstream of(“data”, ios::out); in ios::out mode the file is truncated by default, ofstream implies that the file is
opened as “out” mode. ofstream of(“data”);
Append Mode ofstream of(“data”, ios::app);
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File Opening Modes ios::app //Write output to end of file ios::ate //Move to end of file. Allows
//data to be written anywhere ios::in //Open file for input ios::out //Open file for output ios::trunc //erase file contents ios::nocreate //if file does not exist, the
//operation fails ios::noreplace //if the file exists, the
operation //fails
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The open Member function
An ofstream object can be created without opening a specific file:
ofstream of;
A file can be attached to an ofstream object:
of.open(“data”, ios::out);
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Testing the Success of open
if ( ! of ) ...
true if failbit or badbit are set.
Possible errors: opening a non-existent file attempting to read without permission attempting to write with no disk space
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The void* operator
returns 0 if badbit or failbit has been set:
ios::operator void*() const
{
return fail() ? (void *)0 : (void *) (-1) ;
}
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Closing a File
A file is implicitly closed when the ofstream object is destroyed.
We can explicitly close a file by using:
of.close();
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Members of istream
class istream : public virtual ios {//...
public:int peek();istream& putback(char c);istream& seekg(streampos);istream& seekg(streamoff, seek_dir);streampos tellg();//...
};
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Opening an Input File
By default files are open as “in” mode:
ifstream if(“indata”, ios::in);
ifstream if(“indata”);
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The open Member Function
An ifstream object may be created without specifying a file:
ifstream if;
if.open(“indata”, ios::in);
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Repositioning a File Pointer
Reposition pointer at the beginning:if.seekg(0)
Reposition to the nth byte:if.seekg(n);
Position y bytes back from endif.seekg(y, ios::end);
Position at end of file:if.seekg(0, ios::end);
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tellg and tellp
tellg() and tellp() return the current pointer position:
long pos = if. tellg();//Gets the value for the //stream's get pointer.
long pos = if. tellp();//Gets the value for the stream's put pointer
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Creating an Inventory
#include <iostream.h>#include <fstream.h>#include <stdlib.h>void main(){ char name[10]; int quantity; ofstream outf("inventory", ios::out); if( !outf ) {
cerr << "Cannot open file\n";exit(1);
} cout << "Enter item name and quantity: " << endl; while( cin >> name >> quantity)
outf << name << ' ' << quantity << endl;}//ex10-inventory.cpp ^z to end!!!
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Read/Print Inventory#include <iostream.h>#include <fstream.h>#include <iomanip.h>#include <stdlib.h>void main(){ char name[10]; int quantity; ifstream inf("inventory", ios::in); if( !inf ) { cerr << "Cannot open file\n"; exit(1); } while( inf >> name >> quantity)
cout << setiosflags(ios::left) << setw(10) << name << setiosflags(ios::right) << setw(7) << quantity << endl;
}//ex10-rpinve.cpp
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Query Inventory#include <iostream.h>#include <fstream.h>#include <iomanip.h>#include <stdlib.h>#include <string.h>void main(){ char name[10]; int quantity; char item[10]; ifstream inf("inventory", ios::in); if( !inf ) { cerr << "Cannot open file\n"; exit(1); }
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Query Inventory cout << "item name:"; cin >> item; inf >> name >> quantity; while( !inf.eof() ) {
if( !strcmp(name, item) ) cout << setiosflags(ios::left) << setw(10) << name
<< setiosflags(ios::right)<< setw(7) << quantity << endl;
inf >> name >> quantity; }}//ex10-queryinv.cpp
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Processing String Streams
Supported by the class istrstream. Allows:
inputting from character arrays in memory outputting to character arrays in memory
must include the following header filesin your program: iostream.h strstream.h//strstrea.h for VC++
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Types of ostrstream Objects
Dynamic:
Allocates space as need. The member function str() is used to fix its size.
Static:
The object is initialized with a fixed-size array.
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A Dynamic ostrstream#include <iostream.h>
#include <strstrea.h>
void main()
{ ostrstream s;
char* s1 = "Hello, world";
s << s1 << 123 << '\n';
s << "goodbye\n" << ends;
char* string = s.str();
cout << string;
}//ex10-dyn.cpp
Hello, world123goodbye
Result:
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A Static ostrstream #include <iostream.h> #include <strstrea.h> void main() { const int size=6; int x=12345; char buf[size]; ostrstream s(buf, size, ios::out); s << "x = " << x << ends; cout << "buf contains: " << buf << endl; }//ex10-static.cpp
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Reading
Readings Chapter 2 Section 2.5, Chapter 3 Sections 3.5 - 3.6
