Streams
ENGR File 101, Lecture 4: 20 Sep 10
Announcements
Project 2 is due Wednesday night at 9pm
More on the project later in this lecture
1
Streams
Sequences of data made available over time
Input Stream
Source of data, accessed
incrementally
In C++, accessed by
extraction operator (>>)
C++ standard input: cin
Output Stream
element
by
element
Destination for data,
presented incrementally
In C++, presented by
insertion operator (<<)
C++ standard input: cout
Files
Basic units of persistent information maintained by an
operating system
Organized in a file system, with file names and (typically)
hierarchical directories
Pathname: specifies sequence of contained directories,
culminating in file name
Examples:
/afs/umich.edu/user/w/e/wellman/engr101/myprog.cpp
~/engr101/myprog.cpp
2
The fstream Library
If we include the fstream library by typing
#include <fstream>
then we can define new streams that can get input
from, and direct output to, files.
fstream adds two new types: ifstream and ofstream.
stand for input file stream and output file stream
subtypes of istream and ostream, so they can be used just
like cin and cout
C++ Stream Types
istream
cin
ifstream
operations:
>>
ostream
cout
ofstream
operations:
<<
3
Reading from Files
To read from a file you first must declare a new ifstream
and open it.
Format:
ifstream infile("thisfile.txt");
opens the file at pathname "thisfile.txt"
declares and creates a new ifstream called infile
stream named infile now eligible for extracting input
Writing to Files
To write to a file you first must declare a new ofstream
and open it.
Format:
ofstream outfile("thatfile.txt");
opens the file at pathname "thatfile.txt"
declares and creates a new ofstream called outfile
stream named outfile now eligible for inserting output
4
File Stream Example
istream
x 25
ostream
cin
y 15
thisfile.txt
cout
ifstream
infile
25 15
operations:
>>
ifstream infile("thisfile.txt");
int x, y;
infile >> x >> y;
ofstream
outfile
operations:
thatfile.txt
x = 25
y =15
<<
ofstream outfile("thatfile.txt");
outfile << "x =" << x << endl;
outfile << "y =" << y << endl;
Opening and Closing Files
Recall that the statement
ifstream infile("thisfile.txt");
does two things:
It declares a new input file stream called infile
It opens the file thisfile.txt and associates it with this
stream
This can also be performed in two separate steps
ifstream infile;
infile.open("thisfile.txt");
5
Opening and Closing Files
When we are done using a file it is good practice to
close the file
ifstream infile;
infile.open("thisfile.txt");
infile >> x >> y;
infile.close();
open and close are operations defined on ofstream
and ifstream objects
The fstream Library
istream
cin
ostream
cout
ifstream
ofstream
operations:
open, close
operations:
open, close
operations:
>>
operations:
<<
6
What is in outfile.txt after this
Program Executes?
#include <iostream>
#include namespace <fstream>
using std;
A Alpha
int main( ) {
ofstream outfile("outfile.txt");
B
cerr << "Alpha" << endl;
cout << "Beta" << endl;
C
outfile << "Gamma" << endl;
outfile.close();
return 0;
D
}
Beta
Gamma
Alpha
Beta
Gamma
What is in outfile.txt after the
second program executes?
#include <iostream>
#include <fstream>
using namespace std;
int main( ) {
A
int x=4;
ofstream outfile("outfile.txt");
B
outfile << x << x+1 << x+3;
outfile << x-1 << endl;
outfile.close();
C
return 0;
}
4573
457
3
4587
D 458
7
7
What is in outfile.txt after the
third program executes?
#include <iostream>
#include <fstream>
using namespace std;
void printproc(ofstream & zfile, int a){
zfile << a << a << endl;
return;
}
A
int main( ){
ofstream outfile("outfile.txt");
printproc(outfile, 7590);
outfile.close();
return 0;
}
B 7590
C 75907590
D 7590
7590
Tracing Program 3
#include <iostream>
#include <fstream>
using namespace std;
void printproc(ofstream & zfile, int a){
zfile << a << a << endl;
return;
a
zfile
}
7590
outfile.txt
75907590
int main( ){
ofstream outfile("outfile.txt");
printproc(outfile, 7590);
outfile.close();
return 0;
outfile
}
8
What is output to the screen after
the fourth program executes?
int main( ){
int a, b, c, d, e, f;
ofstream outfile("outfile.txt");
permute(outfile, 1, 2, 3);
outfile.close();
ifstream infile("outfile.txt");
infile >> a >> b >> c >> d >> e
>> f;
infile.close();
cout << f << a << endl;
cout << e << b << endl;
cout << d << c << endl;
return 0;
}
A 21
32
13
B 31
23
12
void permute(ofstream & fout,
int a, int b, int c)
{
fout << a << b << c << endl;
fout << a << c << b << endl;
fout << b << a << c << endl;
fout << b << c << a << endl;
fout << c << a << b << endl;
fout << c << b << a << endl;
return;
}
C 312132
231213
312132
D 321123
312132
231213
Tracing Program 4
int main( ){
int a, b, c, d, e, f;
ofstream outfile("outfile.txt");
permute(outfile, 1, 2, 3);
outfile.close();
ifstream infile("outfile.txt");
infile >> a >> b >> c >> d >> e
>> f;
infile.close();
cout << f << a << endl;
cout << e << b << endl;
cout << d << c << endl;
return 0;
}
void permute(ofstream & fout,
int a, int b, int c)
{
fout << a << b << c << endl;
fout << a << c << b << endl;
fout << b << a << c << endl;
fout << b << c << a << endl;
fout << c << a << b << endl;
fout << c << b << a << endl;
return;
}
123
132
213
231
312
321
a
>> b
>> c
>> d
>> e
>> f
>>
321123
312132
231213
9