Chapter 12

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Chapter 12

Streams and File I/O

This activity contains 10 questions.

The following code intends to read an integer from the file "foo.txt", but has an error. What is it?


ofstream f;
int i;

f.open("foo.txt");
f >> i;
cout << i << endl;
f.close();

The data must be read from the file as a string.The complete pathname to the file must be given.Data must be read using getline.The file variable f should be of type ifstream.

The following code intends to read an integer from the file "foo.txt" in the root of the C:\ drive, but it has an error. What is it?


ifstream f;
int i;

f.open("c:\foo.txt");
f >> i;
cout << i << endl;
f.close();

The file variable must be checked to see if it was opened successfully.The file variable f should be of type ofstream.Use f.open("c:\\foo.txt");

The program below intends to add log messages to a log file. Will this work?


using namespace std;

int main()
{
	ofstream f;
	string message;

	message = "Message to add to the log file";
	f.open("log.txt");
	f << message.c_str() << endl;
	f.close();
	return 0;
}

No; the file variable f should be declared as type istreamNo; without opening the file with ios::app as the second parameter, the program will overwrite the old log file each time the program is run.No; the file must be flushed before closing to ensure that the contents are written to disk.Yes.

Say that we have the file named "foo.txt" in the current directory that contains:


10
20

The code below attempts to read this file as follows, except the file name is misspelled:


ifstream f;
int i=1;

f.open("foobar.txt");    
f >> i;
cout << i << endl;
f.close();

What will the program output when run?

It will give an error message when attempting to read data into i via: f >> i;It will give an error message when attempting to close the file.It will give an error message when attempting to open the (nonexistent) file.1

You have a file of names stored in the file "names.txt". It is organized so that the first line is a number indicating how many names are in the file, followed by the actual names. For example the file could contain the following contents:


4
Fresco, Al
Guini, Lynn
Oki, Kerry
Wright, Eaton

Which program will open the file and correctly input each name into an array?

ifstream f;
string *names;
int numnames;

f.open("foo.txt");
f >> numnames;
names = new string[numnames];
for (int i=0; i<numnames; i++) {
	getline(f, names[i]);
}
f.close();
delete[] names;

ifstream f;
string *names;
int numnames;

f.open("foo.txt");
f >> numnames;
f.ignore();
names = new string[numnames];
for (int i=0; i<numnames; i++) {
	getline(f, names[i]);
}
f.close();
delete[] names;

ifstream f;
string *names;
int numnames;

f.open("foo.txt");
f >> numnames;
names = new string[numnames];
for (int i=0; i<numnames; i++) {
	f >> names[i];
}
f.close();
delete[] names;

ifstream f;
string *names;
int numnames;

f.open("foo.txt");
f >> numnames;
f.ignore();
names = new string[numnames];
for (int i=0; i<numnames; i++) {
	f >> names[i];
}
f.close();
delete[] names;

You have a file of names stored in the file "names.txt". Each name is on a separate eline. For example the file could contain the following contents:


Fresco, Al
Guini, Lynn
Oki, Kerry
Wright, Eaton

Which program will open the file and correctly input each name into a vector?

ifstream f;	
vector<string> names;	
string s;
int num;
	
f.open("foo.txt");
f >> num;
f.ignore();
for (int i=0; i<num; i++) {
	getline(f, s);
	names.push_back(s);		
}
f.close();

ofstream f;	
vector<string> names;	
string s;
	
f.open("foo.txt");
while (!f.eof()) {
	getline(f, s);
	names.push_back(s);		
}
f.close();

ifstream f;	
vector<string> names;	
string s;
	
f.open("foo.txt");
while (!f.fail()) {
	getline(f, s);
	names.push_back(s);		
}
f.close();

ifstream f;	
vector<string> names;	
string s;
	
f.open("foo.txt");
while (!f.eof()) {
	getline(f, s);
	names.push_back(s);		
}
f.close();

What is the output of the following code?


int i;
	
cout.setf(ios::fixed | ios::showpoint | ios::left);
cout.width(10);
cout << "Number";
cout.width(6);
cout << "Amount" << endl;
cout.precision(3);	
for (i=0; i<5; i++) {
	cout.width(10);
	cout << i;
	cout.width(6);
	cout << i*0.1 << endl;
}

Number Amount
0 0.0
1 0.1
2 0.2
3 0.3
4 0.4

Number    Amount
0         0.000
1         0.100
2         0.200
3         0.300
4         0.400

Number    Amount
0         0
1         0.1
2         0.2
3         0.3
4         0.4

Number Amount
0 0.000
1 0.100
2 0.200
3 0.300
4 0.400

Is it possible to directly store binary data to the middle of a file?

No, data may only be stored sequentially.No, data may only be stored sequentially and in text format.Yes, seekp allows positioning the file pointer to a particular byte for reading or writing.No, data may be stored directly to a particular location but must be stored in text format, not binary format.

What is the relationship between the class istream and the class ifstream?

The class ifstream is derived from istream, meaning it is a more specific type of istream.There is no relationship aside from most of the letters are the same.The classes are identical; ifstream does everything that an istream can, and vice versa.The class istream is derived from ifstream, meaning it is a more specific type of ifstream.

The code skeleton below opens a file for output and will loop forever. Inside the loop the program will periodically output some data to the file.


ofstream f;

f.open("log.txt", ios::app);
while (true)
{
	// Some code here to periodically 
	// set the variable message
	f << message << endl;
}

Upon opening the file "log.txt" on the disk, you find that nothing has been appended to the file as expected. What is a likely reason?

The seekp method should be invoked to directly write data to the end of the file.The file is not properly opened for append mode; each time data is written to the file, it overwrites the existing data.The file must be closed in order for data to be physically written to disk.The operating system may cache or buffer the writes for reasons of efficiency. An explicit flush call will force the data to be physically written to the file.

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Chapter 12

The following code intends to read an integer from the file "foo.txt", but has an error. What is it?
ofstream f; int i; f.open("foo.txt"); f >> i; cout << i << endl; f.close();

The following code intends to read an integer from the file "foo.txt" in the root of the C:\ drive, but it has an error. What is it?
ifstream f; int i; f.open("c:\foo.txt"); f >> i; cout << i << endl; f.close();

The program below intends to add log messages to a log file. Will this work?
using namespace std; int main() { ofstream f; string message; message = "Message to add to the log file"; f.open("log.txt"); f << message.c_str() << endl; f.close(); return 0; }

You have a file of names stored in the file "names.txt". Each name is on a separate eline. For example the file could contain the following contents:
Fresco, Al Guini, Lynn Oki, Kerry Wright, Eaton
Which program will open the file and correctly input each name into a vector?

Is it possible to directly store binary data to the middle of a file?

What is the relationship between the class istream and the class ifstream?

Chapter 12

The following code intends to read an integer from the file "foo.txt", but has an error. What is it? ofstream f; int i; f.open("foo.txt"); f >> i; cout << i << endl; f.close();

The following code intends to read an integer from the file "foo.txt" in the root of the C:\ drive, but it has an error. What is it? ifstream f; int i; f.open("c:\foo.txt"); f >> i; cout << i << endl; f.close();

The program below intends to add log messages to a log file. Will this work? using namespace std; int main() { ofstream f; string message; message = "Message to add to the log file"; f.open("log.txt"); f << message.c_str() << endl; f.close(); return 0; }

You have a file of names stored in the file "names.txt". Each name is on a separate eline. For example the file could contain the following contents: Fresco, Al Guini, Lynn Oki, Kerry Wright, Eaton Which program will open the file and correctly input each name into a vector?

Is it possible to directly store binary data to the middle of a file?

What is the relationship between the class istream and the class ifstream?

The following code intends to read an integer from the file "foo.txt", but has an error. What is it?
ofstream f; int i; f.open("foo.txt"); f >> i; cout << i << endl; f.close();

The following code intends to read an integer from the file "foo.txt" in the root of the C:\ drive, but it has an error. What is it?
ifstream f; int i; f.open("c:\foo.txt"); f >> i; cout << i << endl; f.close();

The program below intends to add log messages to a log file. Will this work?
using namespace std; int main() { ofstream f; string message; message = "Message to add to the log file"; f.open("log.txt"); f << message.c_str() << endl; f.close(); return 0; }

You have a file of names stored in the file "names.txt". Each name is on a separate eline. For example the file could contain the following contents:
Fresco, Al Guini, Lynn Oki, Kerry Wright, Eaton
Which program will open the file and correctly input each name into a vector?