15.13. Multiple Choice Exercises

We want to open a file and parse its data into our program. What library do we need to include?

iostream

• This library deals with communication through the standard input and output.

sstream

• This library is used to manipulate string objects as if they were streams.

fstream

• This library is used to manipulate files using streams.

iomanip

• This library is used to modify internal flags and formatting options.

Check answer

The code below reads data from a file called input.txt. What is wrong with the following code?

#include <iostream>
#include <fstream>
using namespace std;

int main() {
  string input;
  string filename = "input.txt";
  ifstream infile(filename.c_str());
  getline(filename, input);
}

We should use ofstream instead of ifstream.

• Since we are dealing with an input file, we should use ifstream.

The arguments in getline are incorrect.

• We are supposed to read input through the ifstream object. This line should be getline(infile, input).

We cannot use a variable to store the filename.

• Since the name of the file is just a string, we can store it in a variable.

There are no errors with this code.

• Take another look at the code. Are we reading the input correctly into input?

Check answer

We want to make sure the file we wanted to open was opened successfully. Which of the following checks this and prints the proper output?

if (infile.good()) { cout << "File opened unsuccessfully" << endl; }

• If the if statement evaluates to true, then the file was opened successfully.

if (!infile.is_open()) { cout << "File opened unsuccessfully" << endl; }

• is_open() is another function that returns true if a file is opened successfully.

if (infile.good() == false) { cout << "File opened unsuccessfully" << endl; }

• If the file isn't opened successfully, an error message is printed.

if (!infile.open()) { cout << "File opened unsuccessfully" << endl; }

• The open() function is different from is_open() and does not return a bool.

Check answer

Which of the following statements are true?

The ifstream constructor expects a C string as an argument.

• Thus you should convert a filename string to a C string using the c_str() function.

We can assume the program opens all files successfully.

• There are times when a file can't be properly opened because they may have not been properly closed the last time they were used or the file is currently used by another program. As a result, it is good practice to include a check in your program.

A stream is an abstract object that represents the flow of data from a source.

• We've used two streams before: the standard input stream and standard output stream (cin and cout).

We cannot use the operators << and >>, as they are for iostream objects only.

• We are also able to use the operators for fstream objects.

Check answer

What are the contents of the output file output.txt after running the code below?

#include <iostream>
#include <fstream>
using namespace std;

int main() {
  ofstream outfile("output.txt");

  if (!outfile.good()) {
    cout << "Unable to open file" << endl;
  }

  cout << "Powers of 2: ";
  outfile << "2 4 8 16 32 64" << endl;
}

2 4 8 16 32 64

• This is the only thing we write to the output file.

Powers of 2: 2 4 8 16 31 64

• Take another look at the stream objects used in the code.

Powers of 2:

• This is printed to standard output, not the output file.

Unable to open file

• Although this may be printed, this is not the contents of the output file.

Check answer

The file scores.txt contains data about the roster number and test scores of students in a class. The output file averages.txt should store each student's roster number and average test score. What should replace the question marks?

#include <iostream>
#include <fstream>
using namespace std;

int main() {
  string junk;
  int studentNum;
  double mid1, mid2, final;
  ifstream infile("scores.txt");
  ofstream outfile("averages.txt");

  if (!infile.good() || !outfile.good()) {
    cout << "Unable to open a file" << endl;
  }

  getline(infile, junk);
  outfile << "Student#\tAverage" << endl;

  while (infile >> studentNum >> mid1 >> mid2 >> final) {
    double avg = (mid1 + mid2 + final) / 3;
    ???
  }
}

cout << avg << endl

• This will output the average to standard output.

outfile << avg << endl

• Take another look at the code. Is there a clue as to what data should be in the output file?

infile << studentNum << "\t" << avg << endl

• The data should be written to the output file.

outfile << studentNum << "\t" << avg << endl

• This properly outputs the student number and the student's average to the output file.

Check answer

What does the following code do?

#include <iostream>
#include <string>
using namespace std;

int main() {
  string original = "430-0444";
  string digitString = "";

  for (size_t i = 0; i < original.length(); i++) {
    if (isdigit(original[original.length() - 1 - i])) {
      digitString += original[original.length() - 1 - i];
    }
  }
  cout << atoi(digitString.c_str()) << endl;
}

The code converts the original string to an integer and outputs the integer.

• Take a closer look at the contents of the for loop.

The code converts an integer to a string and outputs the string.

• What does the atoi function do?

The code outputs the sum of all the original string's digits.

• The digitString variable is a string, not an int.

The code converts the original string to an integer in reverse and outputs the integer in reverse.

• The code in the for loop parses the string in reverse.

Check answer

Which of the following statements are false about the Set data structure?

A set can contain multiple elements with the same value.

• All elements in a set are unique.

We can identify elements of a set by each element's index.

• Each element has an index associated with it.

The elements of a Set are always sorted.

• Although a set is ordered, it is not necessarily sorted.

The size of a Set object is fixed.

• The Set data structure can expand to make room for new elements.

Check answer

There are many ways to construct a matrix. Which of the following are valid constructors of a matrix?

matrix<string> m1(2);

• A matrix has two dimensions and this constructor only has one dimension.

matrix<int> m2(2, 6, 0);

• This creates a matrix with 2 rows and 6 columns with all of its elements equal to 0.

matrix<char> m3(m2);

• This creates m3 to be a copy of m2.

matrix<int> m2(2.4, 2);

• There must be a whole number of rows and columns.

Check answer

What is the output of the following code?

#include <iostream>
#include <vector>
using namespace std;

bool secret_function(int num) {
  if (num % 2 == 0) {
    return true;
  }
  return false;
}

int main() {
  matrix<int> mat(4, 2);
  for (size_t i = 0 i < mat.size(); ++i) {
    for (size_t j = 0; j < mat[i].size(); ++j) {
      if (!secret_function(i + j) {
        mat[i][j] = 0;
      }
      else {
        mat[i][j] = i + j;
      }
    }
  }
  int n;
  for (size_t i = 0 i < mat.size(); ++i) {
    for (size_t j = 0; j < mat[i].size(); ++j) {
      n += mat[i][j];
    }
  }
  cout << n << endl;
}

8

• The matrix only contains even sums of the row and column indices and 0 otherwise. Thus, the sum of all elements is 8.

16

• What does the secret function do?

0

• What are the contents of the matrix?

9

• What are the contents of the matrix?

Check answer