15.12. Multiple Choice Exercises

Q-1: 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.

Q-2: 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?

Q-3: 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.

Q-4: 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.

Q-5: 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.

Q-6: 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.

Q-7: 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.

Q-8: 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.

Q-9: 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.

Q-10: 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?

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