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How To Think Like a Computer Scientist C++ Edition The Pretext Interactive Version

Barbara Ericson, Allen B. Downey, Jason L. Wright (Editor)

Exercises 7.17 Multiple Choice Exercises

1.

Which variables below is declared as a string type?
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int main() {
  int x = 0;
  double y = 4.5;
  string word = "hello";
  string letter = "a";
  char c = 'c';
  bool isPrime = 1;
}
  • x
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  • x is an int.
  • y
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  • y is a double.
  • word
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  • word is a string.
  • "hello"
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  • “hello” is not a variable.
  • letter
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  • letter is a string.
  • c
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  • c is a char.
  • isPrime
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  • isPrime is a bool.
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2.

What value should replace the question mark to output the character ‘p’?
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int main() {
  string quote = "Not my tempo.";
  cout << quote[?];
}
  • 11
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  • Remember that indexing begins at 0 in C++.
  • 10
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  • ‘p’ is located at index 10 in quote.
  • The character ‘m’ is located at index 4.
  • The character “N” is located at index 0.
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3.

What is the output of the code below?
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int main() {
  string quote = "I love you 3000.";
  int x = 3;
  int y = 3 * x;
  int z = 1;
  if (y > 12) {
    z = z + x + y;
  }
  else {
    z = z + y - x;
  }
  cout << quote[z];
}
  • The value of z is not 0.
  • The value of z is not greater than 11.
  • The value of z is not 3.
  • The final value of z is 7, and ‘y’ is at index 7 of quote.
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4.

What is the output of the code below?
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int main() {
  string quote = "Look at me. I'm the captain now.";
  int x = quote.length();
  cout << quote[x];
}
  • -1
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  • -1 is not in quote.
  • x is not the index value of the character ‘w’.
  • x is not the index value of the last period.
  • ‘ ‘
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  • It might be logical to think that memory outside of the string is empty space, but there could be leftover junk values.
  • Error, we are indexing out of bounds.
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  • x has a value of 32 and there is no index 32 in quote.
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5.

What is the output of the code below?
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int main() {
  string quote = "With great power comes great responsiblity.";
  size_t n = 0;
  while (n < quote.length()) {
    if (n % 5 == 0) {
      cout << quote[n];
    }
    n++;
  }
}
  • teeest
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  • Remember that indexing begins at 0 in C++.
  • Wg reeest
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  • If we print out every fifth character, including the first, this is the answer.
  • ith reatpowe coms grat rsponibliy.
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  • This is what we would get if we removed every fifth character.
  • With great power comes great responsiblity.
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  • Take a look at the conditional in the while loop.
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6.

What is the output of the code below?
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int main() {
  string quote = "Why so serious?";
  size_t index = quote.find("a");
  cout << index;
}
  • string::npos
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  • Since ‘a’ is not found in quote, the find function returns string::npos.
  • The character at index 0 is ‘W’.
  • The character at index 8 is ‘e’.
  • 15
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  • There is no index 15 in quote.
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7.

What is the output of the code below?
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int main() {
  string quote = "How much wood could a woodchuck chuck if a woodchuck could chuck wood?";
  size_t index = quote.find("wood");
  cout << index;
}
  • Although “wood” appears four times in the string, that is not what the find function returns.
  • The index of ‘w’ in the first “wood” is at index 9.
  • 10
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  • Remember indexing begins at 0 in C++.
  • 12
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  • The find function returns the index of the first character of the found string.
  • 22
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  • The find function returns the index of the first character of the found string.
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8.

What is the output of the code below?
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int main() {
  string quote = "How much wood could a woodchuck chuck if a woodchuck could chuck wood?";
  size_t index = quote.find('w', quote.find("wood") + 1);
  cout << index;
}
  • Take a closer look at the starting index for where we should start looking.
  • 22
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  • After the first ‘w’, the second ‘w’ appears at index 22.
  • 43
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  • Take a closer look at the find function and its arguments.
  • 65
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  • Take a closer look at the find function and its arguments.
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9.

What is the output of the code below?
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int main() {
  string quote = "Life is like a box of chocolates. You never know what you're gonna get.";
  size_t i = 0;
  size_t count = 0;
  while (i < quote.length()) {
    if (quote[i] == 'e') {
      count++;
    }
    i++;
  }
  cout << count;
}
  • Are there any occurences of the letter ‘e’ in quote?
  • Count the number of ‘e’s in quote.
  • There are 7 occurences of the letter ‘e’ in quote.
  • 12
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  • Count the number of ‘e’s in quote.
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10.

What is the output of the code below?
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int main() {
  string call = "Marco!";
  string response = "Polo!";
  string output = "call" + "response";
  cout << output;
}
  • Marco! Polo!
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  • Take a closer look at the initialization of output.
  • Marco!Polo!
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  • Take a closer look at the initialization of output.
  • call response
    🔗
  • Can we concatenate “call” and “response”?
  • callresponse
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  • Can we concatenate “call” and “response”?
  • Error!
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  • We cannot concatenate native C strings like “call” and “response”, so this code results in an error.
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11.

An error occured while delivering a message. All instances of the letter ‘s’ got replaced by ‘X’s. Can you complete the code below to fix this error by selecting the correct line of code to replace the question marks?
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int main() {
  string question = "Honey? Where'X my Xuper Xuit?";
  size_t i = 0;
  while (i < question.length()) {
    if (question[i] == 'X') {
      ?????
    }
    i++;
  }
  cout << question;
}
  • question['X'] = 's';
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  • The argument in the [] operator should be a position in the string.
  • 's' = question[i];
    🔗
  • Check the order of your assignment.
  • 'X' = 's';
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  • We cannot assign the value of ‘s’ to ‘X’.
  • question[i] = 's';
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  • This will successfully replace all instances of ‘X’ with ‘s’.
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12.

What is the output of the code below?
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int main() {
  cout << ("butter" < "butterfly");
}
  • butterbutterfly
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  • The operator between “butter” and “butterfly” is the < operator, not << .
  • Does “butter” come before or after “butterfly”?
  • “butter” comes before “butterfly” in the dictionary.
  • False
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  • In C++, boolean values are outputted as either a 0 or 1.
  • True
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  • In C++, boolean values are outputted as either a 0 or 1.
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13.

What is the output of the code below?
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int main() {
   string quote = "Suffering builds character";
   size_t count = 0;
   size_t index = 17;
   while ( index != quote.length() ) {
     if ( quote[index] == 'a' || quote[index] == 'e' ) {
       count = count + index;
     }
     index = index + 1;
   }
   cout << count << endl;
}
  • The code is not counting the number of a’s or e’s after position 17. Rather adding up their indices.
  • The code is not counting the number of a’s or e’s. Rather adding up their indices.
  • 64
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  • Correct! the occurences of ‘a’ are 19 and 21, while that of ‘e’ is 24 (after index 17). The total is 64.
  • 68
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  • The first occurence of ‘e’ is at index 4 so it is not counted.
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14.

What is the output of the code below?
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int main() {
   string quote = "Its Bond, James Bond";
   size_t index = 1;
   while( index < quote.length() ) {
     quote[index] = 'M';
     index = index * 2;
   }
   cout << quote << endl;
}
  • “IMM MondM James Mond”
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  • Correct! We change indices 1,2,4,8,16 to M before index becomes > quote.length().
  • “IMMMMMMMMMMMMMMMMMMM”
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  • We are not increasing index by 1, instead we are doubling it.
  • “MMM MondM James Mond”
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  • We don’t start at position 0 this time.
  • “IMsMBMnM,MJMmMsMBMnM”
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  • we are not increasing index by 2, instead we are doubling it.
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