C++ exam on string class implementation

Question

I just took an exam where I was asked the following:

Write the function body of each of the methods GenStrLen, InsertChar and StrReverse for the given code below. You must take into consideration the following;

• How strings are constructed in C++
• The string must not overflow
• Insertion of character increases its length by 1
• An empty string is indicated by StrLen = 0

class Strings {
  private:
    char str[80];
    int StrLen;
  public:

  // Constructor
  Strings() {
    StrLen=0;
  };

  // A function for returning the length of the string 'str'
  int GetStrLen(void) {

  };

  // A function to inser a character 'ch' at the end of the string 'str'
  void InsertChar(char ch) {

  };

  // A function to reverse the content of the string 'str'
  void StrReverse(void) {

  };

};

---

The answer I gave was something like this (see bellow). My one of problem is that used many extra variables and that makes me believe am not doing it the best possible way, and the other thing is that is not working....

class Strings {
private:
    char str[80];
    int StrLen;
    int index; // *** Had to add this ***
public:

    Strings(){
        StrLen=0;
    }

    int GetStrLen(void){
        for (int i=0 ; str[i]!='\0' ; i++)
            index++;
        return index;   // *** Here am getting a weird value, something like 1829584505306 ***
    }

    void InsertChar(char ch){    
        str[index] = ch;  // *** Not sure if this is correct cuz I was not given int index ***
    }

    void StrRevrse(void){
        GetStrLen();
        char revStr[index+1];
        for (int i=0 ; str[i]!='\0' ; i++){
            for (int r=index ; r>0 ; r--)
                revStr[r] = str[i];
        }
    }
};

---

I would appreciate if anyone could explain me roughly what is the best way to have answered the question and why. Also how come my professor closes each class function like " }; ", I thought that was only used for ending classes and constructors only.

Thanks a lot for your help.

Answer 1

First, the trivial }; question is just a matter of style. I do that too when I put function bodies inside class declarations. In that case the ; is just an empty statement and doesn't change the meaning of the program. It can be left out of the end of the functions (but not the end of the class).

Here's some major problems with what you wrote:

1. You never initialize the contents of str. It's not guaranteed to start out with \0 bytes.
2. You never initialize index, you only set it within GetStrLen. It could have value -19281281 when the program starts. What if someone calls InsertChar before they call GetStrLen?
3. You never update index in InsertChar. What if someone calls InsertChar twice in a row?
4. In StrReverse, you create a reversed string called revStr, but then you never do anything with it. The string in str stays the same afterwords.

The confusing part to me is why you created a new variable called index, presumably to track the index of one-past-the-last character the string, when there was already a variable called StrLen for this purpose, which you totally ignored. The index of of one-past-the-last character is the length of the string, so you should just have kept the length of the string up to date, and used that, e.g.

int GetStrLen(void){
  return StrLen; 
}

void InsertChar(char ch){    
  if (StrLen < 80) {
    str[StrLen] = ch;
    StrLen = StrLen + 1; // Update the length of the string
  } else {
    // Do not allow the string to overflow. Normally, you would throw an exception here
    // but if you don't know what that is, you instructor was probably just expecting
    // you to return without trying to insert the character.
    throw std::overflow_error();
  }
}

Your algorithm for string reversal, however, is just completely wrong. Think through what that code says (assuming index is initialized and updated correctly elsewhere). It says "for every character in str, overwrite the entirety of revStr, backwards, with this character". If str started out as "Hello World", revStr would end up as "ddddddddddd", since d is the last character in str.

What you should do is something like this:

void StrReverse() {
   char revStr[80];
   for (int i = 0; i < StrLen; ++i) {
     revStr[(StrLen - 1) - i] = str[i];
   }
}

Take note of how that works. Say that StrLen = 10. Then we're copying position 0 of str into position 9 of revStr, and then position 1 of str into position 9 of revStr, etc, etc, until we copy position StrLen - 1 of str into position 0 of revStr.

But then you've got a reversed string in revStr and you're still missing the part where you put that back into str, so the complete method would look like

void StrReverse() {
   char revStr[80];
   for (int i = 0; i < StrLen; ++i) {
     revStr[(StrLen - 1) - i] = str[i];
   }
   for (int i = 0; i < StrLen; ++i) {
     str[i] = revStr[i];
   }
}

And there are cleverer ways to do this where you don't have to have a temporary string revStr, but the above is perfectly functional and would be a correct answer to the problem.

By the way, you really don't need to worry about NULL bytes (\0s) at all in this code. The fact that you are (or at least you should be) tracking the length of the string with the StrLen variable makes the end sentinel unnecessary since using StrLen you already know the point beyond which the contents of str should be ignored.

Answer 2

int GetStrLen(void){
    for (int i=0 ; str[i]!='\0' ; i++)
        index++;
    return index;   // *** Here am getting a weird value, something like 1829584505306 ***
}

You are getting a weird value because you never initialized index, you just started incrementing it.