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I'll do my best to be brief: So I have an assignment where I am creating a 'Wordlist' class. In which I will store a list of words. These are the member variables

class WordList 
{ //...  
  unsigned int m_count;        // Number of words currently in list    
  unsigned int m_max;  // The total size of the list. 
  char** m_list;         // The list storing the words
};

This is my constructor

WordList::WordList(const int max_words) {
            
    if(max_words < 1){
        m_list = nullptr;
        m_max = 0;
        m_count = 0;
    }
    else
        m_list = new char*[max_words];
        m_count = 0;
        m_max = max_words;
    for (int i = 0; i < max_words; i++) {
        m_list[i] = new char[20];
    }
}

And this is where I start to find problems. The following add function is supposed to add a word in the form of a c-style string that is pointed to from the array of character pointers that is pointed to from **char m_list .

int WordList::add(const char word[]) {
    if (m_count == 0 && m_list != nullptr ) {
        strcpy (m_list[m_count], word);
        m_count++;
        return 0;
    }
    if (m_count < m_max) {
        m_count++;
        strcpy (m_list[m_count], word);
        return 0;
    }
    if (m_count == m_max) {
        m_count++;
        m_max ++;
        strcpy (m_list[m_count], word);
        return 1;
    }
    if (strlen(word)==0) {
        return -2;
      }
      if (m_list == nullptr ){
          return -2;
      }
else
    return -2;
}

So the issue I am having is that I clearly not syntactically correct with my * because I am not getting an array of 5 pointers that point to full words rather I am getting the first letter saved to the final destination char but its not copying over everything like I want.

I'm sure I didn't translate my problem to English as well as I should have but hopefully thats a start. Thank you!

An example of how I will be calling my add function:

WordList *wordlist = new WordList(5);
wordlist->add("harry"); 
wordlist->add("ron"); 
wordlist->add("hermione");

And it should add to the bottom of the pointer array a pointer to each word so

    cout  << wordlist->m_list[0][2] << endl; // Expect 'r'

    cout  << wordlist->m_list[1] << endl; // Expect "ron"

instead I get

r

printed out only

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4
  • First, does the assignment allow you to use std::vector<std::string> as that would be a much better choice here. Second, please show us how you are calling the add() method. Commented Oct 12, 2020 at 2:16
  • 1
    Why do you think you're getting only the first letter? Can you provide a minimal reproducible example that shows the unexpected behavior? Also, unless your assignment doesn't allow it for some reason, just use a std::vector<std::string>. Commented Oct 12, 2020 at 2:16
  • " I start to find problems" --> strcpy (m_list[m_count], word); will have trouble if strlen(word) > 19. Commented Oct 12, 2020 at 2:19
  • Thank you for the quick responses I will post some more example code. Unfortunately I cannot use the string library at all. I think that is so that we really focus on manipulating pointers . Commented Oct 12, 2020 at 2:40

2 Answers 2

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1

I don't see anything wrong with your use of double-pointers.

There are other issues, though:

  1. in your WordList::add you should check for empty word or empty list first, and fail fast. Besides, in your code if the word was empty - you would already added it and returned form that function.
  2. in if (m_count < m_max) block, you pre-increment m_count, leaving one element empty and risking to go out-of-bounds on the last entry.
  3. in if (m_count == m_max) { you are CERTAINLY going out-of-bounds
  4. Suggestion: instead of pre-allocating 20-character arrays, leave them nullptr; when you need to a word - use strdup(word); that would allocated a required space for you.
  5. As for your I am getting the first letter saved - I am guessing you are not checking it right...
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8 Comments

Thank you so much for your help! I am looking over now and really appreciate it.
@StoneCodeStawny - yes, please see en.cppreference.com/w/c/experimental/dynamic/strdup
@StoneCodeStawny you can use realloc to resize your array: en.cppreference.com/w/c/memory/realloc . If there are new questions about that - yes, please create a new one.
@StoneCodeStawny Re: your printing code in the question - as I explained in my answer above, you are skipping element [1] because of your incorrect increments of m_count
@StoneCodeStawny realloc is as much C++ as strcpy :)
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1

The problem is that you add the first word:

if (m_count == 0 && m_list != nullptr ) {
    strcpy (m_list[m_count], word);
    m_count++;
    return 0;
}

Which increments m_count so now m_count is 1.

Then you add the second word:

if (m_count < m_max) {
    m_count++;
    strcpy (m_list[m_count], word);
    return 0;
}

Which increments m_count BEFORE adding the word so the second word is at index 2 and index 1 is skipped altogether.

You need to always increment the count after copying the word because m_count is 1 based and the array is 0 based.

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1 Comment

Wow sometimes it just takes another eye to catch something you are just stuck on. This has been so helpful!

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