Dynamic delaration of a 2D CString Array

[gehring]

Hi All,

I am extremely new to C++ and would greatly appreciate any advice regarding the following question:

I am trying to create a 2D array of CStrings, however it needs to be declared dynamically for my application. I have read up on numerous forums but I just cant seem to find what I am looking for.

From what I have read the best way to do this would be to use a vector of CStrings. However, I am not to sure how to declare this.

What I would like to be able to do is adress any CString in the following way:

myarray[0][0] = "hello";

Each example I have found requires me to first set the size of the array in the declaration before compilation.

Please could someone point me in the right direction?

[Clinton Portis]

What you are asking for can be a very useful data structure... if created dynamically can have about the same performance as a vector and might even be more efficient (a vector might allocate more memory than what you actually need based on the number of push_back() opearations) In either case, here is how you can do both:

The creation of a 2d array is like having a 'pointer to a bunch of pointers':

//create the array pointer (holds the address to the beginning of a block of allocated memory)

//create 2d array of pointers to cstrings
char* words = new char*[wordcount];

//allocate memory for each cstring pointer
for(int i=0; i<wordcount; i++)
{
     wordcount[i] = new char[20];
}
//Now you have words[word#][individual char] array of cstrings, each can hold a word of (in this case 19 chars or less)

//Now you can perform cool cstring operations:
//'Add' to an existing cstring (in this case, the second cstring of the array)
strcat(words[2], " a good day");

If you want to use more modern c++ techniques as opposed to using the older C style character arrays, here is how ye' would declare a 'vector of char vectors'

//Notice that this is one of the very few instances in c++ where spacing matters
//Be sure to add a space in between the > > or else your compiler will think it's a >> extraction operator
vector<char*> words;

//create a temporary cstring we will push into the vector anytime we want to add to it
char* temp = new char[20];

//Once populated, push it back into our vector of cstrings
for(int i=0; i<wordcount; i++)
{
     cout << "Enter a word (19 chars or less): ";
     cin >> temp;

     words.push_back(temp);
}

//Access a cstring from the char* vector:
cout << "Enter element number to display: ";
cin >> element;
cout << words[element];

[pecet]

@Clinton Portis:
I suppose, at line #3, you wanted to write:

vector<vector<char*> > words; // this is vector of char* vectors

if so, this won't work:

words.push_back(temp);

Can be done like this:

vector<char*> v_temp;
for(int i=0; i<wordcount; i++)
{
     cout << "Enter a word (19 chars or less): ";
     cin >> temp;

     v_temp.push_back(temp);
}
words.push_back(v_temp);

[gehring]

Hi Clinton,

Thank you very much for your detailed reply...You have definitely helped me clarify a few things.

However I still have a question or two:

In your examples(for which i am extremely grateful) you have showed me exactly how to create the array as well as add to and read a specific element.

Firstly, am i correct in saying that "MyArray" in the following example is two dimensional:

MyArray[0][1]="hello"

If so how would I access an array using your example above in the same "row", "collum" manner?

For instance the following code was pasted on one of the other forums:

#include <vector>
std::vector<std::vector<CString> > your2darray(width,
std::vector<CString>(height));
your2darray[x][y] = "asdf";

Memory management is done automatically here.

However according to the poster this is supposed to be dynamic, however "Width" and "Height" must be still be declared?

[Clinton Portis]

I was just driving down the road and I was thinking, "Hmm.. i forgot to allocate memory for each char* of the vector... so I think this will work better:

for(int i=0; i<wordcount; i++)
{
     char* temp = new char[20];
     cout << "Enter a word (19 chars or less): ";
     cin >> temp;

     words.push_back(temp);
}

Pecet, I was first considering a 'vector of a vector of char pointers' I soon realized that was overkill.. a simple vector of char* is similar to a char[][] 2d array.

ghering,

MyArray[0][1]="hello"

will give you the 'e' in "hello".. if you wanted to access the entire word, just dereferrence the first dimension, which is a pointer to the beginning of the cstring

cout << MyArray[0];

As long as the 2nd dimension is null terminated, your cout operation will not run out of bounds into the rest of the array.

[gehring]

Thanks again Clinton,

That is now clear to me.

Just for interest sakes, am I missing something with the following code or do you agree that the "width" and "height" must first be defined?

#include <vector>
std::vector<std::vector<CString> > your2darray(width,
std::vector<CString>(height));
your2darray[x][y] = "asdf";

Thanks again for the help!

[Clinton Portis]

I think you mean to do this, if your2darray is an array of cstrings:

//this
your2darray[x][y] = "asdf";

//should be this:
strcpy(your2darray[x], "asdf");