Dynamically allocating a 2d array

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# Thread: Dynamically allocating a 2d array

1. nnp
Registered User
Join Date
Oct 2005
Posts
5

## Dynamically allocating a 2d array

As part of a program i have a function that takes in a word and saves it into a 2d array. Before it saves the word it checks to make sure there is room in the array. If the number of words in the array is equal to the current size of the row dimension then more space needs to be allocated. The problem occurs after the new bigger 2d array is created and i want to assign the address of this array to the original pointer to the smaller array.

The parameters to the procedure are
word = a string holding the new word to be saved
results = a 2d array holding all previously saved words
numWords = the number of words currently in the array
maxWords = the maximum number of words for which there is space. i.e the size of the row parameter to results.

RMAX_LEN is #define - ed earlier in the program to 60

Any help is appreciated.

Code:
```void saveWord(string &word, char results[][RMAX_LEN], int &numWords, int &maxWords)
{
unsigned int i(0), j(0);
//First check there is room for another word
if(numWords == maxWords){
//put the address of the array of char* in tempResults
char **tempResults= new char*[maxWords*2];

//Each element of temp results holds a pointer to a char array
for(i = 0; i < (maxWords*2); i++){
tempResults[i] = new char[RMAX_LEN];
}

//Copy all entries in results into the new array
for(i = 0; i < maxWords; i++){
for(j = 0; j < RMAX_LEN; j++){
tempResults[i][j] = results[i][j];
}// End of for
} //End of for

maxWords *= 2; //change maxwords to reflect new buffer size

for(i = 0; i < maxWords; i++){ //Free up old reserved space
delete [] results[i];
}
delete [] results;
results = tempResults; //results now points to new array
}

//Then copy in the word
j = (unsigned int)word.length();
for(i = 0; i < j; i++){
results[numWords][i] = word[i];
} //End of for

results[numWords][i] = '\0';
numWords++;
}// End of saveWord```
And the compiler error
Code:
```bash-3.00\$ g++ pa1.cpp
pa1.cpp: In function `void saveWord(std::string&, char (*)[60], int&, int&)':
pa1.cpp:119: error: cannot convert `char**' to `char (*)[60]' in assignment
bash-3.00\$```
Thanks,
NNP

By the way, is my method of creating the new array the most desirable method or is there a better one?

2. Senior Member
Join Date
Dec 2003
Posts
3,366
Its usually desirable to use single dimensional and just index it or cast it out as a 2-d. Use of vectors instead is often recommended as well, unless you have a reason not to. the compiler problem can be worked around, however, and your code looks correct except for that one problem.

results = &tempResults[0]; //I think this will cure it? try it and see. I dont have a compiler at home atm as I reloaded recently.

3. nnp
Registered User
Join Date
Oct 2005
Posts
5
thanks for your reply. Im also not at a pc with a compiler so i cant test it as of yet but i do have a question

In a case like char **tempResults arent tempresults and &tempResults[0] the same thing?

4. Senior Member
Join Date
Dec 2003
Posts
3,366
Almost. the address is the same but the assignment should see it as ** instead of * to array of [60].

5. nnp
Registered User
Join Date
Oct 2005
Posts
5
Unfortunately it doesnt seem to be working. I got the exact same error. Any other ideas?

6. Senior Member
Join Date
Dec 2003
Posts
3,366
I will sneak a compile at it tomorrow at work. There is a simple way to trick the compiler into seeing them as the same type, but as I always just use single dimensional arrays, I can't think it up atm.

7. Senior Member
Join Date
Dec 2003
Posts
3,366
void saveWord(string &word, char **results, int &numWords, int &maxWords)

does this work for you? It did over here, or seems to with my test code.

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