C++: access values of 2D array being passed as parameter/pointer in function

Question

I am trying to access values of a 2D array initially passed as an array of pointers in the function. Additionally I would like this function to return a 2D array with the values I have accessed and worked on.

I understand the concept of pointer, however I have trouble with the double pointer concept of 2D arrays. If I understand correctly, iterating over the first dimension of the array points to a position in the memory of the computer, which itself points to another position of the memory in the computer where the value is. This 2 pointing process represents the 2D.

Now, in my code, I don't manage to access the values of this second pointer and I don't understand why.

extern "C" double cfun(double **indatav, int rows, int cols)
{

    double* *outdatav = new double*[rows];
    for (int i=0; i < rows; i++){
        outdatav[i] = new double*[cols];
        }

    for (int i=0; i < rows; i++){
        for (int j=0; j < cols; j++){
            outdatav[i][j] = indatav[i][j];
            }
        }
    return outdatav;
}

The code above is called through ctypes in Python and therefore it has this extern... However the equivalent in C++ would just consist in removing this extern "C" I have spent quite some time now on this problem and I don't think I will be able to solve it alone so any help would be highly appreciated.

*indata[i][j] is not valid. intdatav[i] IS a pointer and points to the start of row I. Just use outdatav[i][j] = indatav[i][j]; — doug
– doug, Commented Sep 27, 2019 at 19:25
Note that you'll also have to free all those pointers. If you're going to access this func from Python only I thought about a workaround that would not require Python code to call the deallocation func. I was preparing it for one of the 2 question you deleted. — CristiFati
– CristiFati, Commented Sep 30, 2019 at 10:16

boby · Accepted Answer · 2019-09-27 17:42:06Z

1

seems wrong memory allocation in double pointer

double* *outdatav = new double*[rows];
for (int i=0; i < rows; i++){
    // outdatav[i] = new double*[cols]; // <- typo here
    outfatav[i] = new double [cols]; // <- replace with this
}

ADD

the return type of the function is not matched with the returned variable, so need to revise the return type to double**

extern "C" double** cfun(double **indatav, int rows, int cols)

edited Sep 27, 2019 at 17:42

answered Sep 27, 2019 at 17:07

boby

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2 Comments

Joachim Over a year ago

Thank you for your response, but it still doesn't entirely solve my problem (see edit)

boby Over a year ago

for *indatav[i][j];, already indatav[i][j] points to the value (type as double), not an address

donjuedo · Accepted Answer · 2019-09-27 17:26:41Z

1

extern "C" double cfun(double **indatav, int rows, int cols)

{

    double**outdatav = new double*[rows];
    for (int i=0; i < rows; i++){
        outdatav[i] = new double[cols];
        }

    for (int i=0; i < rows; i++){
        for (int j=0; j < cols; j++){
            outdatav[i][j] = indatav[i][j];
            }
        }
    return outdatav;
}

This will work properly now.

 outdatav[i] = new double[cols];

In this line of code, you were assigning an array of pointers to a pointer. Instead, you have to assign array to each pointer.

edited Sep 27, 2019 at 17:26

donjuedo

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answered Sep 27, 2019 at 17:03

zarar tariq

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4 Comments

Joachim Over a year ago

Thank you very much for your response and edit, but it still doesn't entirely solve my problem because I want outdatav to be a matrix of values, and not pointers to pointers.

zarar tariq Over a year ago

no problem at all :).. The above code solved your problem right?

Joachim Over a year ago

Well the code now works correctly according to your recommendations, which was the question of this thread. However, I am not able to do what I want but I am not sure where does the problem come from, if it's from Python or from C++, so therefore I did another thread on the subject where I am detailing more precisely what exactly my problem is. stackoverflow.com/questions/58150201/…

CristiFati Over a year ago

@JoachimBsh: Note: It takes an user to have 10k+ reputation in order to be able to view deleted posts.

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