Dynamic 2D array without using new

Question

I've been posed with creating a dynamic 2D array in C++ without using new in C++. I have been trying for a while to make something work but I'm clueless as to what I'm supposed to do.

Edit: Sorry, should have been more specific. Just to be transparent, yes it is homework, and no I don't want it solved I just want to be pointed (no pun intended) in the right direction to code it myself.

The order, for reference, is as follow: Develop a console application to create a type int matrix of size m x n using pointers. The user must input the values for the size of the matrix from the keyboard and its contents must be randomly generated (1 - 100). Then, the transpose of the matrix must be calculated and shown (it's necessary to create classes).

We can't use new, nor vector, as we have to do it just via pointers with uni-dimensional arrays. So far I created a class that represent the "rows", and another class which represents the "columns". The columns go into the rows and the rows go into another class called matrix. That was the idea but was having trouble implementing it.

How about std::make_unique? But seriously, here's the right<sup>1</sup> way. <sup>1</sup>Right for for a common pool of problems. — user4581301
– user4581301, Commented Oct 30, 2019 at 17:56
@tmlen That’s a nested array instead of a 2D array. I know that nested arrays are occasionally used to implement multi-dimensional arrays but they’re a bad implementation. — Konrad Rudolph
– Konrad Rudolph, Commented Oct 30, 2019 at 17:58

eerorika · Accepted Answer · 2019-10-30 20:34:52Z

4

new is the only way to create dynamic objects or arrays in standard C++. So, depending on how you interpret the task, it could be considered impossible.

If we assume that it is OK for you to call a standard function that internally calls new, then the problem is solvable. A commonly used way to create a dynamic array in C++ is to use std::vector. Elements of std::vector may not be arrays however, so a 2D dynamic array is not technically possible using it. One workaround is to wrap the array within a class, and use the class as element of the vector. There is a standard template for such array wrapper: std::array. An example of a vector of array wrappers:

std::vector<std::array<type_of_element, 10>> name_of_vector(number_of_arrays);

The elements of the arrays within the dynamic array managed by the vector will have effectively the same layout as a 2D array would.

edited Oct 30, 2019 at 20:34

answered Oct 30, 2019 at 18:03

eerorika

240k13 gold badges212 silver badges354 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Max Over a year ago

My bad, didn't specify I can't use other libraries, just good old int arr [ ] arrays

eerorika Over a year ago

@Max then your task is impossible in standard c++. new is the only way to create dynamic objects or arrays.

Max · Accepted Answer · 2019-10-30 23:19:18Z

malloc did the trick. Here is the code I used to test it. It was a bit convoluted to figure out how to write the matrix loop but once I got it down I realized how obvious it was.

Matriz::Matriz(int numFil, int numCol)
    :numFil(numFil), numCol(numCol)
{
    mat = (int *)malloc(numFil * numCol * sizeof (int));

    int c = 0;

    for(int i = 0; i < numFil; i++)
    {
        for(int j = 0; j < numCol; j++)
        {
            *(mat + i * numCol + j) = ++c;
        }
    }
}

void Matriz::printMat()
{
    for(int i = 0; i < numFil; i++)
    {
        for(int j = 0; j < numCol; j++)
        {
            std::cout << *(mat + i*numCol + j);
        }
         std::cout << std::endl;
    }
}

Collectives™ on Stack Overflow

Dynamic 2D array without using new

2 Answers 2

2 Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

2 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related