How to convert one-dimensional array into two-dimensional without memory allocating?

Question

I have a filled one-dimensional array double *vals as a class component with sizes Nn[0]*Nn[1]. I need to get 2-dimensional array **w (w[Nn[1]][Nn[0]]) without allocating new memory for it, e.g. i need to represent vals as 2-dimensional array.

Using g++ compiler i can make

double (* w)[Nn[0]] = (double (*)[Nn[0]])val;

But VS compiler or intel compiler don't allow to use non-constant expression as dimension array.

In general, I can just use element in initial vals array converting 2 int indices (i,j) of w[i][j] element into global index and do not declare w at all. But it would be great if it's possible to get 2-dimensional array w on initial memory (with compiling with intel compiler too). So is there any way to do it?

C++ does not allow non-constant expressions as array dimensions, while modern C allows it. So pick one language for the question, instead of two. — Lundin
– Lundin, Commented Dec 7, 2015 at 7:21
What you're describing is the concept of a view into another container. In your case the container is a 1-dimensional array and the view presents it to the caller as a 2-dimensional array. One such implementation of a view is provided by the Boost.MultiArray library. — legalize
– legalize, Commented Dec 10, 2015 at 22:26

Budo Zindovic · Accepted Answer · 2015-12-07 13:36:41Z

1

If wals is a class, you can implement your access operator to function as if it were 2D array.

walsdataType walsclass::operator()(int i, int j){return walsdata[i*N+j]};

with walsdata being the class member for storing data and N being the row length. You should do the bound checking as well.

edited Dec 7, 2015 at 13:36

answered Dec 7, 2015 at 7:22

Budo Zindovic

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4 Comments

RedX Over a year ago

What operator is that supposed to be?

Budo Zindovic Over a year ago

That is the access operator. You use it to access the data stored in your member walsdata like this: walsinstance(i,j)

RedX Over a year ago

I don't know any access operator in c++. Can you be more specific? Did you mean operator()(int i, intj)?

Budo Zindovic Over a year ago

Yes, since you will be making your own class (I've named it walsclass but you can name it whatever you prefer) and you should implement your access operator, namely operator()(int i, int j). I've also realized that I've made a typo and will correct my answer.

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