1D array into a 2D array C++

Question

Here I have an array of 10. How can I make this into a 2D array of say 2 by 5.

double arr1D[10] = {1,2,3,4,5,6,7,8,9,10};

What I want the 2D array to be like:

double arr2D[2][5] = {1,2,3,4,5},{6,7,8,9,10};

Also how would I do it if I had a pointer to an array. So kinda like this:

double arr1D[10];
double*ptr;
ptr = arr1D;

Answer 1

Lets take a closer look at how your arr1D would look like in memory:

+---+---+---+---+---+---+---+---+---+----+
| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 |
+---+---+---+---+---+---+---+---+---+----+

It's a simple and contiguous chunk of memory.

Now how would that arr2D look like in memory? Actually just the same!

So the simplest way is just to use eg std::copy_n to copy from one array to the other:

double arr1D[10] = {1,2,3,4,5,6,7,8,9,10};
double arr2D[2][5];

std::copy_n(&arr1D[0], 10, &arr2D[0][0]);

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For the last code snippet, you don't have a "2D array". Instead the pointer variable ptr will point to the first element of arr1D . You can then use ptr almost as you use arr1D (the biggest difference is that sizeof arr1D != sizeof ptr ).