a problem of 2D array...

[saisai]

hello i am a newbie in c++
and now got confused in the 2D array

i want to represent xy coordinate in a 2D array
however i'm confused with the array representation
actually is that ,let say, a[0][1] is actually (1,0) on the xy plane?
if i want to create a plane with width=5, height=4, is that i should create an array plane[4][5]?
how to understand how a 2D array represent a xy plane more easily?

I m really confused with this>.<
i m working this for finding the shortest path....
i dunno wht to do....
thanks

[Ganon11]

Remember that, since you are the programmer, you can determine if a[0][1] will correspond to the point (0, 1) or the point (1, 0). It's completely up to you. I do know, however, that when printing and processing in the common row-column form, you will be processing in a y, x fashion. If you want your rows to be x's and your columns to be y's (a.k.a. array[X_VARIABLE][Y_VARIABLE]), you can process/display your array in column-row order. But the ultimate decision is up to you.

[weaknessforcats]

Avoid thinking a 2D array corresponds to an x-y coordinate. C++ has only one-dimensional arrays:

int arr[3];
 

is an array of 3 elements where each element is an int.

int arr[3][3];
 

is an array of 3 elements where each element is an array of 3 int.

So, the first index is the number of elements in the array and the other indexes just describe the elements.

int array[9] = {1,2,3,4,5,6,7,8,9};
 

has a memory layout of:

1 2 3 4 5 6 7 8 9

and this array:
[code]
int tictactoe[3][3] = {1,2,3,4,5,6,7,8,9};

has a memory layout of

1 2 3 4 5 6 7 8 9

So, in memory the arrays are identical.

The indexes descirbe how the array is to be viewed. In the case of array[9], the element with the 8 is array[7]. That is, the 8th element (counting from 0). In teh case of tictactoe[3][3], the 8 is tictactoe[2][1]. That is, it is in the 3rd array (which is tictactoe[2]) and it is element 1 in that array giving tictactoe[2][1].

Locating an element in array, like array[7], consists of adding the size of 7 ints to the address of the array: array[7] = address of array + 7 * sizeof(int).

Locating an element in tictactoe, like tictactoe[2][1] consists of adding 2 times the size of an array of 3 int plus the size of 1 int: tictactoe[2][1] = address of tictactoe + 2* sizeof(3* sizeof(int))+ 1*sizeof(int).

In both cases, the starting address of each array has been increased by the size of 7 ints.

So, the "dimensions" are really how you want to view the array in memory. Regardless of your choice of dimensions, the pointer arithmetic must yield the same address for an element or the compiler won't be able to locate elements.