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2 Dimensional Arrays - Behind the Scenes

As with 1D arrays, a declaration such as
     int a[MAX_ROWS][MAX_COLS];

allocates a collection of integer cells, and a pointer which is given the name, a, which is initialized to point to the beginning of the memory allocated. However, a 2D array must still be stored in linear memory. The convention for C is that the 2 dimensions of the array are stored in row major order in memory. This means that all of the elements of row 0 are stored, followed by all the elements of row 1, then row 2, etc.

As with 1D arrays, the name of the array is a pointer to the beginning of the memory block allocated. In order to calculate the address of an arbitrary cell in the array, a[i][j], the compiler uses an expression like:

     *(a + i * MAX_COLS * sizeof(int) + j * sizeof(int))

To evaluate such an expression inside a function, the function must know the number of columns in the structure.

So, to pass the array, we must declare it in the parameter list like:

     process(int a[][MAX_COLS])

NOTE: We MUST provide the number of columns in the declaration.

2D arrays with pointers

As we can see, 2D arrays are implemented and accessed with pointers, just like 1D arrays.
     a         <===>     &a[0][0]

     a[0]      <===>     &a[0][0]

     a[1]      <===>     &a[1][0]

     ...

     a[i]      <===>     &a[i][0]

Essentially, a single subscripted expression for a 2D array refers to a 1D array - i.e. one row.

We rarely use the pointer expressions for individual cells in a 2D array.

C also allows multi-dimension arrays:

     float b[MAX_ROWS][MAX_COLS][MAX_DEPTH];

for a 3D array.

We can now use these 2D arrays to improve the stock program.


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