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16.7 Multidimensional Arrays

Strictly speaking, all arrays in C are unidimensional. However, you can create an array of arrays, which is more or less equivalent to a multidimensional array. For example,

struct chesspiece *board[8][8];

declares an array of 8 arrays of 8 pointers to struct chesspiece. This data type could represent the state of a chess game. To access one square’s contents requires two array index operations, one for each dimension. For instance, you can write board[row][column], assuming row and column are variables with integer values in the proper range.

How does C understand board[row][column]? First of all, board is converted automatically to a pointer to the zeroth element (at index zero) of board. Adding row to that makes it point to the desired element. Thus, board[row]’s value is an element of board—an array of 8 pointers.

However, as an expression with array type, it is converted automatically to a pointer to the array’s zeroth element. The second array index operation, [column], accesses the chosen element from that array.

As this shows, pointer-to-array types are meaningful in C. You can declare a variable that points to a row in a chess board like this:

struct chesspiece *(*rowptr)[8];

This points to an array of 8 pointers to struct chesspiece. You can assign to it as follows:

rowptr = &board[5];

The dimensions don’t have to be equal in length. Here we declare statepop as an array to hold the population of each state in the United States for each year since 1900:

#define NSTATES 50
  int nyears = current_year - 1900 + 1;
  int statepop[NSTATES][nyears];

The variable statepop is an array of NSTATES subarrays, each indexed by the year (counting from 1900). Thus, to get the element for a particular state and year, we must subscript it first by the number that indicates the state, and second by the index for the year:

statepop[state][year - 1900]

The subarrays within the multidimensional array are allocated consecutively in memory, and within each subarray, its elements are allocated consecutively in memory. The most efficient way to process all the elements in the array is to scan the last subscript in the innermost loop. This means consecutive accesses go to consecutive memory locations, which optimizes use of the processor’s memory cache. For example:

int total = 0;
float average;

for (int state = 0; state < NSTATES, ++state)
    for (int year = 0; year < nyears; ++year)
        total += statepop[state][year];

average = total / nyears;

C’s layout for multidimensional arrays is different from Fortran’s layout. In Fortran, a multidimensional array is not an array of arrays; rather, multidimensional arrays are a primitive feature, and it is the first index that varies most rapidly between consecutive memory locations. Thus, the memory layout of a 50x114 array in C matches that of a 114x50 array in Fortran.

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