What is the reason C compiler demands that number of columns in a 2d array will be defined?

Question

given the following function signature:

void readFileData(FILE* fp, double inputMatrix[][], int parameters[])

this doesn't compile.

and the corrected one:

void readFileData(FILE* fp, double inputMatrix[][NUM], int parameters[])

my question is, why does the compiler demands that number of columns will be defined when handling a 2D array in C? Is there a way to pass a 2D array to a function with an unknown dimensions?

thank you

Answer 1

Built-in multi-deminsional arrays in C (and in C++) are implemented using the "index-translation" approach. That means that 2D (3D, 4D etc.) array is laid out in memory as an ordinary 1D array of sufficient size, and the access to the elements of such array is implemented through recalculating the multi-dimensional indices onto a corresponding 1D index. For example, if you define a 2D array of size M x N

double inputMatrix[M][N]

in reality, under the hood the compiler creates an array of size M * N

double inputMatrix_[M * N];

Every time you access the element of your array

inputMatrix[i][j]

the compiler translates it into

inputMatrix_[i * N + j]

As you can see, in order to perform the translation the compiler has to know N, but doesn't really need to know M. This translation formula can easily be generalized for arrays with any number of dimensions. It will involve all sizes of the multi-dimensional array except the first one. This is why every time you declare an array, you are required to specify all sizes except the first one.