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cmultidimensional-array
Mike Egren
Mike Egren

Reputation: 205

Why can we omit the first dimension of a multidimensional array when we are passing it to a function

Why can we omit the first dimension of a multidimensional array when we are passing it to a function?

In my programming class, we were told what when passing a multidimensional array to a function we can omit first dimension, for example, a[10][15][20] can be passed as a[][15][20].

Why?

Upvotes: 10

Views: 4348

Answers (2)

Michael Krelin - hacker
Michael Krelin - hacker

Reputation: 143081

Because the array will decay to pointer and to calculate offset to the elements of the array you do not need to know the innermost dimension. Offset to a[i][j][k] is i*nj*nk+j*nk+k (where nj and nk are corresponding dimensions).

Upvotes: 11

CB Bailey
CB Bailey

Reputation: 791719

You cannot pass arrays to functions. Any function declared as taking an array type is automatically adjusted to take the corresponding pointer type. When you use the name of an array in a function call expression it decays to a pointer to its first element.

These four declarations declare the same function:

void f(int b[10][15][20]);
void f(int (*b)[15][20]);
void f(int b[][15][20]);
void f(int b[100][15][20]);

and, given int a[10][15][20];, these function calls are identical:

f(a);
f(&a[0]);

Upvotes: 4

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