CODEWITHSUNDEEP
carrays
Hairi
Hairi

Reputation: 3715

Why I can't dereference a two dimentional array?

I have the following code:

void main()
{
  int a[3][4] = {{1,2,3,4},{11,12,13,14},{21,22,23,24}};   
  printf("%d\n",*a);
  system("pause");  
}

The output is the address of the first element of the array. Why? I expect this address to be dereferenced.

Upvotes: 0

Views: 102

Answers (2)

PU.
PU.

Reputation: 148

A multidimensional array in C is contiguous. The following:

int a[4][5];

consists of 4 int[5]s laid out next to each other in memory.

An array of pointers:

int *a[4];

is jagged. Each pointer can point to (the first element of) a separate array of a different length.

a[i][j] is equivalent to ((a+i)+j). See the C11 standard, section 6.5.2.1:

The definition of the subscript operator [] is that E1[E2] is identical to (*((E1)+(E2)))

Thus, a[i][j] is equivalent to (*(a+i))[j], which is equivalent to ((a+i)+j).

This equivalence exists because in most contexts, expressions of array type decay to pointers to their first element (C11 standard, 6.3.2.1). a[i][j] is interpreted as the following:

a is an array of arrays, so it decays to a pointer to a[0], the first subarray. a+i is a pointer to the ith subarray of a. a[i] is equivalent to *(a+i), dereferencing a pointer to the ith subarray of a. Since this is an expression of array type, it decays to a pointer to a[i][0]. a[i][j] is equivalent to ((a+i)+j), dereferencing a pointer to the jth element of the ith subarray of a.

Note that pointers to arrays are different from pointers to their first element. a+i is a pointer to an array; it is not an expression of array type, and it does not decay, whether to a pointer to a pointer or to any other type.

and for some reason printing *a in single dimensional array will print the first element of the array whereas *a in a multidimensional array will print a random number. Why is this so?

The output is the address of the first element of the array. Why?

printf("%d\n",*a);

*a will be equivalent to a[0] that prints first element of the array.

Upvotes: 1

Steffen
Steffen

Reputation: 2431

To get the content you might want to dereference twice. The first element can be accessed by:

**a

or

*a[0]

Upvotes: 3

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