About pointers in C

Question

I started to read a few articles about pointers in C and I've got one example that I don't understand.

The example is from here: http://en.wikibooks.org/wiki/C_Programming/Pointers_and_arrays

Here it is:

Let's look at a slightly different problem. We want to have a two dimensional array, but we don't need to have all the rows the same length. What we do is declare an array of pointers. The second line below declares A as an array of pointers. Each pointer points to a float. Here's some applicable code:

float  linearA[30];
 float *A[6];

 A[0] = linearA;              /*  5 - 0 = 5 elements in row  */
 A[1] = linearA + 5;          /* 11 - 5 = 6 elements in row  */
 A[2] = linearA + 11;         /* 15 - 11 = 4 elements in row */
 A[3] = linearA + 15;         /* 21 - 15 = 6 elements        */
 A[4] = linearA + 21;         /* 25 - 21 = 4 elements        */
 A[5] = linearA + 25;         /* 30 - 25 = 5 elements        */

 A[3][2] = 3.66;          /* assigns 3.66 to linearA[17];     */
 A[3][-3] = 1.44;         /* refers to linearA[12];           
                             negative indices are sometimes useful.
                             But avoid using them as much as possible. */

My question is why A[0] is a pointer only to five elements and not to ALL of linearA, since the name of an array is a pointer to its first member.

And A[1] = linearA + 5; is 6 elements in a row -- for the same reason? Isn't A[1] supposed to be a pointer to the 6th member of linearA?

Can someone explain where is my mistake?

Answer 1

Except in a few exceptions, in C an array name is converted to a pointer to the first element of the array. linearA is an array 30 of float and in the expression:

A[0] = linearA;

it is converted to a pointer to float.

A is an array 6 of pointer to float. The element of A are of type pointer to float. So A[0] is a pointer to float and not a pointer to an array.

And A[i][j] in C is equivalent to *(A[i] + j) so A[i][j] is a float (dereferencing a pointer to float yields a float).