C Language Basics Tutorial (iv) Pointers, structures, unions, and enumerations (5)

2.2. Pointers to two-dimensional arrays
Address of 2.2.1 Two-D array elements
To illustrate the problem, we define the following two-D arrays:
int a[3][4]={{0,1,2,3},{4,5,6,7},{8,9,10,11}};
A is a two-dimensional array name, which has 3 rows, 4 columns, and a total of 12 elements. But it can also be understood that array a consists of three elements: a[0],a[1],a[2]. And it evenly each element is a one-dimensional array, and contains 4 elements (the equivalent of 4 columns), for example, a[0] represented by the one-dimensional array contains 4 elements of a[0][0],a[0][1],a[0][2], a[0][3]. As shown in Figure 5:
┏━━━━┓┏━┳━┳━┳━┓
A─→┃a[0]┃─→┃0┃1┃2┃3┃
┣━━━━┫┣━╋━╋━╋━┫
┃a[1]┃─→┃4┃5┃6┃7┃
┣━━━━┫┣━╋━╋━╋━┫
┃a[2]┃─→┃8┃9┃10┃11┃
┗━━━━┛┗━┻━┻━┻━┛
Figure 5.
But from the point of view of the two-dimensional array, a represents the first address of the two-dimensional array and, of course, the first address of the No. 0 row of the two-dimensional array. A+1 on behalf of the first address of line 1th, a+2 on behalf of the first address of line 2nd. If the first address of this two-dimensional array is 1000, because the No. 0 row has 4 integer elements, a+1 is 1008,a+2 1016. As shown in Figure 6.
A[3][4]
A┏━┳━┳━┳━┓
(1000) ─→┃0┃1┃2┃3┃
A+1┣━╋━╋━╋━┫
(1008) ─→┃4┃5┃6┃7┃
A+2┣━╋━╋━╋━┫
(1016) ─→┃8┃9┃10┃11┃
┗━┻━┻━┻━┛
Figure 6.
Since we see a[0],a[1],a[2] as a one-dimensional array name, we can assume that they represent the first address of their corresponding array, which means that a[0] represents the address of the No. 0 column element in line No. 0, that is, &a[0][0],a[1] is the address of the No. 0 column element in line 1th. &a[1][0], according to the address Operation rule, a[0]+1 represents the address of the 1th column element in line No. 0, that is &a[0][1], in general, A[i]+j represents the address of the J-Column element in line I, that is, &a[i][j.
In addition, in a two-dimensional array, we can also represent the addresses of each element in the form of pointers. As mentioned earlier, A[0] is equivalent to * (a+0) equivalence, a[1] and * (a+1), so a[i]+j is equivalent to * (A+i) +j, which represents the address of the array element A[i][j].
Therefore, two-dimensional array element A[i][j] can be expressed as either * (A[I]+J) or * (* (a+i) +j), which are equivalent to A[i][j] or written (* (A+i)) [j].
In addition, to add, if you write a program output print a and *a, you can find that their values are the same, this is why? We can understand this: first of all, in order to illustrate the problem, we consider the two-dimensional array as a a[0],a[1],a[2 of three array elements, and consider a[0],a[1],a[2 as an array name and a one-dimensional array consisting of 4 elements respectively. Therefore, a represents the address of row No. 0 of the array, and *a is a[0], which is the array name, or, of course, the address, which is the address of the No. 0 column element in the array No. 0 row.