C Language Notes (two-dimensional array and numerical pointer), two-dimensional array pointer

C Language Notes (two-dimensional array and numerical pointer), two-dimensional array pointer

I. Differences between two-dimensional arrays and two-dimensional arrays

(1) One-dimensional arrays are stored continuously in the memory. Likewise, two-dimensional arrays are stored continuously in the memory. Therefore, from the perspective of memory, there is no essential difference between a one-dimensional array and a two-dimensional array.

(2) Two-dimensional arrays can be replaced by one-dimensional arrays. However, in practical applications, it is more intuitive to use a two-dimensional array to facilitate program programming.

(3) The two have the same memory usage efficiency.

 

2. concepts of the first dimension and the second dimension of a two-dimensional array

(1) For example, In int a [2] [5], 2 indicates the first dimension; 5 indicates the second dimension.

(2) The first dimension of a two-dimensional array represents the most external layer. The first dimension is an array and contains two elements, which are the second-dimensional arrays. The second-dimensional array is also an array. The elements in the array are common int variables.

3. subscript access to two-dimensional arrays

Example 1:

1 int a [2] [5] = {{1,2,3,4,5}, {6,7,8,9,10}};
2 int (* p) [5]; // define an array pointer
3 p = a;
4
5 printf ("a [1] [2] =% d. \ N", a [1] [2]); // a [1] [2] = 8
6 printf ("(* (p + 1) +1) =% d. \ N", * (* (p + 1) +2)); // a [1] [2]

Running result:

 

4. symbols that must be understood about two-dimensional data

Example 2: Understanding a, & a, a [0], & a [0], a [0] [0], & a [0] [0]

1 /*
  2 Testing of several symbols in a two-dimensional array
  3 1, a is equivalent to & a [0]
  4 2. a [0] is equivalent to & a [0] [0]
  5 3. In terms of value, a, & a, a [0], & a [0], & a [0] [0] are equal, but there are differences in types.
  6 * /
  7 int a [2] [5] = {{1,2,3,4,5}, {6,7,8,9,10}};
  8     
  9 printf ("a =% p. \ N", a); // a is of type int (*) [5]
10 printf ("& a =% p. \ N", & a); // & a is of type int (*) [2] [5]
11 printf ("a [0] =% p. \ N", a [0]); // a [0] is of type int *
12 printf ("& a [0] =% p. \ N", & a [0]); // & a [0] is of type int (*) [5]
13 printf ("a [0] [0] =% d. \ N", a [0] [0]); // a [0] [0] is of type int
14 printf ("& a [0] [0] =% p. \ N", & a [0] [0]); // & a [0] [0] is of type int *

Running result:

 

Example 3: Use of array pointers in dimension 1 and dimension 2

1 // Combination of two-dimensional array and pointer
  2 int a [2] [5] = {{1,2,3,4,5}, {6,7,8,9,10}};
  3
  4 int (* p1) [5]; // array pointer
  5 int * p2; // general pointer
  6 p1 = a; // equivalent to p1 = & a [0]; // array name pointing to a two-dimensional array
  7 p2 = a [0]; // equivalent p2 = & a [0] [0]; // first-dimensional array pointing to two-dimensional array
  8 
  9 printf ("a [0] [2] =% d. \ N", * (* (p1 + 0) +2)); // a [0] [2] = 3
10 printf ("a [1] [2] =% d. \ N", * (* (p1 + 1) +2)); // a [1] [2] = 8
11
12 printf ("a [0] [2] =% d. \ N", * (p2 + 2)); // a [0] [2] = 3
13 printf ("a [0] [4] =% d. \ N", * (p2 + 4)); // a [0] [4] = 5 

Running result:

 

V. Summary

(1) understand the essence of a two-dimensional array and the meanings of several symbols.

(2) Two-dimensional arrays and array pointers are closely related. Learn to use array pointers to operate two-dimensional arrays, and focus on practical operations to deepen understanding.

 

Note: The study notes are extracted from the IOT video tutorial by instructor Zhu youpeng.