# Array pointer, pointer array, and two-digit Array

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Int * P [3] and INT (* P) [3]

* P [3] is a pointer array, which means that every element in the array is a pointer variable, and (* P) [3], P is a pointer variable that points to a one-dimensional array containing three integer elements.

View code

`Int I, j; int A [2] [3] = {3, 4, 5, 6, 7, 8}; // int * P [3]; // represents an array, the elements in the array are pointer types. There are three elements in total: int (* q) [3]; // a pointer, point to an array containing three int types (q + 1) three array elements are skipped. // the address of the three elements in the first row is stored in the p pointer array for (I = 0; I <3; ++ I) P [I] = & A [0] [I]; // The value of the address in the output pointer array for (j = 0; j <3; ++ J) cout <* P [J] <""; // The output result is 3, 4, and 5 cout <Endl; q =; // assign the starting address of array A to the Q; for (I = 0; I <2; I ++) for (j = 0; j <3; j ++) cout <* (q + I) + J) <""; // output the element system ("pause") in the array ");`
PS:, see C ++ Primer

Strictly speaking, there is no multi-dimensional array in C ++. Generally, the multi-dimensional array is actually an array, such as int arry [3] [4]; represents an array with a length of 3. Each element in the array is an array with a length of 4. When using multi-dimensional arrays, remember this to understand its application.

The following describes the relationship between multi-dimensional arrays and pointers. Like an ordinary array, when a multi-dimensional array is used, it is actually automatically converted to a pointer pointing to the first element of the array. That is to say, the array name is a pointer to the first element in the array. In a one-dimensional array, arry = & arry [0], the two addresses are the same. In a two-dimensional array, the array name points to the first element, and the first element is an array with a length of 4. We define a pointer int (* P) [4] to an array with a length of 4, and then assign the first address of the Two-dimensional array to it, P = Arry. In this way, values can be assigned. Arry = & arry [0].

Knowing the relationship between two-dimensional array names and pointers, we will understand much better when passing parameters in two-dimensional arrays. Passing parameters in two-dimensional arrays has always been a headache. Here we still pass the two-dimensional array name as a real parameter. In the form parameter that accepts the function, we only need to define a pointer pointing to an array with a specific length, for example, here we use int (* P) [4] to accept parameters such as Arry. The following code example is provided.

View code

`# Include <iostream> # include <stdlib. h> using namespace STD; // The array name is a pointer to the first element of the array. Here we define a pointer to the array to accept arry // R to indicate the number of rows in the two arrays, C indicates the number of columns in a two-dimensional array. Void printarry (INT (* arry) [4], int R, int c) {for (INT I = 0; I <r; I ++) {for (Int J = 0; j <C; j ++) {cout <arry [I] [J] <";}cout <Endl ;}} void main () {int arry [3] [4] = {,}, {,}, {,}; printarry (Arry, 3, 4); // equivalent to printarry (& arry [0], 3, 4); System ("pause ");}`

The preceding simple example of printing a two-dimensional array focuses on passing parameters of a two-dimensional array.

More optimized methods

In the above example, the form parameter must specify the arry pointer pointing to an array of the maximum length,For example, INT (* arry) [4] must be specified as 4There are some limitations, so is there a better way. The answer is yes. ConsideringTwo-dimensional arrays occupy continuous space in the memoryWe can use an array to express two-digit arrays. Rewrite the printarry method as follows:

View code

`# Include <iostream> # include <stdlib. h> using namespace STD; // input the array pointer. The number of rows and columns of the Two-dimensional array void printarry2 (int * Arry, int R, int c) {for (INT I = 0; I <r * C; I ++) {cout <arry [I] <"" ;}cout <Endl ;} void main () {int arry [4] [4] = {1, 2, 8, 9}, {2, 4, 12}, {4, 7, 10, 13}, {6, 8, 11, 15 }}; printarry2 (& arry [0] [0],); // enter the address of the first element in the first array in the array system ("pause ");}`

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