指向二维动态数组指针的指针

Question

How to create a pointer to the following 2D array pointer int **arr. I am looking for something similar to int*** arr and its allocation, deallocation, accessing and usage.

int main()
{
  int** arr;

  int row = 11;
  int col = 2;
  int i,j;

  arr = (int**)malloc(sizeof(int*)*row);
  for (i=0; i<row; ++i){
    arr[i] = (int*)malloc(sizeof(int)*col);
  }

  for (i = 0; i < row; i++) {
    for (j = 0; j < col; j++) {
        arr[i][j] = i;
        printf("arr[%d][%d] : %d\n",i,j,arr[i][j]);
    }
  }

  return 0;
}

Previous references:

Dynamic 2D array Pointer:

C programming initialize 2D array dynamically

Pointer to dynamic 2D array, But the explanation here is using new keyword and I want to use malloc instead:

How to allocate a 2D array of pointers in C++

Additional Info:

Consider following scenario:

XYZ* arr[A][B];

"XYZ" is a custom datatype, "A" & "B" are dynamic variables at runtime

In this case, how can we define A & B at runtime and access arr[A][B] element values using XYZ* type.

Answer 1

Further down the answer, you find more possible solutions.

First, I want to present a solution to you, where you manage a dynamic 2d array with a 1d pointer, in case of same-length columns.

#include <stdlib.h>
#include <stdio.h>

struct XYZ
{
    int someValue;
};

struct XYZ* GetArrayItem(struct XYZ* itemArray, int numCols, int row, int col)
{
    return &itemArray[row * numCols + col];
}

int main()
{
    int A = 5;
    int B = 4;
    struct XYZ* arr = (struct XYZ*)calloc(A*B, sizeof(struct XYZ));

    for (int i = 0; i < A; i++)
    {
        for (int j = 0; j < B; j++)
        {
            GetArrayItem(arr, B, i, j)->someValue = 1;
        }
    }

    free(arr);

    return 0;
}

With columns of different length, a double pointer might be a viable solution.

struct XYZ
{
    int someValue;
};

int main()
{
    int i;
    int j;
    // row count
    int A = 5;
    // column count per row
    int B[] = { 3, 4, 3, 2, 4 };
    struct XYZ** arr = (struct XYZ**)calloc(A, sizeof(struct XYZ*));
    for (i = 0; i < A; i++)
    {
        // initialize column for each row
        arr[i] = (struct XYZ*)calloc(B[i], sizeof(struct XYZ));
    }

    for (i = 0; i < A; i++)
    {
        for (j = 0; j < B[i]; j++)
        {
            // access items
            arr[i][j].someValue = 1;
        }
    }

    for (i = 0; i < A; i++)
    {
        free(arr[i]);
    }

    free(arr);

    return 0;
}

However, I would advise you to create a more explicit object structure in case where you need 2d data. This makes the design more explicit and the column count per row more transparent.

struct XYZ
{
    int someValue;
};

struct MyDomainSpecificRow
{
    int numColumns;
    struct XYZ* myRowData;
};

int main()
{
    int i;
    int j;
    // row count
    int A = 5;
    // column count per row
    int B[] = { 3, 4, 3, 2, 4 };
    // 1d array of rows, each containing 1d array of cells
    struct MyDomainSpecificRow* arr = (struct MyDomainSpecificRow*)calloc(A, sizeof(struct MyDomainSpecificRow));

    for (i = 0; i < A; i++)
    {
        // initialize column for each row
        arr[i].numColumns = B[i];
        arr[i].myRowData = (struct XYZ*)calloc(B[i], sizeof(struct XYZ));
    }

    for (i = 0; i < A; i++)
    {
        for (j = 0; j < arr[i].numColumns; j++)
        {
            // access items
            arr[i].myRowData[j].someValue = 1;
        }
    }

    for (i = 0; i < A; i++)
    {
        free(arr[i].myRowData);
    }

    free(arr);

    return 0;
}

Answer 2

Solution: At last, I managed to get what I was looking for.

Declaration:

NodeClass* currentNode;     //Single Pointer Declaration
NodeClass*** triplePtrNode; //Triple Pointer Declaration

Definition & Allocation:

//row, col are dynamic values at runtime

//row memory allocation
triplePtrNode = (NodeClass***)malloc(sizeof(NodeClass)*row); 
for (i=0; i<row; ++i){
    //col memory allocation
    triplePtrNode[i] = (NodeClass**)malloc(sizeof(NodeClass)*col); 
}

Usage:

for (i=0; i<memberCounts; ++i){
    *(*(triplePtrNode+i)+0) = currentNode->member1;
    *(*(triplePtrNode+i)+1) = currentNode->member2;
}