How to dynamically allocate a 2D array in C?

• Difficulty Level : Hard
• Last Updated : 12 Jan, 2022

Following are different ways to create a 2D array on the heap (or dynamically allocate a 2D array).
In the following examples, we have considered ‘r‘ as number of rows, ‘c‘ as number of columns and we created a 2D array with r = 3, c = 4 and the following values

1  2  3  4
5  6  7  8
9  10 11 12

1) Using a single pointer and a 1D array with pointer arithmetic:
A simple way is to allocate a memory block of size r*c and access its elements using simple pointer arithmetic.

C

 #include #include  int main(void){    int r = 3, c = 4;     int* ptr = malloc((r * c) * sizeof(int));     /* Putting 1 to 12 in the 1D array in a sequence */    for (int i = 0; i < r * c; i++)        ptr[i] = i + 1;     /* Accessing the array values as if it was a 2D array */    for (int i = 0; i < r; i++) {        for (int j = 0; j < c; j++)            printf("%d ", ptr[i * c + j]);        printf("\n");    }     free(ptr);     return 0;}

Output:

1 2 3 4
5 6 7 8
9 10 11 12

2) Using an array of pointers
We can create an array of pointers of size r. Note that from C99, C language allows variable sized arrays. After creating an array of pointers, we can dynamically allocate memory for every row.

C

 #include #include  int main(){    int r = 3, c = 4, i, j, count;     int* arr[r];    for (i = 0; i < r; i++)        arr[i] = (int*)malloc(c * sizeof(int));     // Note that arr[i][j] is same as *(*(arr+i)+j)    count = 0;    for (i = 0; i < r; i++)        for (j = 0; j < c; j++)            arr[i][j] = ++count; // Or *(*(arr+i)+j) = ++count     for (i = 0; i < r; i++)        for (j = 0; j < c; j++)            printf("%d ", arr[i][j]);     /* Code for further processing and free the      dynamically allocated memory */     for (int i = 0; i < r; i++)        free(arr[i]);     return 0;}

Output:

1 2 3 4 5 6 7 8 9 10 11 12

3) Using pointer to a pointer
We can create an array of pointers also dynamically using a double pointer. Once we have an array pointers allocated dynamically, we can dynamically allocate memory and for every row like method 2.

C

 #include #include  int main(){    int r = 3, c = 4, i, j, count;     int** arr = (int**)malloc(r * sizeof(int*));    for (i = 0; i < r; i++)        arr[i] = (int*)malloc(c * sizeof(int));     // Note that arr[i][j] is same as *(*(arr+i)+j)    count = 0;    for (i = 0; i < r; i++)        for (j = 0; j < c; j++)            arr[i][j] = ++count; // OR *(*(arr+i)+j) = ++count     for (i = 0; i < r; i++)        for (j = 0; j < c; j++)            printf("%d ", arr[i][j]);     /* Code for further processing and free the       dynamically allocated memory */     for (int i = 0; i < r; i++)        free(arr[i]);     free(arr);     return 0;}

Output:

1 2 3 4 5 6 7 8 9 10 11 12

4) Using double pointer and one malloc call

C

 #include#include int main(){    int r=3, c=4, len=0;    int *ptr, **arr;    int count = 0,i,j;     len = sizeof(int *) * r + sizeof(int) * c * r;    arr = (int **)malloc(len);     // ptr is now pointing to the first element in of 2D array    ptr = (int *)(arr + r);     // for loop to point rows pointer to appropriate location in 2D array    for(i = 0; i < r; i++)        arr[i] = (ptr + c * i);     for (i = 0; i < r; i++)        for (j = 0; j < c; j++)            arr[i][j] = ++count; // OR *(*(arr+i)+j) = ++count     for (i = 0; i < r; i++)        for (j = 0; j < c; j++)            printf("%d ", arr[i][j]);     return 0;}

Output:

1 2 3 4 5 6 7 8 9 10 11 12

Thanks to Trishansh Bhardwaj for suggesting this 4th method.

5) Using a pointer to Variable Length Array.

The dimensions of VLA are bound to the type of the variable. Therefore one form a pointer to an array with run-time defined shape.
The pointer has to be dereferenced before subscripting with syntax (*arr)[i][j].

C

 #include #include  int main(){    int row = 3, col = 4, i, j, count;     int (*arr)[row][col] = malloc(sizeof *arr);         count = 0;    for (i = 0; i < row; i++)        for (j = 0; j < col; j++)            (*arr)[i][j] = ++count;     for (i = 0; i < row; i++)        for (j = 0; j < col; j++)            printf("%d ", (*arr)[i][j]);     free(arr);         return 0;}

6) Using a pointer to the first row of VLA

Similar to 5 but allows arr[i][j] syntax.

C

 #include #include  int main(){    int row = 3, col = 4, i, j, count;     int (*arr)[col] = calloc(row, sizeof *arr);         count = 0;    for (i = 0; i < row; i++)        for (j = 0; j < col; j++)            arr[i][j] = ++count;     for (i = 0; i < row; i++)        for (j = 0; j < col; j++)            printf("%d ", arr[i][j]);     free(arr);         return 0;}

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