Following are different ways to create a 2D array on 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 following values
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1) Using a single pointer:
A simple way is to allocate memory block of size r*c and access elements using simple pointer arithmetic.
#include <stdio.h> #include <stdlib.h> int main() { int r = 3, c = 4; int *arr = ( int *) malloc (r * c * sizeof ( int )); int i, j, count = 0; for (i = 0; i < r; i++) for (j = 0; j < c; j++) *(arr + i*c + j) = ++count; for (i = 0; i < r; i++) for (j = 0; j < c; j++) printf ( "%d " , *(arr + i*c + j)); /* Code for further processing and free the dynamically allocated memory */ return 0; } |
Output:
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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.
#include <stdio.h> #include <stdlib.h> 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 */ return 0; } |
Output:
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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.
#include <stdio.h> #include <stdlib.h> 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 */ return 0; } |
Output:
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4) Using double pointer and one malloc call
#include<stdio.h> #include<stdlib.h> 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 = 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:
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Thanks to Trishansh Bhardwaj for suggesting this 4th method.
This article is attributed to GeeksforGeeks.org
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