How to dynamically allocate a 2D array in C?
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
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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 <stdio.h> #include <stdlib.h> 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; } |
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Time Complexity : O(R*C), where R and C is size of row and column respectively.
Auxiliary Space: O(R*C), where R and C is size of row and column respectively.
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 <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 */ for ( int i = 0; i < r; i++) free (arr[i]); return 0; } |
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Time Complexity: O(R*C)
Here R and C is size of row and column respectively.
Auxiliary Space: O(R*C)
The extra space is used to store the elements of the matrix.
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 <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 */ for ( int i = 0; i < r; i++) free (arr[i]); free (arr); return 0; } |
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Time Complexity: O(R*C)
Here R and C is size of row and column respectively.
Auxiliary Space: O(R*C)
The extra space is used to store the elements of the matrix.
4) Using double pointer and one malloc call
C
#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 = ( 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; } |
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Time Complexity: O(R*C)
Here R and C is size of row and column respectively.
Auxiliary Space: O(R*C)
The extra space is used to store the elements of the matrix.
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 <stdio.h> #include <stdlib.h> 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; } |
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Time Complexity: O(R*C)
Here R and C is size of row and column respectively.
Auxiliary Space: O(R*C)
The extra space is used to store the elements of the matrix.
6) Using a pointer to the first row of VLA
Similar to 5 but allows arr[i][j] syntax.
C
#include <stdio.h> #include <stdlib.h> 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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Time Complexity: O(R*C)
Here R and C is size of row and column respectively.
Auxiliary Space: O(R*C)
The extra space is used to store the elements of the matrix.