One-dimensional Array Dynamic Memory Allocation

First, let's start with a simple dynamic memory allocation.

#include "stdio.h"
#include "stdlib.h"
#define N 5

int main() {
    int* arr = (int*)malloc(sizeof(int)*N);

    for(int i = 0; i < N; i++) {
        arr[i] = i * i;
    }

    for(int i = 0; i < N; i++) {
        printf("\n%d", arr[i]);
    }

    free(arr);
    arr = NULL;
    for(int i = 0; i < N; i++) {
        printf("\n%d", arr[i]);
    }
}

Dynamic memory allocation for one-dimensional arrays requires only the required memory size (sizeof(int)*N) to allocate array elements as needed.

2-D Array Dynamic Memory Allocation

First, for a neat two-dimensional array, there are two methods of allocation:

• Allocate all memory space at once
• Allocate all memory space twice

One allocation

Assuming we want to allocate memory space for an array of m*n, we can allocate all memory space at once.

#include "stdio.h"
#include "stdlib.h"
#define m 5
#define n 4

int main() {
    int* arr = (int*)malloc(sizeof(int)*m*n);

    for (int i = 0; i < m; ++i) {
        for (int j = 0; j < n; ++j) {
            arr[i*n + j] = 99;
        }
    }

    for (int i = 0; i < m; ++i) {
        for (int j = 0; j < n; ++j) {
            printf("\n%d", arr[i*n + j]);
        }
    }

}

The memory allocation of the above two-dimensional array is simple, but it is not very convenient to use, so this method is rarely used.

Secondary Allocation

#include "stdio.h"
#include "stdlib.h"
#define m 5
#define n 4

int main() {

    int** arr = (int**)malloc(sizeof(int*)*m);

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

    for (int i = 0; i < m; ++i) {
        for (int j = 0; j < n; ++j) {
            arr[i][j] = 99;
        }
    }

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

First, an array of pointers is allocated, each element in the array is an array pointer, which points to a one-dimensional array. Then, memory space is allocated to the array pointer. Compared with the memory allocation of the previous one-dimensional array, this two-dimensional memory allocation is allocated twice, so the allocated memory is not contiguous.

From the memory allocation method above, we know that we can create an irregular two-dimensional array.

#include "stdio.h"
#include "stdlib.h"
#define m 5
#define n 4

int main() {
    int** arr = (int**)malloc(sizeof(int*)*m);

    arr[0] = (int*)malloc(sizeof(int*)*1);
    arr[1] = (int*)malloc(sizeof(int*)*2);
    arr[2] = (int*)malloc(sizeof(int*)*3);
    arr[3] = (int*)malloc(sizeof(int*)*4);
    arr[4] = (int*)malloc(sizeof(int*)*5);

    for (int i = 0; i < 1; i++) {
        arr[0][i] = 9;
    }
    for (int i = 0; i < 2; i++) {
        arr[1][i] = 9;
    }
    for (int i = 0; i < 3; i++) {
        arr[2][i] = 9;
    }
    for (int i = 0; i < 4; i++) {
        arr[3][i] = 9;
    }
    for (int i = 0; i < 5; i++) {
        arr[4][i] = 9;
    }

    for (int i = 0; i < m; ++i) {
        for (int j = 0; j < i+1; ++j) {
            arr[i][j] = 99;
        }
    }

    for (int i = 0; i < m; ++i) {
        for (int j = 0; j < i+1; ++j) {
            printf("\n%d", arr[i][j]);
        }
    }
}

Dynamic memory allocation with structs and two-dimensional arrays

#include "stdio.h"
#include "stdlib.h"
#define m 5
#define n 4

typedef struct {
    int** sums;
    int sumsSize;
} NumMatrix;

int main() {
    int row = 5;
    int col[5] = {1, 2, 3, 4, 5};

    NumMatrix* ret = (NumMatrix*)malloc(sizeof(NumMatrix));
    ret->sums = (int **)malloc(sizeof(int*)*row);

    for (int i = 0; i < row; ++i) {
        ret->sums[i] = (int*)malloc(sizeof(int)*col[i]);
    }

    for (int i = 0; i < row; ++i) {
        for (int j = 0; j < col[i]; ++j) {
            ret->sums[i][j] = 99;
        }
    }

    for (int i = 0; i < row; ++i) {
        for (int j = 0; j < col[i]; ++j) {
            printf("\n%d", ret->sums[i][j]);
        }
    }
}

First, let's look at this code. In a structure, there is a two-dimensional array pointer.

So the order in which we allocate memory should follow these steps:

• Allocate memory for the structure first
• Then allocate memory as previously allocated for a two-dimensional array