Inserting 1D array to 2D array

Question

Can we directly insert a 1D array to a 2D array?

For example I have this code:

void insert(int[]data , int**collection)
{
collection[1] = data
}
int main()
{
int data[2]= {1,3}
int collection[2][2];
insert(data,&collection);
}

Will this work?

`int **` is not a 2D array. It may be used to manage data in a "2d-like" fashion. But it is still something totally different than `int [2][2]`. Oh, and you cannot do array-assignments in C (unless it is part of a struct assignment), only pointer assignments. — HAL9000, Jan 06 '22 at 02:48
@HAL9000 how do you make a call by reference in c for 2D arrays? — Miss Mulan, Jan 06 '22 at 03:07
@HAL9000 "Oh, and you cannot do array-assignments in C (unless it is part of a struct assignment), only pointer assignments" Do you mean this line of code is wrong?"collection[1] = data"? — Miss Mulan, Jan 06 '22 at 03:09
This may help explain why your code does not work: [What is array to pointer decay?](https://stackoverflow.com/q/1461432/12149471) — Andreas Wenzel, Jan 06 '22 at 04:39

score 0 · Accepted Answer · answered Jan 06 '22 at 03:16

You cannot insert a 1D array to 2D array the way you are doing. Use memcpy to copy the elements of 1D array to 2D array, like this:

memcpy(collection[1], data, 2 * sizeof(int));

this will copy the 2 integer elements of data array to collection[1].

Besides, a couple of problems in your code. Lets discuss them:

First:

insert(data,&collection);
            ^

You don't need to pass the address of collection. Note that, an array, when used in an expression, will convert to pointer to its first element (there are few exceptions to this rule). That means, when you pass collection, it will convert to type int (*)[2]. Just do:

insert(data, collection);

Second:

void insert(int[]data , int**collection)

int[]data is wrong. The first parameter of insert() should be int data[2] or int data[], both are equivalent to int * data. You can use either of them.

The second argument to insert() is collection array which is a 2D array of integers. When you pass it to insert(), it will decay to pointer whose type is int (*)[2]. The type of second parameter of insert() is int ** which is not compatible with the argument that you are passing to insert() function. The second parameter of insert() function should be int collection[2][2] or int collection[][2], both are equivalent to int (*collection)[2].

Putting these altogether, you can do:

#include <stdio.h>
#include <string.h>

#define ROW 2
#define COL 2

void insert(int data[ROW], int collection[ROW][COL]) {
    //for demonstration purpose, copying elements of data array 
    //to all elements (1D array) of collection array.
    for (int i = 0; i < ROW; ++i) {
        memcpy(collection[i], data, COL * sizeof(int));
    }
}

int main(void) {
    int data[COL] = {1, 3};
    int collection[ROW][COL];

    insert(data, collection);

    for (int i = 0; i < 2; ++i) {
        for (int j = 0; j < 2; ++j) {
            printf("collection[%d][%d] : %d ", i, j, collection[i][j]);
        }
        printf("\n");
    }

    return 0;
}

Output:

# ./a.out
collection[0][0] : 1 collection[0][1] : 3 
collection[1][0] : 1 collection[1][1] : 3

Inserting 1D array to 2D array

1 Answers1