I have two 2D array in C. Let's call them a[row][col] and b[row][col]. I generated random values in array a. Now i want to write a function that will count values, return 2D array and the output would be assigned to array b (it's a convoy game of life).

#include <stdio.h>
#include <stdlib.h>

int row, col = 0;

//creates row boundary
int create_row_line()
{
    printf("\n");
    for (int i = 0; i < col; i  )
    {
        printf(" -----");
    }
    printf("\n");
}

//returns the count of alive neighbours
int count_alive_neighbours(int a[row][col], int r, int c)
{
    int i, j, count = 0;
    for (i = r - 1; i <= r   1; i  )
    {
        for (j = c - 1; j <= c   1; j  )
        {
            if ((i < 0 || j < 0) || (i >= row || j >= col) || (i == r && j == c))
            {
                continue;
            }
            if (a[i][j] == 1)
            {
                count  ;
            }
        }
    }
    return count;
}

int print_state(int x[row][col]){
    create_row_line();
    for (int i = 0; i < row; i  )
    {
        printf(":");
        for (int j = 0; j < col; j  )
        {
            printf("  %d  :", x[i][j]);
        }
        create_row_line();
    }
}

int count_values(int x[row][col]){
    int y[row][col];
    for (int i = 0; i < row; i  )
    {
        for (int j = 0; j < col; j  )
        {
            int neighbours_alive = count_alive_neighbours(x, i, j);
            // printf("%d" ,neighbours_alive);
            if ((x[i][j] == 0) && (neighbours_alive == 3))
            {
                y[i][j] = 1;
            }
            else if ((x[i][j] == 1) && (neighbours_alive == 3 || neighbours_alive == 2))
            {
                y[i][j] = 1;
            }
            else
            {
                y[i][j] = 0;
            }
        }
    }
    return y;
}

int main()
{
    //change row and column value to set the canvas size
    printf("Enter number of rows: ");
    scanf("%d", &row);
    printf("Enter number of cols: ");
    scanf("%d", &col);

    int a[row][col], b[row][col];
    int i, j, neighbours_alive;

    // generate matrix
    for (i = 0; i < row; i  )
    {
        for (j = 0; j < col; j  )
        {
            a[i][j] = rand() % 2;
        }
    }

    // print initial state
    printf("Initial state:");
    print_state(a);

    char quit;
    do {
        printf("Press enter to print next generation. To quit, enter 'q'.");
        scanf("%c", &quit);
        int** b = count_values(a);
        print_state(b);
        int** a = b;
    } while(quit != 'q');

    return 0;
} 

But there's an error. I'm a poor C programmer and don't know what should be done to get desirable effect. After assigning the output from function to array b, I want to assign a=b and make a loop from this to make the next generations.

So the question is how to assign values from one 2D array in C to another and how to return 2D array from a function and assign the result to existing 2D array. Thank you for any help

CodePudding user response：

A simple but not efficient way is you can define a struct to hold 2d array. Then you can simply return struct object from function and assign to each other.

typedef struct {
    int array[row][col];
} S2dArray;

S2dArray count_values(S2dArray x){
   S2dArray y;
   ....
   return y;
}

int main() {
    S2dArray a, b;
    ....
    b = count_values(a);
    print_state(b);
    a = b;
    ....
}

CodePudding user response：

Arrays are "non-modifiable l-values" thus the cannot be a left argument of the assignment operator. To copy the array just use memcpy(). For example:

int a[row][col], b[row][col];

memcpy(a, b, sizeof(int[row][col]));

CodePudding user response：

"How to return 2D array in C with a function"

Pass VLA array as argument in VLA arrangement.

Note the position of the array sizes before the array itself:

void populate2D(int x, int y, int (*arr)[x][y]);

int main(void)
{
    int x=3, y=4;
    int arr[x][y];
    populate2D(x, y, &arr);
    
    return 0;
}

void populate2D(int x, int y, int (*arr)[x][y])
{
    for(int i=0;i<x;i  )
    {
        for(int j = 0;j<y;j  )
        {
            (*arr)[i][j] = i*j;
        }
    }
}