Passing Array as Reference in c++

Question

I have been trying to use an 2D array as Reference into a function. Thanks for the help.

#include <iostream>
using namespace std;
void dfs(int *G[],int i,int *visited,int size) {
    visited[i]=1;
    int j;
    for(j=0;j<size;j++) {
        if(!visited[j]&& G[i][j] == 1)
            dfs(G,j,visited);
    }
}

@mello I think the last one addresses exactly your question. — vsoftco, Apr 05 '15 at 18:44
Both do, the reason why you get the problem is because of static/dynamic array mismatch, this problem has been answered in both linked answers. — Peter Uhnak, Apr 05 '15 at 18:44

score 1 · Answer 1 · edited Apr 05 '15 at 19:45

1

The issue is in the signature of your dfs function

void dfs(int *G[],int i,int *visited)

It takes a pointer to pointer to int. You are however passing an array to it

dfs(Array_From_file, 0, visited);

where Array_From_file is declared as

int Array_From_file[ROWS][COLUMNS];

Such conversion is not possible. Quick fix: change the signature of the function to:

void dfs(int G[][COLUMNS],int i,int *visited)

Better, use a std::vector<std::vector<int>> that you pass by reference. Here is an example:

#include <iostream>
#include <fstream>
#include <vector>

using namespace std;

int const COLUMNS = 100;
int const ROWS = 100 ;
typedef std::vector<std::vector<int>> int_mat;

void dfs(const int_mat& G, int i, vector<int>& visited) {
    int size = ROWS * COLUMNS ;
    visited[i] = 1;
    for (int j = 0; j < size; j++) {
        if (!visited[j] && G[i][j] == 1)
            dfs(G, j, visited);
    }
}

edited Apr 05 '15 at 19:45

mello

75
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8

answered Apr 05 '15 at 18:37

vsoftco

52,188
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so i should change the void pdfs (int G[][],int i , int *visitded) – mello Apr 05 '15 at 18:40
you should either change the signature to `void(int G[][COLUMNS], int, int*)` (you have to specify the second dimension), or use `int** Array_from_file`. – vsoftco Apr 05 '15 at 18:42
so i should take the int size variable into the function and put a nested for loop with the const max right – mello Apr 05 '15 at 18:47
@mello, yes, loop as you'd do over any bi-dimensional array, with a nested loop. – vsoftco Apr 05 '15 at 18:50
@mello see the updated edit for a modification of your code so it uses `std::vector`. – vsoftco Apr 05 '15 at 19:01

Jonathan Bedard · Answer 2 · 2015-04-05T19:10:53.873

-1

I believe your (heavily edited) fix is this:

int one_to_two(int* a, int x, int y)
{
    return a[x*ROWS+y];
}
void dfs(int** G,int i,int *visited) {
    int size = ROWS * COLUMNS ;
    visited[i]=1;
    int j;
    for(j=0;j<size;j++) {
        if(!visited[j]&& one_to_two(G,i,j) == 1)
            dfs(G,j,visited);
    }
}

That being said, I don't think that this is a well-structured method of solving this problem. I don't know why you're using recursion to do this, an iterative loop would be much simpler. This method has the possibility to blow-up the stack. You are NOT using tail recursion here, so this method will cause memory problems if ROWS or COLUMNS gets very large.

I apologize for my screw-ups, the typing didn't work quite the way I thought it did. This will require casting your 2-d array to an int pointer before sending it into the function. I'm not sure I like this solution, its not very elegant, but it should at least compile.

On another note, not quite sure what you're trying to do with depth-first search, but I don't think this algorithm does quite what you you think it does (for one, malloc doesn't zero the memory it allocates, which is of particular concern for this algorithm)

edited Apr 05 '15 at 19:10

answered Apr 05 '15 at 18:47

Jonathan Bedard

169
6

this declares `G` as an array of references, which is an error (caught at compile time) – vsoftco Apr 05 '15 at 18:48
that just happened ... the program crashed .. DO you have any subjections ? Please – mello Apr 05 '15 at 18:49
Just changed it. I thought the reference would work, it does not. The double pointer should do the trick though. – Jonathan Bedard Apr 05 '15 at 18:51
@JonathanBedard it will still not work, `int G[][]` is the same as `int** G`, the problem is that you pass an array to it. It works only if you pass a double pointer, or change the signature to `int G[][COLUMNS]` – vsoftco Apr 05 '15 at 18:52
Ok, so everything I said before was fantastically messed up. Posting one that will definitely compile, although is a bit of a hack. – Jonathan Bedard Apr 05 '15 at 19:03
@JonathanBedard the last line `dfs(G,j,visited)` is still not going to pass... You just cannot pass the array `int[ROWS][COLS]` to `int**`. – vsoftco Apr 05 '15 at 19:28
@JonathanBedard yeah my DFS function is not good ? do you have any ideas how to implement it to with 2D array ? – mello Apr 05 '15 at 19:49
Why don't you just use a nested loop? I can post another answer, but it will be a rebuild of your entire application – Jonathan Bedard Apr 05 '15 at 20:40

Passing Array as Reference in c++

2 Answers2