cannot convert from int[][] to int**?

Question

I have the following code in the caller function

int matrix[3][3] = {{1, 2, 3},{4, 5, 6},{7, 8, 9}};
find_index(matrix);

and the find_index's prototype is:

void find_index(int** m);

and I got compiler error: cannot convert 'int (* )[3]' to ‘int**’

Anyway I can fix this? Thanks!

update:

is there a way that I can then use m[a][b] operator in the second function instead of passing the column number and do m[a * col_num + b]?

where in matrix is an array of pointers? Of course they are not compatible types... — tenfour
– tenfour, Commented Dec 2, 2011 at 0:08
@tenfour the confusion obviously comes from the fact that you can pass an int[] to a function expecting an int*, but not an int[][] to a function expecting an int**. — Seth Carnegie
– Seth Carnegie, Commented Dec 2, 2011 at 0:10

TJD · Accepted Answer · 2011-12-02 00:10:38Z

6

A double indexed array still just corresponds to a single pointer, not a pointer to a pointer.

answered Dec 2, 2011 at 0:10

TJD

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3 Comments

Rachel Over a year ago

is there a way that I can then use m[a][b] operator in the second function instead of passing the column number and do m[a * col_num + b]? Thanks!

TJD Over a year ago

if you really want to use m[a][b] inside the function, you can declare it as find_index(int m[][3])

TJD Over a year ago

you definitely don't need to pass column number. if you pass the array as int*, then you just need to understand how the columns/rows are laid out linearly. so if you pass an int* m, you would find m[0] = 1, m[3] = 4, m[6] = 7, etc. The memory is laid out linearly with each row following the previous

Seth Carnegie · Accepted Answer · 2011-12-02 00:14:00Z

4

In memory, an int** looks like

[0] ----> [0] ---> int
          [1] ---> int
          [2] ---> int

[1] ----> [0] ---> int
          [1] ---> int
          [2] ---> int

[2] ----> [0] ---> int
          [1] ---> int
          [2] ---> int

But an int[][] looks like

[0][1][2][0][1][2][0][1][2]
(0)      (1)      (2)

So you can see how indexing them would cause different things to happen (one dereferences pointers, one performs the arithmetic differently), and they are not compatible. Trying to index an int[][] like an int** would cause one of the integers you are storing to be dereferenced, causing Undefined Behaviour.

answered Dec 2, 2011 at 0:14

Seth Carnegie

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