How to create function with input is 2D array in C++

Question

I have a 2D array that defined as

int P[5][10];
for (int i=0;i<N;i++)
{
    for(int j=0;j<L;j++)
    {
        if(random()>0.5)
            P[i][j]=1;
        else
            P[i][j]=0;
    }
}

I want to make a function with input is P. The function allows us show the value of P. How to defined that function. I tried such as

void show_P(int P[][], int numcols,int numrows)

However, it is wrong. Could you help me fix it? Thanks

Don't use 2D arrays. Use 1D arrays, perhaps encapsulating them in some class which has member functions to access or modify them. — Basile Starynkevitch, Mar 31 '15 at 05:56
Declare a 1D array or vector, and use `a[i*width+j]` instead of `a[i][j]` — Basile Starynkevitch, Mar 31 '15 at 05:58
You should avoid 2D arrays. It is very likely that you'll get confused. And there is no need for them. — Basile Starynkevitch, Mar 31 '15 at 06:01
You could always search the net for a proper answer. One I could easily found is here, on stackoverflow: http://stackoverflow.com/a/8767247/1432385 — Lucaci Andrei, Mar 31 '15 at 06:02
@LucaciAndrei It is too bad that the selected and most up-voted answer is full of wrong / misleading statements. Otherwise I would have used it to close this as duplicate. — juanchopanza, Mar 31 '15 at 06:04

score 3 · Accepted Answer · edited Mar 31 '15 at 14:19

3

If you want to restrict the arguments to 5 by 10 2D arrays, you can pass by reference like this:

void show_P(int (&P)[5][10])

This will fail for any other type of array. If you want the function to work for other sizes, you can make it a template.

template <size_t N, size_t M>
void show_P(int (&P)[N][M])

edited Mar 31 '15 at 14:19

Paul R

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answered Mar 31 '15 at 05:57

juanchopanza

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Is it safety way? That means we don't worry about memory problem. Someone suggest to me using 2D points array. However, I think your suggestion is very simple. Thanks – Jame Mar 31 '15 at 06:00
@user8430 This has type safety, as mentioned in the answer. A pointer to pointer doesn't have that. – juanchopanza Mar 31 '15 at 06:01
Good knowledge. Thank you. – Jame Mar 31 '15 at 06:06
Can your way apply for returning 2D array? That means if I want to write a function to return 2D array, can I apply template void set_P(int (&P)[N][M]) – Jame Mar 31 '15 at 06:31
@user8430 That isn't "returning a 2D array", but modifying an input parameter. Yes, you can kind of do that, but only by setting the elements of the array. Arrays are not assignable or copyable like, say, `int`s or `double`s are. – juanchopanza Mar 31 '15 at 06:34
Could you show to me one way to apply the work "return 2D array"?Thanks – Jame Mar 31 '15 at 06:35
1

@user8430 There isn't a way to do that, because arrays are not copyable. – juanchopanza Mar 31 '15 at 06:36

score 1 · Answer 2 · answered Mar 31 '15 at 05:52

1

You could just change it to:

void show_P(int** P, int numcols, int numrows)

Passing 2D array always using the pointer.

Hope this will help.

answered Mar 31 '15 at 05:52

WindMemory

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This does not solve the OP's problem - you can't pass a 2D array of int as an `int **`. – Paul R Mar 31 '15 at 09:56

score 1 · Answer 3 · answered Mar 31 '15 at 06:17

Or using the std::array

void printArray(array<array<int,2>,3>& arr)
{
  for (auto x : arr)
  {
    for (auto y : x)
    {
      cout << y << endl;
    }
  }
}

int main()
{
  array<array<int,2>,3> arr{{{1, 2}, {2, 3}, {3, 4}}};
  arr[0][1] = 5;
  printArray(arr);
}

would give you:

1

5

2

3

4

Cpp Programmer · Answer 4 · 2015-03-31T06:31:35.030

0

void print( int (&ref)[5][10]) {

for( auto &lm: ref) // get the first array from the multidimensional array and initialize lm with it 
    for( auto &elem: lm) // get the first element from lm 
        std::cout << elem << " "; // print the element 

}

This only works with an array with dimensions as P i.e, [5][10]

edited Mar 31 '15 at 06:31

answered Mar 31 '15 at 06:14

Cpp Programmer

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I have made 'lm' a reference because if we don't then lm will not be a reference to an array but rather have a type of int * which is not that we want it to be. – Cpp Programmer Mar 31 '15 at 06:22

John · Answer 5 · 2015-03-31T06:15:39.883

-1

N=5;
L=10;    
void show_P( int ( &P )[N][L] )

edited Mar 31 '15 at 06:15

answered Mar 31 '15 at 05:55

John

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How to create function with input is 2D array in C++

5 Answers5