How to return a 2D array in C++ with simple way

Question

I have to implement a code to generate a 2D array. Given N and L that are size of array. I need to generate matrix A such as

for i=1 to N
 for j=1 to L
   if(random(0,1)>0.5)
     A[i][j]=1;
   else 
     A[i][j]=0;
   end
 end
end

Let call all above code is in intial() function. Let N=10; L=100; How to make the function initial() that return a 2D array A. Thanks This is my code

const int N=10;
const int L=100;
int A[N][L]; 

int [][] initialization()
{
  for (int i=0;i<10;i++)
    for (int j=i;j<100;j++)
      {
       if(random(0,1)>0.5)
         A[i][j]=1;
       else
         A[i][j]=0;
       }
  return A;
}

Please show concise code samples, when tagging concrete languages. Also pick one c or c++. It makes a big difference for applicable solutions. — πάντα ῥεῖ, Mar 30 '15 at 17:43
define the array in calling function and then pass the array as argument to initial() — GorvGoyl, Mar 30 '15 at 17:43
That depends on what `A` is. Is it an array of arrays, or is it an array of pointers (that's indistinguishable from your code)? — cmaster - reinstate monica, Mar 30 '15 at 17:43
@NathanOliver I wouldn't necessarily consider the answers offered there, to be good c++ code. — πάντα ῥεῖ, Mar 30 '15 at 17:45
@πάνταῥεῖ: I wouldn't either. I would suggest they use a 2d vector instead of arrays. As the OP had it initially I figured the dupe was what they were looking for. — NathanOliver, Mar 30 '15 at 17:48
@user So you might get well off with returning a [`std::array,L>,N>`](http://en.cppreference.com/w/cpp/container/array) by value from your function. — πάντα ῥεῖ, Mar 30 '15 at 17:48

Vlad from Moscow · Accepted Answer · 2015-03-30T18:08:11.657

2

You already declared a two-dimensional array

const int N=10;
const int L=100;
int P[N][L];

So declare the function like

void initial( int a[][L], size_t n );

and call it like

initial( P, N );

Or in C++ you could define the function like

void initial( int ( &a )[N][L] );

Or even you may return reference to the array

int ( & initial( int ( &a )[N][L] ) )[N][L];

If N and L are not constants then maybe it is better to use std::vector<std::vector<int>> instead of the array.

Or if the number of columns is known at compile time then you can define a vector of std::array. For example

std::vector<std::array<int, L>>

If it is the function that has to create the array then you can dynamically allocate an array. There are two approaches. The first one

int ** initial( size_t n, size_t m )
{
    int **a = new int * [n];

    for ( size_t i = 0; i < n; i++ ) a[i] = new int [m];

    return a;
}

This function is unsafe so it is better to use smart pointers like std::unique_ptr.

Or if the number of columns is a constant then

int  ( * initial( size_t n ) )[L]
{
    int ( *a )[L] = new int [n][L];

    return a;
}

edited Mar 30 '15 at 18:08

answered Mar 30 '15 at 17:49

Vlad from Moscow

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Sorry. Let see my update question. I missing between A and P – Jame Mar 30 '15 at 17:58
@user8430 I have not understood you nevertheless see my updated post. – Vlad from Moscow Mar 30 '15 at 18:06
Previous code, I use notation P for int P[N][L]. However, It is wrong. It must be int A[N][L]; – Jame Mar 30 '15 at 18:07
Anh your code is just initial 2D array. How to set up the value for each element. It must be scan again. Right? – Jame Mar 30 '15 at 18:10
@user8430 You may name the array as you like. It is totally unimportant. – Vlad from Moscow Mar 30 '15 at 18:11
@user8430 Include in the function the code that initialize elements of the array. What is the problem? – Vlad from Moscow Mar 30 '15 at 18:11
Because, my algorithm requires set up the values for each element base on the random number. Then I saw your code, it only has initial part. It does not have setup part. – Jame Mar 30 '15 at 18:13

How to return a 2D array in C++ with simple way

1 Answers1