填充数组的C ++函数

Question

What I'm trying to do:

User inputs two numbers. Array is declared using those numbers as dimensions. Function outside main() is filling the array. Array is accessed in main() for further thingies.

What I have problem with:

Function + array combination doesn't seem to work as I think.

What I did:

void tablica1(int h, int w)
{
    int m,n;

    for(m=0; m<h; m++)
        for(n=0; n<w; n++) 
        {
            arr[h][w]=1;
        }
}

What happens:

array arr is inaccessible in tablica1() because it has not been declared in that function.

Of course, when I declare the array in tablica1() it becomes inaccessible in main() .

Possible solutions:

• Passing arr to tablica1() as a reference - no idea how to do that
• Declaring arr in tablica1() and somehow passing it to main() - no idea how to do that

Other possible solutions?

Answer 1

You can declare the array outside of both, at compilation unit level:

int arr[10][10];

void func() {
    for (int i=0; i<10; i++) {
        for (int j=0; j<10; j++) {
            arr[i][j] = i + j;
        }
    }
}

int main(int argc, const char *argv[]) {
    func();
    std::cout << arr[3][4] << "\n"; // Output will be 7
    return 0;
}

If you want handle a dynamically-sized matrix the most common pattern is to use an std::vector of std::vector s (it's a little more general and therefore a little less efficient than a 2d matrix because each row can have a different length, but in most cases the cost difference is not a big issue)

#include <vector>

std::vector< std::vector< int > > arr;

void func() {
    int height = arr.size();
    int width = arr[0].size();
    for (int i=0; i<height; i++) {
        for (int j=0; j<width; j++) {
            arr[i][j] = i + j;
        }
    }
}


int main() {
    int height = 13;
    int width = 7;

    arr = std::vector< std::vector<int> >(height, std::vector<int>(width));

    func();

    ...
}

Answer 2

the two solutions you mentioned

1、Passing arr to tablica1() as a reference

void tablica1(int h, int w,int **arr)
{
    int m,n;

    for(m=0; m<h; m++)
        for(n=0; n<w; n++) 
        {
            arr[h][w]=1;
        }
}

void main()
{
  const int h=100,w=100;
  int arr[h][w];
  tablica1(h,w,arr);
}

2、Declaring arr in tablica1() and somehow passing it to main()

int **tablica1(int h, int w)
{
    int m,n;
    int **arr=new int*[h];
    for(int i=0;i<h;i++)
    {
       arr[i]=new int[w];
    }
    //it is best to initialize arr by setting each element 0
    for(m=0; m<h; m++)
        for(n=0; n<w; n++) 
        {
            arr[h][w]=1;
        }
   return arr;

}

void main()
{
  const int h=100,w=100;
  int **arr=tablica1(h,w);
  //do somting

  //delete arr
  for(int i=0;i<h;i++)
  {
    delete []arr[i];
  }
  delete []arr;
}

Answer 3

If you want to declare a dynamic multidimensional array you can do that with the template give below.

     #include<iostream.
     #include <vector>
     using namespace std;
     typedef vector<int> vi;

     vector<vi> arr; // arr is a dynamic two dimensional array.
     vi.assign(10,vi());//10 rows of type vi .

If you want to enter values in arr you can do that by

    vi[0].push_back(a);
    vi[0].push_back(b);  // a,b,c are some example values..
    vi[1].push_back(c);

Answer 4

You can understand using this code

     #include <iostream>
      #include <cstdio>
     #include <vector>
     using namespace std;

     typedef vector <int> vi;

     vector<vi> arr;// dynamic multidimensional array

     int main(){
     cout<<"enter array size\n";
     int h,w,temp;;
     cin>>h>>w;

     arr.assign(h,vi());
     int i, j;
     for(i=0; i < h ;i++)
      for(j=0; j < w; j++)
       {
       cin>>temp;
       arr[i].push_back(temp);
       }

       // for printing
       for(i=0; i < h ;i++){
       for(j=0; j < w; j++)
        {
        cout<<arr[i][j]<<" ";

        }
     cout<<endl;
   }
    return 0;

    }