c++ function that fills array - c++

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::vectors (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;
}