如何在 c++ 中为 2d arrays 使用迭代器？

Question

I want to use an iterator for a 2D array but I don't know how.

My purpose is to access the columns and not the rows.

I found out that to access the rows I can use this auto it=begin(arr); but I think it isn't the right way to do it, however it seems to work.

#include <iostream>
#include <iterator>

using namespace std;

int main(){
  int arr[3][3];
  for (int i=0;i!=3;i++)
    for (int j=0;j!=3;j++)
      arr[i][j]=rand()%9;

  for (int i=0;i!=3;i++)
    for (int j=0;j!=3;j++){
      cout<<arr[i][j]<<' ';
      if (j==2)
        cout<<'\n';
    }

  auto it=begin(arr);
  cout<<**it<<endl;

Is this correct?

Thanks in advance:D

Answer 1

If you want to iterate over a column, you can do something like:

for(auto it = std::begin(arr); it != std::end(arr); it++)
    cout << **it;

This will print the first column.

To iterate over the array in column order you would need somenthing like:

for (size_t i = 0; i < std::size(arr); i++) {
    for (auto it = std::begin(arr); it != std::end(arr); it++) {
        cout << *(*it + i) << " ";
    }
    cout << endl;
}

This will print the array in column order. Note that std::size is only available after C++17, you may need to use std=c++17 compiler flag.

Live demo

Two side notes:

• You are missing a seed for your random engine:

  #include <time.h>
  //...
  srand(time(0)); //seed
  //...

• You should not use using namespace std;

  Why is "using namespace std;" considered bad practice? .

Answer 2

Here's a simple pair of iterators that go in row-major and column-major order.

class row_iterator
{
    int (&arr)[3][3];
    std::size_t pos;
public:
    row_iterator(int (&arr)[3][3], std::size_t pos) : arr(arr), pos(pos) {}

    row_iterator& operator++() { ++pos; return *this; }
    row_iterator operator++(int) { auto that = *this; ++pos; return that; }
    int & operator*() { return arr[pos / 3][pos % 3]; }
    int * operator->() { return &arr[pos / 3][pos % 3]; }

    friend bool operator==(row_iterator lhs, row_iterator rhs) { return (lhs.arr == rhs.arr) && (lhs.pos == rhs.pos); }
    friend bool operator!=(row_iterator lhs, row_iterator rhs) { return !lhs == rhs; }
};

row_iterator row_begin(int (&arr)[3][3]) { return row_iterator(arr, 0); }
row_iterator row_end(int (&arr)[3][3]) { return row_iterator(arr, 9); }

class col_iterator
{
    int (&arr)[3][3];
    std::size_t pos;
public:
    col_iterator(int (&arr)[3][3], std::size_t pos) : arr(arr), pos(pos) {}

    col_iterator& operator++() { ++pos; return *this; }
    col_iteratoroperator++(int) { auto that = *this; ++pos; return that; }
    int & operator*() { return arr[pos % 3][pos / 3]; }
    int * operator->() { return &arr[pos % 3][pos / 3]; }

    friend bool operator==(col_iterator lhs, col_iterator rhs) { return (lhs.arr == rhs.arr) && (lhs.pos == rhs.pos); }
    friend bool operator!=(col_iterator lhs, col_iterator rhs) { return !lhs == rhs; }
};

col_iterator col_begin(int (&arr)[3][3]) { return col_iterator(arr, 0); }
col_iterator col_end(int (&arr)[3][3]) { return col_iterator(arr, 9); }

Answer 3

You can just do pointer arithmatic to access the column.

cout<<"Column : "<<*(*it+1)<<endl;

Here, 1 is the column number.

Live example here: https://gcc.godbolt.org/z/WXKLCk