Clean ways to write multiple 'for' loops

Question

For an array with multiple dimensions, we usually need to write a for loop for each of its dimensions. For example:

vector< vector< vector<int> > > A;  for (int k=0; k<A.size(); k++) {     for (int i=0; i<A[k].size(); i++)     {         for (int j=0; j<A[k][i].size(); j++)         {             do_something_on_A(A[k][i][j]);         }     } }  double B[10][8][5]; for (int k=0; k<10; k++) {     for (int i=0; i<8; i++)     {         for (int j=0; j<5; j++)         {             do_something_on_B(B[k][i][j]);         }     } } 

You see this kind of for-for-for loops in our code frequently. How do I use macros to define the for-for-for loops so that I don't need to re-write this kind of code every time? Is there a better way to do this?

Answer 1

The first thing is that you don't use such a data structure. If you need a three dimensional matrix, you define one:

class Matrix3D {     int x;     int y;     int z;     std::vector<int> myData; public:     //  ...     int& operator()( int i, int j, int k )     {         return myData[ ((i * y) + j) * z + k ];     } }; 

Or if you want to index using [][][], you need an operator[] which returns a proxy.

Once you've done this, if you find that you constantly have to iterate as you've presented, you expose an iterator which will support it:

class Matrix3D {     //  as above...     typedef std::vector<int>::iterator iterator;     iterator begin() { return myData.begin(); }     iterator end()   { return myData.end();   } }; 

Then you just write:

for ( Matrix3D::iterator iter = m.begin(); iter != m.end(); ++ iter ) {     //  ... } 

(or just:

for ( auto& elem: m ) { } 

if you have C++11.)

And if you need the three indexes during such iterations, it's possible to create an iterator which exposes them:

class Matrix3D {     //  ...     class iterator : private std::vector<int>::iterator     {         Matrix3D const* owner;     public:         iterator( Matrix3D const* owner,                   std::vector<int>::iterator iter )             : std::vector<int>::iterator( iter )             , owner( owner )         {         }         using std::vector<int>::iterator::operator++;         //  and so on for all of the iterator operations...         int i() const         {             ((*this) -  owner->myData.begin()) / (owner->y * owner->z);         }         //  ...     }; };