Split array into sub blocks - c++

Question

What I am trying to achieve is this:
I have an image and I need to split it into sub blocks of 16x16 and I am working on the algorithm for this. For testing purposes though, I am using a small matrix:
A = {1, 2, 3, 4}
Now what I want to end up is this: 2 blocks containing:
A[1] = {1 2};
A[2] = {3, 4};
I have tried to use the following:
double matrix[4] = {1, 2, 3, 4};
for(int i = 0; (i < 4); i++)
{
for(unsigned j=i; (j < 2); j +=2)
{
std::cout << j << ' ';
}
std::cout << std::endl;
}
My thought process was to loop through the entire array (4) and then increment by 2 each time to create the 1x2 block. This did not work however.
Where am I going wrong here?

Answer 1

Something like that? (Does both output and assignment)
int LEN = 4;
int INNER = 2;
int OUTER_LEN = LEN/INNER_LEN;
double matrix[LEN] = {1, 2, 3, 4};
double* matrix2[OUTER_LEN];
for(int i = 0; i < OUTER_LEN; i++)
{
matrix2[i] = &matrix[i*INNER_LEN];
for(unsigned j=0; j < INNER_LEN; j++)
{
std::cout << matrix[i*INNER_LEN+j] << ' ';
}
std::cout << std::endl;
}

Answer 2

Just for output you could do something like that:
#include <iostream>
int main(){
const size_t SIZE = 4;
const size_t PART_SIZE = 2;
double matrix[4] = {1, 2, 3, 4};
for(int i = 0; (i < SIZE); i += PART_SIZE)
{
for(size_t j = i; (j < i + PART_SIZE) && j < SIZE; j += 1)
{
std::cout << matrix[j] << ' ';
}
std::cout << std::endl;
}
}
To add another matrix:
#include <iostream>
int main(){
const size_t SIZE = 4;
const size_t PART_SIZE = 2;
size_t partsNumber = SIZE / PART_SIZE; // Beware of SIZE that is not divisible by PART_SIZE - partsNumber will be too small
double matrix[4] = { 1, 2, 3, 4 };
// To do it properly I should make it dynamic array with size of partsNumber instead of the 2 literals
double parts_matrix[2][PART_SIZE];
for (int i = 0; (i < SIZE); i += PART_SIZE) {
for (size_t j = i; (j < i + PART_SIZE) && j < SIZE; j += 1) {
std::cout << matrix[j] << ' ';
parts_matrix[j / partsNumber][j % PART_SIZE] = matrix[j];
}
std::cout << std::endl;
}
std::cout << parts_matrix[0][0] << " " << parts_matrix[0][1] << std::endl << parts_matrix[1][0] << " " << parts_matrix[1][1]; // Check if it works
}

Answer 3

The following is a demo of how to do the splitting for custom block size (rough cut though, corner cases and input verification are ommited) using boost range and the boost::slice functionality (here "output creation" is presented)
#include <iterator>
#include <iostream>
#include <boost/range/adaptor/sliced.hpp>
#include <boost/range/algorithm/copy.hpp>
using namespace std;
using namespace boost::adaptors;
template<typename T, size_t N>
void split(T (&input)[N], size_t block_size)
{
for (size_t i(0); i <= N-block_size; i += block_size)
{
cout << "{ ";
boost::copy(input | sliced(i, i+block_size),
std::ostream_iterator<int>(std::cout, " "));
cout << "}\n";
}
}
int main()
{
int A[] = {1, 2, 3, 4};
split(A, 2);
}
Demo
Output
{ 1 2 }
{ 3 4 }
What if I don't want to do output
To some the following may look more readable
template<typename T, size_t N>
void split(T (&input)[N], size_t block_size)
{
for (size_t i(0); i <= N-block_size; i += block_size)
{
cout << "{ ";
// do whatever with the i slice (again I'm showing output)
for (auto k : (input | sliced(i, i+block_size))) cout << k << " ";
cout << "}\n";
}
}