Is there an example for accumarray() in C/C++ - c++

Question

We are trying to understand accumarray function of MATLAB, wanted to write C/C++ code for the same for our understanding. Can someone help us with a sample/pseudo code?

Answer 1

According to the documentation,
The function processes the input as follows:
Find out how many unique indices there are in subs. Each unique index defines a bin in the output array. The maximum index value in
subs determines the size of the output array.
Find out how many times each index is repeated.
This determines how many elements of vals are going to be accumulated at each bin in the output array.
Create an output array. The output array is of size max(subs) or of size sz.
Accumulate the entries in vals into bins using the values of the indices in subs and apply fun to the entries in each bin.
Fill the values in the output for positions not referred to by subs. Default fill value is zero; use fillval to set a different
value.
So, translating to C++ (this is untested code),
template< typename sub_it, typename val_it, typename out_it,
typename fun = std::plus< typename std::iterator_traits< val_it >::value_type >,
typename T = typename fun::result_type >
out_it accumarray( sub_it first_index, sub_it last_index,
val_it first_value, // val_it last_value, -- 1 value per index
out_it first_out,
fun f = fun(), T fillval = T() ) {
std::size_t sz = std::max_element( first_index, last_index ); // 1. Get size.
std::vector< bool > used_indexes; // 2-3. remember which indexes are used
std::fill_n( first_out, sz, T() ); // 4. initialize output
while ( first_index != last_index ) {
std::size_t index = * first_index;
used_indexes[ index ] = true; // 2-3. remember that this index was used
first_out[ index ] = f( first_out[ index ], * first_value ); // 5. accumulate
++ first_value;
++ first_index;
}
// If fill is different from zero, reinitialize untouched values
if ( fillval != T() ) {
out_it fill_it = first_out;
for ( std::vector< bool >::iterator used_it = used_indexes.begin();
used_it != used_indexes.end(); ++ used_it ) {
if ( * used_it ) * fill_it = fillval;
}
}
return first_out + sz;
}
This has a few shortcomings, for example the accumulation function is called repeatedly instead of once with the entire column vector. The output is placed in pre-allocated storage referenced by first_out. The index vector must be the same size as the value vector. But most of the features should be captured pretty well.

Answer 2

Many thanks for your response. We were able to fully understand and implement the same in C++ (we used armadillo). Here is the code:
colvec TestProcessing::accumarray(icolvec cf, colvec T, double nf, int p)
{
/* ******* Description *******
here cf is the matrix of indices
T is the values whose data is to be
accumulted in the output array S.
if T is not given (or is scaler)then accumarray simply converts
to calculation of histogram of the input data
nf is the the size of output Array
nf >= max(cf)
so pass the argument accordingly
p is not used in the function
********************************/
colvec S; // output Array
S.set_size(int(nf)); // preallocate the output array
for(int i = 0 ; i < (int)nf ; i++)
{
// find the indices in cf corresponding to 1 to nf
// and store in unsigned integer array q1
uvec q1 = find(cf == (i+1));
vec q ;
double sum1 = 0 ;
if(!q1.is_empty())
{
q = T.elem(q1) ; // find the elements in T having indices in q1
// make sure q1 is not empty
sum1 = arma::sum(q); // calculate the sum and store in output array
S(i) = sum1;
}
// if q1 is empty array just put 0 at that particular location
else
{
S(i) = 0 ;
}
}
return S;
}
Hope this will help others too!
Thanks again to everybody who contributed :)

Answer 3

Here's what I came up with. Note: I went for readability (since you wanted to understand best), rather than being optimized. Oh, and I've never used MATLAB, I was just going off of this sample I saw just now:
val = 101:105;
subs = [1; 2; 4; 2; 4]
subs =
1
2
4
2
4
A = accumarray(subs, val)
A =
101 % A(1) = val(1) = 101
206 % A(2) = val(2)+val(4) = 102+104 = 206
0 % A(3) = 0
208 % A(4) = val(3)+val(5) = 103+105 = 208
Anyway, here's the code sample:
#include <iostream>
#include <stdio.h>
#include <vector>
#include <map>
class RangeValues
{
public:
RangeValues(int startValue, int endValue)
{
int range = endValue - startValue;
// Reserve all needed space up front
values.resize(abs(range) + 1);
unsigned int index = 0;
for ( int i = startValue; i != endValue; iterateByDirection(range, i), ++index )
{
values[index] = i;
}
}
std::vector<int> GetValues() const { return values; }
private:
void iterateByDirection(int range, int& value)
{
( range < 0 ) ? --value : ++value;
}
private:
std::vector<int> values;
};
typedef std::map<unsigned int, int> accumMap;
accumMap accumarray( const RangeValues& rangeVals )
{
accumMap aMap;
std::vector<int> values = rangeVals.GetValues();
unsigned int index = 0;
std::vector<int>::const_iterator itr = values.begin();
for ( itr; itr != values.end(); ++itr, ++index )
{
aMap[index] = (*itr);
}
return aMap;
}
int main()
{
// Our value range will be from -10 to 10
RangeValues values(-10, 10);
accumMap aMap = accumarray(values);
// Now iterate through and check out what values map to which indices.
accumMap::const_iterator itr = aMap.begin();
for ( itr; itr != aMap.end(); ++itr )
{
std::cout << "Index: " << itr->first << ", Value: " << itr->second << '\n';
}
//Or much like the MATLAB Example:
cout << aMap[5]; // -5, since out range was from -10 to 10
}

Answer 4

In addition to Vicky Budhiraja "armadillo" example, this one is a 2D version of accumarray using similar semantic than matlab function:
arma::mat accumarray (arma::mat& subs, arma::vec& val, arma::rowvec& sz)
{
arma::u32 ar = sz.col(0)(0);
arma::u32 ac = sz.col(1)(0);
arma::mat A; A.set_size(ar, ac);
for (arma::u32 r = 0; r < ar; ++r)
{
for (arma::u32 c = 0; c < ac; ++c)
{
arma::uvec idx = arma::find(subs.col(0) == r &&
subs.col(1) == c);
if (!idx.is_empty())
A(r, c) = arma::sum(val.elem(idx));
else
A(r, c) = 0;
}
}
return A;
}
The sz input is a two columns vector that contain : num rows / num cols for the output matrix A. The subs matrix is a 2 columns with same num rows of val. Num rows of val is basically sz.rows by sz.cols.
The sz (size) input is not really mandatory and can be deduced easily by searching the max in subs columns.
arma::u32 sz_rows = arma::max(subs.col(0)) + 1;
arma::u32 sz_cols = arma::max(subs.col(1)) + 1;
or
arma::u32 sz_rows = arma::max(subs.col(0)) + 1;
arma::u32 sz_cols = val.n_elem / sz_rows;
the output matrix is now :
arma::mat A (sz_rows, sz_cols);
the accumarray function become :
arma::mat accumarray (arma::mat& subs, arma::vec& val)
{
arma::u32 sz_rows = arma::max(subs.col(0)) + 1;
arma::u32 sz_cols = arma::max(subs.col(1)) + 1;
arma::mat A (sz_rows, sz_cols);
for (arma::u32 r = 0; r < sz_rows; ++r)
{
for (arma::u32 c = 0; c < sz_cols; ++c)
{
arma::uvec idx = arma::find(subs.col(0) == r &&
subs.col(1) == c);
if (!idx.is_empty())
A(r, c) = arma::sum(val.elem(idx));
else
A(r, c) = 0;
}
}
return A;
}
For example :
arma::vec val = arma::regspace(101, 106);
arma::mat subs;
subs << 0 << 0 << arma::endr
<< 1 << 1 << arma::endr
<< 2 << 1 << arma::endr
<< 0 << 0 << arma::endr
<< 1 << 1 << arma::endr
<< 3 << 0 << arma::endr;
arma::mat A = accumarray (subs, val);
A.raw_print("A =");
Produce this result :
A =
205 0
0 207
0 103
106 0
This example is found here : http://fr.mathworks.com/help/matlab/ref/accumarray.html?requestedDomain=www.mathworks.com
except for the indices of subs, armadillo is 0-based indice where matlab is 1-based.
Unfortunaly, the previous code is not suitable for big matrix. Two for-loop with a find in vector in between is really bad thing. The code is good to understand the concept but can be optimized as a single loop like this one :
arma::mat accumarray(arma::mat& subs, arma::vec& val)
{
arma::u32 ar = arma::max(subs.col(0)) + 1;
arma::u32 ac = arma::max(subs.col(1)) + 1;
arma::mat A(ar, ac);
A.zeros();
for (arma::u32 r = 0; r < subs.n_rows; ++r)
A(subs(r, 0), subs(r, 1)) += val(r);
return A;
}
The only change are :
init the output matrix with zero's.
loop over subs rows to get the output indice(s)
accumulate val to output (subs & val are row synchronized)
A 1-D version (vector) of the function can be something like :
arma::vec accumarray (arma::ivec& subs, arma::vec& val)
{
arma::u32 num_elems = arma::max(subs) + 1;
arma::vec A (num_elems);
A.zeros();
for (arma::u32 r = 0; r < subs.n_rows; ++r)
A(subs(r)) += val(r);
return A;
}
For testing 1D version :
arma::vec val = arma::regspace(101, 105);
arma::ivec subs;
subs << 0 << 2 << 3 << 2 << 3;
arma::vec A = accumarray(subs, val);
A.raw_print("A =");
The result is conform with matlab examples (see previous link)
A =
101
0
206
208
This is not a strict copy of matlab accumarray function. For example, the matlab function allow to output vec/mat with size defined by sz that is larger than the intrinsec size of the subs/val duo.
Maybe that can be a idea for addition to the armadillo api. Allowing a single interface for differents dimensions & types.