Cartesian on NULL arrays

Question

I need a cartesian product of 6 arrays - the catch is that at any time upto 5 arrays could be null. It works great when all arrays are populated but bombs when any of the arrays are null

my arrays are something like this

MatrixArray_1[0] = 1
MatrixArray_1[1] = 2

MatrixArray_2[0] = null
MatrixArray_2[1] = null

MatrixArray_n[0] = 2
MatrixArray_n[1] = 2

etc.

I am using this code currently...derived from http://blogs.msdn.com/b/ericlippert/archive/2010/06/28/computing-a-cartesian-product-with-linq.aspx

var product1 =  from first in MatrixArray_1
                from second in MatrixArray_2
                from third in MatrixArray_3
                from fourth in MatrixArray_4
                from fifth in MatrixArray_5
                from sixth in MatrixArray_6
                select new[] { first, second, third, fourth, fifth, sixth };
            string[][] myCombos_linq = product1.ToArray();

I have tried putting MatrixArray_n where first != null but that stops at the first null array and does not read thru all the remaining arrays so my return array is always 0 rows even though array1 and array 3 are populated.

Change of code/logic anything is appreciated at this point in time! TIA

Answer 1

Since Eric's approach is to use an IEnumerable<IEnumerable<T>>, you must be doing something like this:

Eric's code:

static IEnumerable<IEnumerable<T>> CartesianProduct<T>(this IEnumerable<IEnumerable<T>> sequences) 
{ 
  IEnumerable<IEnumerable<T>> emptyProduct = new[] { Enumerable.Empty<T>() }; 
  return sequences.Aggregate( 
    emptyProduct, 
    (accumulator, sequence) => 
      from accseq in accumulator 
      from item in sequence 
      select accseq.Concat(new[] {item})); 
}

Call site:

var sequences = new int?[][] { MatrixArray_1, MatrixArray_2, ..., MatrixArray_6 };
var cartesianSequence = sequences.CartesianProduct();

Change the call site:

var cartesianSequence = sequences.Where(a => a.Any(e => e != null)).CartesianProduct();

The Where call will exclude sequences where all elements are null.

To exclude null arrays as well as arrays containing only null values:

var cartesianSequence = sequences.Where(a => a != null && a.Any(e => e != null)).CartesianProduct();

Or, with query comprehension:

var filteredSequences = 
    from sequence in sequences
    where sequence != null && sequence.Any(e => e != null)
    select sequence
var cartesianSequence = filteredSequences.CartesianProduct();

EDIT

Another possibility is that you want to exclude null elements of each sequence, even if some elements are non-null:

var filteredSequences = 
    from sequence in sequences
    where sequence != null && sequence.Any(e => e != null)
    select (from v in sequence where v.HasValue select s)
var cartesianSequence = filteredSequences.CartesianProduct();

OR

var cartesianSequence = sequences
    .Where(s => s != null && s.Any(e => e != null))
    .Select(s => s.Where(v => v != null))
    .CartesianProduct();

But it's hard to know exactly what to advise since we don't know what you're doing with the result.