Question

My ultimate aim is to convert the below code in python to C#, but I’d like to do it my self by learning the python syntax. I understand that the code is recursive.

The code produces polynomials of degree n with k variables. More specifically the list of exponents for each variable.

def multichoose(n,k):
    if k < 0 or n < 0: return "Error"
    if not k: return [[0]*n]
    if not n: return []
    if n == 1: return [[k]]
    return [[0]+val for val in multichoose(n-1,k)] + \
        [[val[0]+1]+val[1:] for val in multichoose(n,k-1)]

Here is the conversion I have so far:

public double[] MultiChoose(int n, int k)
{
    if (k < 0 || n < 0)
    {
         throw new Exception();
    }

   if (k == 0)
   {
       return [[0]*n]; // I have no idea what the [[0]*n] syntax means
   }

   if (n == 0)
   {
       return new double[0]; // I think this is just an empty array
   }

   if (n == 1)
   {
       return new double[1] {k}; // I think this is just an empty array
   }

   //Part I don't understand
   return [[0]+val for val in MultiChoose(n-1,k)] + \
        [[val[0]+1]+val[1:] for val in MultiChoose(n,k-1)]
} 

My question is: How do I convert the python code?

Answer 1

I would use LINQ in C# to translate the code:

• [] is the empty list.

  Enumerable.Empty<T>()
  

• [x] is the list containing a single item, x.

  Enumerable.Repeat(x, 1)
  

• [[0]*n] is the list containing the list containing n copies of 0.

  Enumerable.Repeat(Enumerable.Repeat(0, n), 1)
  

• [X for Y in Z] is a list comprehension.

  from Y in Z select X
     - or -
  Z.Select(Y => X);
  

• X + Y (where X and Y are lists) is list concatenation.

  Enumerable.Concat(X, Y)
  

The signature of MultiChoose would be:

 public IEnumerable<IEnumerable<double>> MultiChoose(int n, int k);