MoreLinq's Scan and For Loop returns different results

Question

I need to compute outputs from inputs using the formula:

Output(i) = inputs(i) * factor + outputs(i - 1) * (1 - factor)

I implemented this using a for loop and MoreLinq's Scan extension:

Int32 p = 5;

Decimal factor = (Decimal) 2 / (p + 1);

List<Decimal?> inputs = Enumerable.Range(1, 40).Select(x => (Decimal?)x).ToList(); 

// Using Scan extension  

List<Decimal?> outputs1 = inputs.Scan((x, y) => x * factor + (y * (1 - factor)) ?? 0).ToList(); 

// Using for loop

List<Decimal?> outputs2 = new List<Decimal?> { inputs[0] };

for (int i = 1; i < inputs.Count(); i++) 
  outputs2.Add(inputs[i] * factor + (outputs2[i - 1] * (1 - factor)) ?? 0);

However, I get different outputs results. What am I missing?

Am I using Scan incorrectly?

Answer 1

You are interpreting parameters of transformation function in the wrong order (check out the source code to see how transformation is invoked). Change your code to:

inputs.Scan((x, y) => y * factor + x * (1 - factor) ?? 0).ToList()

x is aggregator, i.e. previous value, y is the current one.