Question

I would like to filter collection, so distance between adjacent elements would be at least 5.

So List(1, 2, 3, 4, 5, 6, 7, 11, 20) will become List(1, 6, 11, 20).

Is it possible to achieve in one pass using filter? What would be scala-way?

Answer 1

Try with foldLeft():

val input = List(1, 2, 3, 4, 5, 6, 7, 11, 20)

input.tail.foldLeft(List(input.head))((out, cur) => 
  if(cur - out.head >= 5) cur :: out else out
).reverse

If it’s not obvious:

1. Algorithm starts with first element (probably you need some edge cases handled) in the output collection

2. It iterates over all remaining elements from the input. If the difference between this element (cur) and first element of input is greater than or equal than 5, prepend to input. Otherwise skip and proceed

3. input was built by prepending and examining head to get better performance. .reverse is needed in the end

This is basically how you would implement this in imperative way, but with more concise syntax.