How-To Exclude results from List Comprehensions

You can have a filter in your comprehension:

for n <- 1..4, rem(n, 2) == 0, do: n

EDIT:

I think I misread the question. I’d probably just use Enum.reduce instead

1. In the example you wrote the list comprehension would return the nil and other values i.e. the case does not filter out nil
2. Given the nil values are a result of _compute unless you know the exact values of x which would result in a _computer(x) == nil you can not prefilter the input.
3. It is very common to generate a list of results and post-filter the results

I feel the post filter is a wasted step, if we can have a way to exclude the nil values directly, that’s my point…

still wont help… you mean Enum.filter

don’t know if you consider this better, but here’s a solution

List.foldr([1, 2, nil, 4], [], fn x, acc -> if is_nil(x), do: acc, else: [x*2|acc] end)

iex(1)> List.foldr([1, 2, nil, 4], [], fn x, acc -> if is_nil(x), do: acc, else: [x*2|acc] end)
[2, 4, 8]
iex(2)> 

edit: it’s also not clear to me whether it had to be a comprehension or not, so maybe this isn’t a solution?

Got this elegant solution from StackOverflow:

for x <- list, y = _compute(x), do: y

I think the idea is if _compute(x) is truthy, return y, else if false / nil, it will be skipped, and you only have to evaluate _compute(x) once

No, I mean Enum.reduce though it will look similar to your solution.

Enum.reduce(some_list, fn(x, acc)->
  case do_stuff(x) do
    nil -> acc
    other -> [other | acc]
  end
end)

That’s a nice solution