`for` special form

Questions / Help
1

White wandering through exercism.io solutions, I just stumbled upon this:

# rows/1, and columns/1 are functions that return lists of columns and rows, respectively

def saddle_points(str) do
  for {row, ri} <- Enum.with_index(rows(str)),
      {col, ci} <- Enum.with_index(columns(str)),
      max = Enum.max(row),
      min = Enum.min(col),
      min == max,
      do: {ri, ci}
end

In my opinion this is a very smart solution to finding saddle points in a matrix (my own was extremely over-engineered). Now that looked at this code, I realize I might not have a good understanding of how for special form works, and what else you can do with it.

I knew that one can specify filters (such as ==, for example) and multiple dependent lists to be read from, e.g.:

for my_list <- list_of_lists,
    element <- my_list,
    element != 3,
    do: element

But this example in the beginning of the post suggests there are more powerful ways of using for:

  1. Lists may not be dependent at all – I’ve never thought about it,
  2. One may use assignment (pattern matching), such as max = Enum.max(row), too – how/why does this even work?

So, my mind is blown, and I just wanted to share this. Also, is there anything else I’m missing from fully understanding the for special form?

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2

The name to google you are looking for is elixir list comprehensions. Read this first:

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