How to Control Map Key Order in Elixir for Custom Report Formatting?

If you need sorted keys then a Map is the wrong data structure. There are lists, keyword lists, records, custom data structures, etc.

Search for “map with ordered keys” as this has been discussed on here a few times.

Have you considered using structs with a custom inspection protocol implementation?

Maps with string keys deep into the program always smell.

Maps are an unordered dataformat. The BEAM just has a default sort order given it has a global ordering between any term it can represent, which is used to order things where they need to become ordered. So the solution here is to use a datatype which does retain order (lists) instead of maps.

For your usecase I’d even argue that this is a pure view layer concern – as in how data is presented – so your view layer code could simply order the columns as needed before rendering the data.

I haven’t tried this. It looks like I just need to use lists.

It’s not entirely a view issue. I have a pivot table module I pass any list of map and specify what should be rows, colum to transpose and colum to use as value for sum, count etc… then I render it as html, csv, pdf etc… to download or render in a browser.

I will try with lists and see how it works. So far it looks like a list of tuples maintains the order.

Sounds like presentation layer…

I always seem to struggle with choosing the right data structure for modeling data. My (probably naive) way of thinking about it that maps are for data that will need fast insert, fast retrieval, and fast updates but for which order is not (as) important. Lists (including keyword lists) are for things that need fast appends, LIFO order is important, random access does not need to be fast. Key-value pairs that need ordering but also reasonably fast updates, inserts, and lookups I tend to reach for a gb_tree.

Also, you can implement a custom sort function and run your map through as seen in this thread Enum.sort_by to sort a record set (i.e. a list of maps) based on 2 "columns" - #2 by benwilson512

Historical trivia: back before maps were introduced into Erlang, Elixir had a module named ListDict which managed the data structure you describe – an ordered list of two element tuples that could be treated like a map.

To make matters worse (? ) the ordering of elements in maps is completely undefined and changes depending on the number of elements. It use to be that for less than 32 elements the ordering was term ordering and for more elements it was “random” depending on the internal search algorithm used.

The thing to remember is that the actual ordering is purely internal. There are iterator functions which can give you some control of which order you want to step over the elements.

Convert to a list of tuples, then sort using a comparator function

map
  |> Map.to_list()
  |>  |> Enum.sort_by(fn {key, _value} -> key end)