How to extract info from an Enum.reduce function other than a single item

lc0815

I have a need to extract the individual items from an Enum.reduce function but I get is the single value from the acc.

Here is the sample code:

defmodule Demo do
  def add_sequence_number(business_card) do

    business_card
        |> String.split("\n")
        |> Enum.reduce(1, fn (x, acc) -> IO.inspect "#{acc}$#$#{x}"; acc + 1 end)

  end
end

business_card = """
Bruce Waine,Batman
Bat cave
Gotham City, NY 12345
987.123.5984

Gotham Security Services Co
Crime fighter
Wonder Woman
"""

new_card = Demo.add_sequence_number(business_card)

IO. puts "\nthis is the result:  #{new_card}\n"

here is a screen shot of the result:

what I need is the array from the inner reduce function but I am getting is the result of 10 and I need to access the list as in the image.

any tips, thank you

13 comments

#troubleshooting #reduce #elxir_code

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2022-02-02 16:39:05 UTC

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kip

ex_cldr Core Team

Whatever the function returns become acc on the next iteration. And whatever acc is at the end is returned by Enum.reduce/3. Therefore you will be accumulating a list of results in many common cases, including this one.

Think more along the lines of:

{count, results} =
  Enum.reduce(cards, {0, []}, fn (x, {seq, acc) ->
    seq = seq + 1
    {seq, "#{seq}$#$#{x}"}
  end)

Post #1

dimitarvp

You don’t exactly need Enum.reduce then, you likely need Enum.with_index plus Enum.map, but it can be done with Enum.reduce by using a tuple accumulator as @kip showed you.

Post #2

lc0815

thanks kip, will try that…

Post #3

lc0815

thanku dimitarvp working on kip’s suggestion. I’m trying to wrap my head around the reduce function…

thanks again

Post #4

lud

Also Enum.map_reduce !

Post #5

lc0815

thanks lud, that was the answer…

Post #6

eksperimental

also list comprehensions, (for)

Post #7

al2o3cr

A helpful way to think about these situations is to think about the types.

Enum.reduce expects a function of type (element, accumulator) -> accumulator and returns a result of type accumulator.

Enum.scan expects a function of type (element, accumulator) -> accumulator and returns a result of type [accumulator], so it’s worth considering if you’re expecting a list of results.

A good example for comparing the two is with lists of integers:

numbers = [1, 2, 3, 4]

Enum.reduce(numbers, 0, fn n, acc ->
  n + acc
end)
# returns the TOTAL: 10

Enum.scan(numbers, 0, fn n, acc ->
  n + acc
end)
# returns the list of SUBTOTALS: [1, 3, 6, 10]

In your example, using binaries (like "1$#$Bruce Waine, Batman") as an accumulator type is less than ideal. A better intermediate format would be a tuple like {1, "Bruce Waine, Batman"}, resulting in code like:

business_card
|> String.split("\n")
|> Enum.scan({0, ""}, fn line, {acc_number, _} ->
  {acc_number+1, line}
end)
|> Enum.map(fn {n, line} ->
  "#{n}$#$#{line}
end)

Post #8

lc0815

@al2o3cr thanks Matt great explanation, I’m starting from scratch again. this little exercise was to practice pattern matching using a sample business card and reformat two sections the personal profile and the company profile. so much for easy examples…

thanks for you help, I guess I have to go back and ready all of the Enums, Maps and related docs… it never ends…

Post #9