Search code examples
arraysnestedfilteringapldyalog

Dyalog APL: How to filter an array like filter()

I need to filter out from the array (namespaces) all values that do not fulfil the required condition: field 'status' = 'TRADING'

I use this variant

tmpSymbolsT←(data←⎕JSON(HttpCommand.Get'api.binance.com/api/v3/exchangeInfo').Data).symbols
tmpSymbols←{(⍵.status≡'TRADING')⌿⍵}⍨¨tmpSymbolsT

However, it returns every row in the wrapper, and I want clean rows

      tmpSymbols[1] ⍝ I have
┌─────────────────────────────────┐
│┌───────────────────────────────┐│
││ #.[JSON object].[JSON object] ││
│└───────────────────────────────┘│
└─────────────────────────────────┘


      tmpSymbolsT[1] ⍝ I need like this
#.[JSON object].[JSON object]

How to do it?

Solution

  • The problem here is that you filter each (¨) object separately, rather than filtering the entire collection.

    (By the way, you also have an unnecessary commute (⍨) which doesn't do anything in this context.)

    So, you really want something along the lines of tmpSymbols←{(⍵.status≡'TRADING')⌿⍵}tmpSymbolsT but now ⍵.status is a vector (list) of statuses, which of course won't match (≡) the single status 'TRADING'. You have a few options for fixing this:

    1. ⍵.status≡¨⊂'TRADING' uses an explicit each and encloses (⊂) the single status so it is used for every comparison.
    2. 'TRADING'∘≡¨⍵.status binds (∘) the single status to the match function, thus deriving an "is trading" function, which is then applied to each status.
    3. ⍵.status∊⊂'TRADING' which encloses the single status and checks all the statuses for which ones are a member of (∊) the array of a single status.

    Here is an example of code that does what you want:

          tmpSymbolsT←(data←⎕JSON(HttpCommand.Get'api.binance.com/api/v3/exchangeInfo').Data).symbols
      tmpSymbols←{(⍵.status∊⊂'TRADING')⌿⍵}tmpSymbolsT
      tmpSymbols[1]
#.[JSON object].[JSON object]