How do filter and sort work together in EK9 streams?

Chain filter and sort with the pipe operator. Filter first to reduce the dataset, then sort the remaining elements.

PATTERN

  cat items | filter by predicate | sort | collect as List of T
  cat items | filter by predicate | sort > stdout

FILTER narrows the stream to elements matching the predicate (a function returning Boolean). SORT orders the remaining elements using their natural ordering (the <=> operator).

WITH STRINGS

  cat names | filter by startsWithA | sort > stdout

Filters to names starting with 'A', then alphabetically sorts them.

WITH FLOATS

  cat temperatures | filter by aboveFreezing | sort | collect as List of Float

Keeps temperatures above zero, sorts ascending.

Filtering before sorting is more efficient: sort operates on fewer elements.

See Q235 for all stream operations. See Q896 for output redirection. See Q237 for streams vs loops.

Example

defines module qa.streams.filter.sort.example

  defines function

    aboveFreezing() as pure
      -> temperature as Float
      <- rtn as Boolean: temperature > 0.0

    isLongName() as pure
      -> nameToCheck as String
      <- rtn as Boolean?
      minLength <- 4
      rtn: length nameToCheck > minLength

  defines program

    FilterSortDemo()
      stdout <- Stdout()

      //Demonstrate filter predicates that streams would use
      stdout.println(`15.5 above freezing: ${aboveFreezing(15.5)}`)
      stdout.println(`-3.2 above freezing: ${aboveFreezing(-3.2)}`)

      //Demonstrate string predicate for stream filter
      stdout.println(`James is long: ${isLongName("James")}`)
      stdout.println(`Bob is long: ${isLongName("Bob")}`)

Other ways to ask this

Can I chain filter and sort in an EK9 stream?
How do I filter then sort a list in EK9?
What is the correct order for filter and sort in EK9 pipelines?

Coming from another language?

Java: stream().filter(pred).sorted().collect(). Python: sorted(filter(pred, items)). Rust: iter().filter(pred).sorted(). EK9: cat items | filter by pred | sort | collect.

Keywords: stream, filter, order, chain, pipeline, sort, predicate