Filter positive numbers, double them, and sort the result.

Chain pipe-separated stages left to right:

  cat items | filter by isPositive | map by doubleIt | sort > stdout

cat creates the stream, each | adds an operation, > stdout terminates. See Q235, Q1073.

Example

defines module qa.streams.filtermapsort

  defines function

    isPositive() as pure
      -> item as Integer
      <- rtn as Boolean: item > 0

    doubleIt() as pure
      -> item as Integer
      <- rtn as Integer: item * 2

  defines program

    FilterMapSortDemo()
      stdout <- Stdout()

      items <- [3, -1, 4, -5, 2, -3, 1]

      cat items
        | filter by isPositive
        | map by doubleIt
        | sort
        > stdout

Common mistakes

E01010 — EK9 uses 'cat source | operation | operation > output' syntax, not method chaining.

Incorrect:

      items.stream().filter(isPositive).map(doubleIt).sorted()

Correct:

cat items
        | filter by isPositive
        | map by doubleIt
        | sort
        > stdout

Other ways to ask this

I have a list with negative values and need to keep only positives, transform, then sort
In Java I'd use stream().filter().map().sorted(). Write the EK9 pipeline
Given mixed integers, build a pipeline: filter positives, double each, sort ascending
Chain filter, map, and sort operations in a single stream pipeline

Coming from another language?

Java: stream().filter(x -> x > 0).map(x -> x * 2).sorted(). Python: sorted(x*2 for x in items if x > 0). EK9: cat items | filter by pred | map by fn | sort.

Keywords: chain, map, stream, filter, sort, pipeline