Sort a list of Item records by price and collect the sorted result.

To sort by price, give the stream a comparator function and use 'sort by':

  sorted <- cat items | sort by comparePrice | collect as List of Item

A bare 'sort' uses the class's 'default operator', which generates <=> comparing fields in declaration order. For Item, name is declared first, so a bare sort orders by name, not price. Use 'sort by' with a comparator (or declare price first) to order by price. See Q1085, Q1073.

Example

defines module qa.streams.streamrecordpipeline

  defines record

    Item
      name as String: String()
      price as Float: 0.0

      Item()
        ->
          name as String
          price as Float
        this.name :=: name
        this.price :=: price

      default operator

  defines function

    comparePrice() as pure
      ->
        left as Item
        right as Item
      <- rtn as Integer: left.price <=> right.price

  defines program

    StreamRecordPipelineDemo()
      stdout <- Stdout()

      items <- List() of Item
      items += Item("Monitor", 299.99)
      items += Item("Mouse", 19.99)
      items += Item("Keyboard", 49.99)

      //Sort by price with a comparator (a bare sort would use the default <=> and order by name)
      sorted <- cat items | sort by comparePrice | collect as List of Item
      stdout.println("Sorted:")
      cat sorted > stdout

Common mistakes

E50001 — EK9 uses stream pipelines for sorting: write a comparator function and use cat | sort by comparator.

Incorrect:

      items.sort(Comparator.comparing(Item::getPrice))

Correct:

      sorted <- cat items | sort by comparePrice | collect as List of Item

Other ways to ask this

I have product objects and need to sort them by price using a stream
In Java I'd use stream().sorted(). Write the EK9 record sort pipeline
Given a list of items with names and prices, sort and output them
Build a stream pipeline that sorts custom records by their natural ordering

Coming from another language?

Java: stream().sorted(Comparator.comparing(Item::getPrice)). Python: sorted(items, key=lambda x: x.price). EK9: cat items | sort | collect.

Keywords: stream, record, pipeline, sort, order, collect