Remove all negative numbers from a list, keeping only positives.

← Collections and Data Structures · Ref: Q1162

Use a stream pipeline to filter and collect into a new list:

  positives <- cat numbers | filter by isPositive | collect as List of Integer

EK9 does not have a .removeIf() method on lists. Filtering is done through stream pipelines, creating a new collection with only the desired elements. See Q235 for stream operations. See Q122 for collect as.

Example

defines module qa.collections.removematching

  defines function

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

    intToString() as pure
      -> num as Integer
      <- rtn as String: $num

  defines program

    RemoveMatchingDemo()
      stdout <- Stdout()

      numbers <- [5, -3, 8, -1, 4, -7, 2, -9, 10]
      stdout.println(`Original: ${numbers}`)

      // Filter to keep only positives
      positives <- cat numbers | filter by isPositive | collect as List of Integer
      stdout.println(`Positives only: ${positives}`)

      // Stream filtered results directly
      stdout.println("Streamed positives:")
      cat numbers | filter by isPositive | map with intToString > stdout

Common mistakes

E50001 — EK9 lists do not have a removeIf() method. Use a stream pipeline with filter and collect to create a new list with the desired elements. See ek9 -h E50060 for details.

Incorrect:

numbers.removeIf(isNegative)

Correct:

positives <- cat numbers | filter by isPositive | collect as List of Integer

Other ways to ask this

Write code to filter out unwanted elements from a list
I have a list of integers and need a new list with only positive values
Given a list with mixed positive and negative numbers, produce a positives-only list
In Java I'd use stream().filter(n -> n > 0).collect(). Write the EK9 equivalent

Coming from another language?

Java: list.removeIf(n -> n < 0) or stream().filter().collect(). Python: [n for n in nums if n > 0]. Rust: vec.retain(|n| *n > 0). Go: manual loop. EK9: cat | filter by | collect as — always creates a new list.

Keywords: pipeline, remove, negative, filter, positive, collect, list, stream