How do I remove an item from a list in EK9?

EK9 provides two removal operators: -= for mutating the list and - for creating a new list without the item.

MUTATING REMOVE (-=)

Remove the first occurrence of an item:

  numbers -= 3

Modifies the list in place. If the item is not found, no change.

NEW LIST (-)

Create a new list without the item:

  filtered <- numbers - 3

The original list is unchanged.

STREAM FILTER FOR COMPLEX REMOVAL

Remove items matching a condition using stream pipeline:

  cat numbers | filter by isPositive | collect as List of Integer

See Q45 for List basics. See Q88 for List operations. See Q89 for stream pipelines.

Example

defines module qa.collectiontasks.listremove

  defines function

    isGreaterThanTwo() as pure
      -> num as Integer
      <- rtn <- Boolean()
      expectedSize <- 2
      rtn: num > expectedSize

  defines program
    ListRemoveDemo()
      stdout <- Stdout()

      // === MUTATING REMOVE (-=) ===

      numbers <- [1, 2, 3, 4, 5]
      stdout.println(`Before: ${numbers}`)

      numbers -= 3
      stdout.println(`After -= 3: ${numbers}`)

      // Remove item not in list — no change
      numbers -= 99
      stdout.println(`After -= 99 (no change): ${numbers}`)

      // === NEW LIST (-) ===

      original <- [10, 20, 30, 40]
      without30 <- original - 30
      stdout.println(`Original: ${original}`)
      stdout.println(`Without 30: ${without30}`)

      // === STREAM FILTER FOR COMPLEX REMOVAL ===

      // Keep only items greater than 2
      mixed <- [1, 2, 3, 4, 5]
      filtered <- cat mixed | filter by isGreaterThanTwo | collect as List of Integer
      stdout.println(`Filtered (> 2): ${filtered}`)

      // === REMOVE FROM STRINGS ===

      names <- ["Alice", "Bob", "Charlie"]
      names -= "Bob"
      stdout.println(`After removing Bob: ${names}`)

Common mistakes

E50060 — EK9 uses the -= operator to remove items from a list, not a remove() method. See ek9 -h E50060 for details.

Incorrect:

numbers.remove(3)

Correct:

numbers -= 3

Other ways to ask this

How do I delete an element from a list in EK9?
What is the EK9 equivalent of list.remove()?
How do I drop an item from a list in EK9?

Coming from another language?

Java: list.remove(item), list.removeIf(predicate). Python: list.remove(item), list comprehension for filtered. Rust: vec.retain(predicate), vec.remove(index). Go: manual loop and slice manipulation. JavaScript: arr.filter(), splice for in-place. Kotlin: list.minus(item), mutableList.remove(). EK9: list -= item for mutating, list - item for new list, stream filter for complex removal.

Keywords: collection, remove, mutate, element, minus, delete, task, drop, list, item, filter