Can I use ++ in an assignment like y <- x++ in EK9?

← Operators and Expressions · Ref: Q940

EK9 has ++ and -- but they are STATEMENT-ONLY. They modify the variable in place and produce no value. You cannot use them inside expressions.

STATEMENT-ONLY MEANS

  counter++          CORRECT — standalone statement
  counter--          CORRECT — standalone statement
  y <- counter++     WRONG — ++ produces no value to assign
  if counter++       WRONG — ++ produces no Boolean

WHY THIS DESIGN

In C and Java, x++ returns the OLD value while ++x returns the NEW value. This causes subtle bugs:

  // C or Java: which updates first?
  array[i++] = array[++j];

EK9 eliminates this entire bug category by making ++ and -- void operations.

IF YOU NEED THE OLD VALUE

  saved <- Integer(counter)
  counter++
  // saved has old value, counter has new value

Take a COPY with Integer(counter). A plain 'saved <- counter' names the SAME Integer object, and ++ changes that object in place, so both names would show the new value (see Q905).

This is the same design choice as Go, which also makes ++ and -- statements only.

See Q780 for detailed explanation. See Q238 for operator overview.

Example

defines module qa.operators.incrementstmt

  defines program

    IncrementDemo()
      stdout <- Stdout()

      counter <- 0

      //Correct: ++ as standalone statement
      counter++
      counter++
      counter++
      stdout.println(`After 3 increments: ${counter}`)

      //Correct: -- as standalone statement
      counter--
      stdout.println(`After decrement: ${counter}`)

      //Copy before incrementing if you need the old value:
      //'previous <- counter' would share the object and ++ would change both
      previous <- Integer(counter)
      counter++
      stdout.println(`Was ${previous}, now ${counter}`)

Common mistakes

E07950 — The ++ operator is statement-only in EK9 — it returns void and cannot be used in expressions. Take a copy with Integer(counter) on its own line, then increment on the next. See ek9 -h E07950 for details.

Incorrect:

      previous <- counter++

Correct:

      previous <- Integer(counter)

Coming from another language?

C/C++: pre/post increment as expressions, undefined behavior in complex expressions. Java: pre/post increment as expressions. Go: ++ and -- are statements only (same as EK9). Python: no ++ or -- at all. Rust: no ++ or -- at all. EK9: ++ and -- are statements only, like Go.

Keywords: statement, increment, assign, decrement, void, bug, expression, E07950