Show me how to use the ++ and -- operators on an Integer counter, demonstrating that they are statement-only and modify the variable in place.

← Operators and Expressions · Ref: Q1259

EK9 supports ++ and -- on Integer (and many other types). They are STATEMENT-ONLY operators — they modify the variable in place and produce NO value. You cannot use them inside expressions.

BASIC USAGE

  visitorCount <- 0
  visitorCount++       // OK: standalone statement
  visitorCount++       // OK: each call increments by 1
  visitorCount--       // OK: decrement by 1

WHAT YOU CANNOT DO

  copy <- visitorCount++   // ERROR: ++ produces no value to assign
  if visitorCount++        // ERROR: ++ has no Boolean result
  total <- a + b++         // ERROR: ++ cannot be inside an expression

WHY STATEMENT-ONLY

In C/Java/JavaScript, ++ returns the OLD value (postfix) or NEW value (prefix). This creates subtle bugs:

  x = counter++   // C: x gets old value
  x = ++counter   // C: x gets new value

EK9 eliminates this ambiguity by making ++ purely a side-effect statement. To get the value, use the variable separately AFTER the increment.

CORRECT PATTERN

  visitorCount <- 0
  visitorCount++
  current <- visitorCount       // Read the value separately
  stdout.println(`After ++: ${current}`)

USAGE IN A LOOP

  retries <- 0
  maxRetries <- 3
  while retries < maxRetries
    retries++
    stdout.println(`Attempt ${retries}`)

This QA uses Integer specifically. See Q1260 for Float ++/--, Q1261 for Date ++/--, and Q1262 for Enumeration ++/--. The same statement-only semantics apply to every type that supports ++ and --.

See Q241 for mutation operators overview. See Q238 for the complete operator set.

Example

defines module qa.operators.incrementinteger

  defines program

    IncrementIntegerDemo()
      stdout <- Stdout()

      visitorCount <- 0
      visitorCount++
      visitorCount++
      visitorCount++
      stdout.println(`After three ++: ${visitorCount}`)

      visitorCount--
      stdout.println(`After one --: ${visitorCount}`)

      retries <- 0
      maxRetries <- 3
      while retries < maxRetries
        retries++
        stdout.println(`Attempt ${retries}`)

Common mistakes

E07510 — ++ is statement-only. It does not return a value, so it cannot be used as the right-hand side of an assignment. Increment first, then read the variable. See ek9 -h E07510 for details.

Incorrect:

current <- visitorCount++

Correct:

      visitorCount++
      visitorCount++

Other ways to ask this

Increment and decrement an Integer using ++ and --.
Write a small program that uses ++ to count up and -- to count down.
Show me how Integer ++ works in EK9 — is it the same as Java?
Demonstrate that ++ on an Integer is statement-only and not an expression.

Coming from another language?

C/C++: ++ and -- are expressions returning a value. Java: same as C, with prefix/postfix distinction. JavaScript: same. Python: no ++ or --, use += 1. Go: ++ is statement-only (no value), like EK9. Kotlin: inc()/dec() functions or += 1. Rust: no ++, use += 1. EK9: matches Go's design — statement-only, no value, no prefix/postfix ambiguity.

Keywords: decrement, in place, --, ++, increment, Integer, statement only, counter