What should a counter or a running total start from in EK9?

← Operators and Expressions · Ref: Q2252

Start a counter or a sum from 0, and a product from 1 - a literal, which is a SET value - then change it with '++', '+=' or '*='.

  total <- 0
  for amount in amounts
    total += amount

  count <- 0
  for amount in amounts
    if amount > threshold
      count++

WHY NOT Integer()
'Integer()' makes an Integer that is present but UNSET - no value yet. '+=', '-=', '*=', '/=', '++' and '--' on an unset value keep it unset whatever is added: the operator never invents the missing starting value. So a total started from 'Integer()' stays unset however many amounts are added. The compiler is sure of that here, so it reports E08096 at the '+=' rather than let the loop run for nothing:

  total <- Integer()
  for amount in [10, 20, 30]
    total += amount     //E08096: 'total' is always unset here

The same holds for Float (start from 0.0), Duration (PT0S), Millisecond (0ms), String ("", see Q2251) and every other built-in type with these operators.

The compiler reports it only when it is SURE: a value that came from a function call may be set, so 'total <- supplied()' then 'total += 1' is never reported. See Q2253, and Q2251 for building up a String.

Example

#!ek9
defines module qa.operators.countandsumfromzero

  defines constant
    THRESHOLD <- 5

  defines program

    CountAndSumDemo()
      stdout <- Stdout()
      amounts <- [2, 4, 6, 9]

      total <- 0
      for amount in amounts
        total += amount
      stdout.println(`total: ${total}`)

      count <- 0
      for amount in amounts
        if amount > THRESHOLD
          count++
      stdout.println(`above threshold: ${count}`)

      product <- 1
      for factor in 1 ... 5
        product *= factor
      stdout.println(`product: ${product}`)

Common mistakes

E08096 — Integer() makes an unset Integer and '+=' keeps an unset Integer unset, so the total never gets a value. Start from 0. See ek9 -h E08096 for details.

Incorrect:

total <- Integer()

Correct:

total <- 0

E08096 — '++' on an unset Integer keeps it unset, so the count never starts. Start from 0. See ek9 -h E08096 for details.

Incorrect:

count <- Integer()

Correct:

count <- 0

E08096 — '*=' on an unset Integer keeps it unset. A product starts from 1. See ek9 -h E08096 for details.

Incorrect:

product <- Integer()

Correct:

product <- 1

Coming from another language?

Java: int total = 0; an Integer total left null throws NullPointerException on 'total += x'. Python: total = 0. Kotlin: var total = 0. EK9: 'total <- 0'. 'Integer()' is an UNSET Integer, which '+=' keeps unset, so the compiler reports E08096.

Keywords: *=, ++, +=, E08096, Integer(), accumulate, counter, start from zero, sum, total, unset