How do I compare two money amounts and find the cheaper price in EK9?

← Operators and Expressions · Ref: Q1189

EK9 Money uses the same comparison and coalescing operators as every other type. Literals use amount#CURRENCY format.

MONEY LITERALS

  priceA <- 29.99#USD
  priceB <- 45.50#USD

COMPARISON

  if priceA < priceB          cheaper price
  ordering <- priceA <=> priceB  negative = cheaper, zero = same, positive = more expensive

COALESCING

  cheaper <- priceA <? priceB    returns the lower price
  dearer <- priceA >? priceB     returns the higher price

IMPORTANT: Comparison only works between same-currency Money values. Comparing 10#GBP with 20#USD returns unset — EK9 prevents accidental cross-currency comparison.

See Q35 for Money basics. See Q149 for currency safety. See Q243 for coalescing operators.

Example

defines module qa.operators.moneycompare

  defines function

    cheaperOf() as pure
      ->
        left as Money
        right as Money
      <- rtn as Money: left <? right

    dearerOf() as pure
      ->
        left as Money
        right as Money
      <- rtn as Money: left >? right

  defines program

    MoneyCompareDemo()
      stdout <- Stdout()

      // === MONEY LITERALS ===

      priceA <- 29.99#USD
      priceB <- 45.50#USD
      sameAsA <- 29.99#USD

      // === COMPARISON ===

      if priceA < priceB
        stdout.println("Price A is cheaper")

      require priceA < priceB
      require priceB > priceA
      require priceA == sameAsA
      require priceA <> priceB

      // === SPACESHIP ORDERING ===

      ordering <- priceA <=> priceB
      stdout.println(`PriceA <=> PriceB: ${ordering}`)
      require ordering < 0

      // === COALESCING: FIND CHEAPER AND DEARER ===

      cheaper <- cheaperOf(priceA, priceB)
      dearer <- dearerOf(priceA, priceB)
      stdout.println(`Cheaper: ${cheaper}`)
      stdout.println(`Dearer: ${dearer}`)
      require cheaper == priceA
      require dearer == priceB

      // === STRING CONVERSION ===

      priceStr <- $priceA
      stdout.println(`Price as string: ${priceStr}`)

      // === ISSET ===

      require priceA?
      unsetMoney <- Money()
      require ~unsetMoney?

      // === COPY OPERATOR ===

      backup <- 0.00#USD
      backup :=: priceA
      require backup == priceA
      stdout.println(`Copied price: ${backup}`)

Common mistakes

E50001 — EK9 has no Math type, so Math.min will not resolve (E50001); use the <? coalescing operator to select the lesser value. See ek9 -h E50001 for details.

Incorrect:

cheaper <- Math.min(priceA, priceB)

Correct:

cheaper <- cheaperOf(priceA, priceB)

Other ways to ask this

Compare two Money values and select the lesser amount.
I need to find the cheapest item from two prices using operators.
In Java I used BigDecimal.compareTo for money — what does EK9 use?

Coming from another language?

Java: BigDecimal.compareTo(), no literal syntax, manual RoundingMode. Python: decimal.Decimal comparison. Rust: no built-in money type. Go: no built-in money type. EK9: 29.99#USD literals, direct < > == operators, <? coalescing, automatic HALF_UP rounding, cross-currency safety.

Keywords: currency, usd, price, cost, compare, operator, coalescing, gbp, money, cheaper