> Many people know that you shouldn’t do decimal calculations, such as those involving U.S. dollars and cents, with the floating-point numbers in most programming languages. This is because decimal numbers can’t be expressed exactly as such floating-point numbers, so you will encounter rounding errors.

Why not, rounding to the nearest cent is going to be much less precise for any realistic amount of dollars when using 64 bit floats (the type of float Javascript uses in every browser).

▲ sluukkonen an hour ago | parent | next [-]

It’s easier just to use a proper decimal type than to remember all of the ways where using floats will bite you in the ass. The behavior is unintuitive even in trivial examples. Code like

  amount_due = 0.10 + 0.20
  amount_paid = 0.30
  
  if amount_paid >= amount_due:
      print("PAID")
  else:
      print("OUTSTANDING")

will print “OUTSTANDING”.

	▲	adrian_b an hour ago \| parent [-]
		True, but those who do not use decimal floating-point numbers do not use such binary floating-point numbers. They use binary fixed-point numbers/integers, where such computations are exact, while not having the huge computational overhead of decimal floating-point numbers.

▲ glimshe 2 hours ago | parent | prev | next [-]

The rounding behavior with true decimals is easy to control and understand across number magnitudes and doesn't suffer from platform specific quirks like floats/doubles.

The article's images clearly show the rounding error mess your get without decimals.

▲

sobriquet9 43 minutes ago | parent | next [-]

There's still catastrophic cancellation, where subtracting two large numbers that are very close results in an answer with reduced precision.

▲

amelius 2 hours ago | parent | prev [-]

But the rounding errors are way down in the nano-cents. Not worrying about them is cheaper!

	▲	andreareina an hour ago \| parent \| next [-]
		The decimal library worries about rounding, that's basically free. With floats you need to decide when to use rounding (because you don't want to show a balance of -0.000000086) and it's more difficult to use automatic checks for the same reason.
	▲	glimshe an hour ago \| parent \| prev [-]
		If you're calculating your monthly expenses that's okay, but across millions of transactions of arbitrary amounts, something financial systems do regularly, aggregates start not adding up and you don't know where the money went.

▲ knorker an hour ago | parent | prev [-]

"The nearest cent" is already a bad assumption. Should IEEE 754 representation dictate taxes and money splitting?

You could end up with splitting an account down the middle, and ending up with an extra cent being created out of thin air, or one destroyed. In billions of transactions each day, this could be a problem for balancing books when there is no longer any equality check.

When not using floats, the rules and checks become more… deterministic, if you don't mind stretching the definition of that word a bit.