Jumbo Calculator for iPad is Beautiful but Slightly Flawed

The Jumbo Calculator is a simple four-function calculator, plus square and square root, that fills the iPad screen. It’s great for casual calculations, but that’s about all.

Jumbo Calculator by Christopher Weems is number #2 on the Top Free list at the App Store. It’s a great calculator with big buttons, perfect for doing simple arithmetic, say balancing a checkbook (the old fashioned way) or casual calculations for, say, carpeting calculations, shopping and so on. The big buttons make it easy and fun to use.

One problem I noted with the calculator, however, was in the square root calculations. Because all computers use base-2 arithmetic, but our number system is base-10, there are always rounding issues when converting decimal numbers to binary and then doing calculations.

For example, if you take the square root of 2, Jumbo Calculator displays 1.414214. If you then square that, you’ll get the answer shown in the display above. It’s a tiny error, but that’s not the point. People who have been both building real calculators and simulators on computers have dealt with this issue for 30 years. There are ways to fix this problem, that is, display 2.0 again, without resorting to BCD arithmetic in the simulator.

Another potential problem I found is the sequence 3 + =.  The provided answer is 6 when I think it should be 3. The Texas Instruments BAII+, in either AOS or Chn mode, gives the correct answer of 3. (Three plus nothing is three.) However, other iPhone calculators I tested, including PCalc Lite, provide 6. So there are some convention issues to be aware of.

The moral of the story is that when building a calculator for the iPhone or iPad, it’s not just a matter of sitting down with the SDK and calling the arithmetic APIs. Writing calculator simulations is a job fraught with pitfalls.

The solar panel is for effect and isn’t functional. Also, Jumbo Calculator doesn’t have a CE or backspace key and doesn’t have any display customization. You get what you pay for.

On the whole, Jumbo Calculator is a friendly, very usable calculator, great for kids and grandmoms. While my gripe above about the square root is a nit, it’s also danger sign for other developers.

Be aware that there are professional-grade calculators out there for the iPad, like PCalc and PCalc Lite (which is also free) that also have big displays and don’t suffer from casual oversights that trip up less experienced developers.

Jumbo Calculator 1.0.2 is free and requires iPhone OS 3.2 or later.