A calculator can be just a calculator — except when it gets the wrong answers. This algebraic calculator tries to rise above being a mundane calculator by adding a touch of class, a scribble pad, a paper tape, and even some desktop decorations. Unfortunately, it has a serious mathematical problem.

**Basic Mode**

First, the good news. Calculators on the iPhone don’t have much room for anything but a keyboard and a display. However, the iPad offers just enough screen real estate that developers can rethink the classic calculator and add some additional functionality. Calculator HD for iPad does that.

It’s easy to tell when the developer has thought through the design process. This calculator is a prime example, and here are the important features:

- User definable calculator mode for both portrait and landscape
- Four modes: Basic, notes, paper tape and scientific
- Optional paper tape print in all modes
- E-mail the paper tape output
- A one page scribble pad for notes or a sketch
- A sexy desktop background, called “decorations”

**Scientific Mode**

Where this calculator excels in design, it fails in terribly as a scientific calculator. Writing any calculator, whether it’s a Texas Instruments calculator bought a Wall-Mart for high school trig or an iPad calculator is tricky business. Extreme attention must be paid to mathematical accuracy in all modes, but especially scientific mode. *This calculator fails.*

Things started out okay in my initial tests. I got some confidence when the developer seemed to pay attention to something like 83/17 and then the result multiplied by 17 to get an integer 83 back. Some calculators exhibit rounding issues and will return, say, 83.000000001. This calculator handles that nicely. It’s also comforting that this app could handle something like the sine of 40,000 degrees.

I went through some typical scientific single function calculations (log, sin) to see how this calculator compared to my Hewlett Packard HP-35s (physical) calculator and didn’t find any technical problems with those mathematical functions in brief tests.

**Settings**

**However, I quickly found a major problem. **

First, it should be noted that the x! (factorial) function is limited to integral values. At least the developer didn’t wander into unknown territory like the developer of Calculator Pro. (Those problems have since been fixed.) But there is a way to handle non-integer values for x! with the Gamma function, and modern calculators just do it.

The big problem I found was with the data entry and handling of very small numbers. I first verified the dynamic range for large numbers. It seems to be 10^{308.} However, if one tries to calculate the inverse (1/x) of say, 10^{27}, the result is displayed as *zero* instead of 10^{-27}. One can’t even enter small numbers. So, for example, entering 1, EE, 27, +/- should result in 1 x 10^{-27}. Instead the input is displayed as zero. The developer explained to me that “our implementation considers numbers smaller than about 10^{-15} as zero. It is not bad approximation for usual usage and it gives us some benefits in calculation speed etc.” I disagree. *This is a fatal flaw for a scientific calculator, and the behavior is not like any serious scientific calculator, within the limits of its dynamic range.* I note that this has to do with the display, and the value seems to be maintained internally. So for example, if I multiply the zero result above by 10^{30}, the calculator displays 1,000. Astounding.

Continuing my tests with other functions, I tried to calculate e^{-52} using the e^{x} function. The answer should be 2.6102791x 10^{-23}, confirmed with the HP-35s.^{ } Calculator HD, as explained above, displays zero. When you get into physics work, with things like Planck’s constant and the Gravitational constant, it’s easy to get into very small positive and negative numbers as a *final result*, so there’s no excuse for this behavior.

I also noted a nuance in convention for the percent usage. This is not an error, just a note to users. In calculating 6% of 200, the key sequence is:

200 , x , 6 , % , =

After you hit the % key, you’ll see 0.06 (confirmed by the TI BA II Plus for iPhone.) Some other calculators, like PCalc, will show the final result, 12, right after striking the % key. This is an arbitrary convention, according to James Thomson, the PCalc developer.

Finally, just about every modern calculator provides a backspace key to allow the user to fix keying errors. This calculator has none, so if you make a mistake, you must hit the Clear (C) key and start all over. That’s so yesteryear.

Prospective buyers should also note that this is an algebraic-only calculator and doesn’t support the RPN input mode. It requires iPad iOS 3.2 or later.

**Do I Recommend it?**

No. Given the problem above, found with just a few minutes of experimentation, I wonder what other deeper problems lurk. A calculator either inspires total confidence or, with one miscue, it fails your trust. This calculator failed my tests of trust, and I must once again warn our readers about these ninety-nine cent calculators that don’t attend to the details. I recommend PCalc instead.

**Calculator HD doesn’t display small numbers. (Showcasing the notepad)**