The Cally app for iOS is a units and currency converter plus a scientific calculator. It is brilliant at conversions, but flubs badly as a calculator. In fact, the calculator part needs a complete overhaul.
The Great Stuff
As someone who has spent his whole adult life using scientific calculators, I can really appreciate a great tool for units conversion. There are some significant nuances associated with that task. Perhaps the largest of them is the proper sense of the conversion. That is, it's easy to apply a conversion factor, but it's no so easy to visually and unambiguously make the iron-clad association: "THIS is so many of THAT." And then do it both ways without a fuss.
Cally does that beautifully and with style as shown below. The blue bar suggests the direction, and that's especially helpful with currency. One dollar is worth 0.663 pounds (as of this writing).
Conversions are clear, unambiguous.
Tap the blue bar and the conversion instantly goes the other way: One pound sterling is worth US$1.508. There is zero ambiguity.
Tap the blue bar to reverse the conversion.
If conversions is the default page, and you can opt to make it so, then swiping to the right will reveal the available conversions. Recently used ones will be at the top.
Conversion options are extensive.
I tested some common conversions and didn't find any problems. I think the author has done a great job with this. The visual presentation and the details of the units in small but readable type is especially nice. That's the only good news.
The Not So Great Stuff
Swiping to the left will being up, at first, a general purpose calculator that works fairly well as an algebraic calculator, but even there, the calculator falls into some intricate user interface pitfalls. For example, if I'm preparing to enter 2 + 9 x 4, I know full well that precedence will force the multiplication to happen first. Yet the calculator jumps the gun and provides an intermediate result of 11 after I enter 2 + 9.
Some problems as a simple arithmetic calculator.
That's not wrong; but it could surprise and annoy the expert user when the 11 is erased and 38 finally replaces it. It actually seems like an attempt to emulate RPN usage in which intermediate results are supplied -- but fails -- in comparison to an excellent iOS calculator like PCalc which waits for the Equal sign to be pressed. All in all, I think I'd rather let the calculator show the expression, then compute the final result when it's time.
Another problem is that the developer has elected to make the entire calculator display live for both swiping and the specific key presses. That means that if you touch the key incorrectly, say, with a glancing blow, the app can be confused about whether that's a swipe or a key press. The display will jump and no key press will be registered.
There are larger issues, and they have to do with the second page of scientific functions, accessed with the "ADV" key.
Most scientific calculators, and this is one, allow the entering of an exponent with an "E" key. For example, the gravitational constant, 6.67 x 10-11 would be entered as 6.67, E (or Exp) , 11, +/-. The lack of an exponent key forces the user into a very complex sequence of keystrokes to achieve the same thing.
Scientific functions are on a separate page: bad design.
Most any scientific calculator would have the scientific functions on the same display. There may be a shift key for special cases like sin/sinh or sin/sin-1. But the sequence of keystrokes for either Algebraic or RPN notation has become fairly standardized over the years. (This calcultor has no RPN option.)
Cally, however, forces the user to frequently go back and forth between the ADV page of scientific functions and the main numerical data entry page. That requires a lot of key pressing, back and forth between the two displays, to calculate simple scientific expressions. In addition, if you make a mistake on the scientific functions page, there is no backspace key to correct the mistake.
Next, there is some developer inconsistency between postfix and prefix notation. Does the function operate on the number in the display? Sometimes not. For example, with "2" in the display, x2 calculates an immediate result, "4". But en (should probably be ex) opens an expression instead of producing 7.389. I think this will be very confusing to the average user.
The one thing I did like was the paper tape function.
A clearable history is a nice feature.
Finally there are some other omissions in this scientific calculator. The Factorial (or better, the Gamma) function is missing and is just as essential as trig or log functions. There is no way to set the precision or boundaries for scientific notation. As I mentioned, entering numbers with scientific notation is tedious. Separating the scientific functions on another page makes the evaluation of expressions more difficult than it should be. As a scientific calculator, it flubs.
Settings are minimal and can only be accessed from the conversion categories page. You can opt to have the calculator or the conversions be the default at launch. The only other option is to turn the key click on and off. This is a seriously limited set of options for a scientific calculator.
The clock symbol on the bottom right is called " last-five-saved-calculation traverser." It worked fine on my iPhone but consistently crashed the app on my iPad 3 with iOS 6.1.2. However, I was locked into version 1.0 and could not upgrade to 1.0.1 thanks to some kind of App Store or developer issue. It's not yet been resolved.
Also, unlike the DEG/RAD key which changes to reflect the mode, the ADV key does not. That can be confusing.
Cally is a Universal app. It is compatible with iPhone back to 3GS, iPod touch back to 3G, and any iPad. It requires iOS 5.0 or later. It's available only in English.
The conversions part of this app is absolutely amazing in its thoughtfulness and presentation. However, the calculator part seems to go its own way and flies in the face of 40 years of calculator design and tradition. That means the average user will probably be confused and frustrated. I can't recommend Cally at all for this reason.