There are calculators of all kinds for the iPhone. Some merely compute tips, but only a few are designed for serious work by scientists and engineers. Calc Zero is one of those and not only includes an option for units associated with each number, but is a new breed of calculator designed to fully exploit the capabilities of the iPhone and iPod touch.

Calc Zero will be of interest to any researcher, scientist or engineer who has an iPhone or iPod touch.

Shipping version: 1.3

Calculator Background

There are, in my experience, three kinds of calculators for the iPhone. The first class consists of simple calculators, like the one that ships with the iPhone. (You can throw that away now, even though, in horizontal mode, it's a decent Algebraic scientific calculator.) Or tip calculators or those that partition a restaurant bill. Or even some of the very basic engineering calculators that support RPN entry logic.

The second class contains the throwbacks, working replicas of the classic Hewlett Packard physical calculators. I reviewed the HP-15C here recently. While very nostalgic and fully functional, along with being a bit pricey, they don't reflect the most modern thinking in calculator design made possible by all the features of the iPhone: soft key board, accelerometer, gestures, extensive math libraries and so on.

The third class has those calculators that are best of breed for the iPhone. Examples are PCalc and the calculator reviewed here, Calc Zero (formerly OneCalc) from Uplake Media, LLC.

Let's start with the developer's list of features.

• 21st Century interface, including multiple pop-up keys
• Units optionally linked to each number and carried along
• Extensive unit conversions
• Optionally, enter and manipulate numbers as fractions
• Algebraic or RPN entry modes
• Soft groups: scientific, trig, time, length, speed, currency, and more
• Swipe display to change decimal precision
• Key clicker: on or off
• Shake to clear
• User set threshold for Engineering notation
• Infinite stack in RPN mode, X and Y registers shown

One of the things I like most about Calc Zero is the carrying of units. For example, one can enter, in RPN mode,

32 m ENTER 32 m x

and obtain 1024 m^2. However, the calculator display is much more modern than my simple abbreviation, so one actually sees:

There are limits to this unit operation, however. Mixed units are not possible, but the author says he's considering that capability.

Also implicit in the units is a conversion. For example, one could enter, in RPN mode:

1 h ENTER year

The conversion is done and shows that one hour is 0.000114 year, approximately.

Another thing I like is that pressing and holding some keys brings up additional key functions. The effect is shown below, which also happens to show the new keyboard look in version 1.4, due out soon.

Version 1.4 (submitted to App Store)

The trick here is that one doesn't have to slide the finger up and over. Simply sliding to the right is sufficient to highlight the desired function and select it.

Accuracy

The developer described his implementation as follows: "Calc Zero uses NSDecimalNumbers to represent all numeric values. Under the hood, NSDecimalNumbers actually implements base-10 math and can represent any number that can be expressed as mantissa x 10^exponent where mantissa is a decimal integer up to 38 digits long, and exponent is an integer from –128 through 127. I also find that I frequently want to vary the number of digits that are displayed by the calculator, so I allow the user to quickly adjust the display precision by swiping horizontally in the number display area at the top of the screen.

"In Calc Zero, I make extensive use of NSNumberFormatters to circumvent floating point math errors. When you adjust the display precision (e.g., by swiping left/right across the number display area), I generate a number of new formatters appropriate to the number of digits past the decimal you want to look at."

NSDecimalNumbers isn't quite the same as binary code decimal (BCD) used in the HP calculators, according to the developer. Some casual tests I conducted, sin, logs, sqrt, showed identical results on a (real) HP-67 and Calc Zero out to 9 decimal places.

Soft Groups

Rather than wrestle with which functions will be on the fixed face of the calculator, Calc Zero uses the multiple functions per key approach combined with user selectable groups at the bottom. In the screen shots above, I selected Scientific, Trig, Time and Currency. The More... key is an is a standard iPhone convention that brings up more options and allows the user to reassign those groups just by dragging into the slots. Shown below are most of the options. Tiny Area, Inductance and RLC are at the bottom, along with Help.

Unit Groups

Documentation

The Help page has nine chapters, including notes on user settings and supported gestures. The documentation is rather minimalist in that it describes the operation of the calculator but doesn't always go into detail. For example, some keys, like the %chg key need to have the specific formula documented.

Currently, when in Help, pressing "Back" goes back to the calculator instead of the Help table of contents. The author said that will be fixed soon. After all, one shouldn't yank the reader abruptly out of the documentation.

Modern Features

As mentioned above, the user can simply swipe a finger in the display to change the numeric accuracy. In RPN mode, swiping up and down rolls the stack. In algebraic mode, swiping to the right or left and down closes or opens parentheses. This is excellent use of the iPhone's gesture capabilities to build a modern calculator.

This calculator has no horizontal mode. Because of the way the soft groups at the bottom can be changed so easily, there isn't really a need for it, and users haven't pushed for it, according to the author. Given how the calculator works, I don't see a need for it either. Besides, the upright position is the natural orientation for an iPhone user.

Shake to clear, typical on many other iPhone calculators, is supported. One feature I haven't seen before is an automatic computation page. For example, if you enter pi in the display and are in scientific mode, pressing (...) brings up whole page of additional pre-computed values and, in RPN mode, shows the x & y stack values at the top.

Precomputed values

Summary

The developer has had a lot of experience with calculators, having used HP calculators himself in school. That's important because there are many pitfalls in mathematical algorithms that can trip up a casual developer who doesn't have a background in numerical analysis and computational algorithms. After all, the calculator could be pressed into serious scientific use, as I described in my review of the HP-15C. Small mistakes in the calculator algorithms could lead to disastrous results in analyzing data or conducting engineering calculations.

In my extensive discussions with the developer, I got the feeling that he's aware of these issues.

If you're in a technical field and need a scientific calculator for your iPhone, Calc Zero is a terrific choice. The author is responsive and has big plans for future versions. With the new keyboard design in version 1.4, it is, along with PCalc, a charming, good looking, ultra-modern and capable scientific calculator. And the price? Much too low in my opinion.