You can sort each list of conversion options alphabetically or by size, and the most-recently used conversions in each category are conveniently listed at the top of the list. If you don't find a particular conversion listed, you can add your own. For many people, PCalc's conversions feature alone will be worth the price of admission.
The app also includes a generous assortment of predefined functions, which are more-complex calculations. These are sorted across six categories: Complex Numbers (for example, finding the square root of [X+iY]), Financial, Memory (functions that use numbers stored in memory), Special (for example, for finding permutations), Trigonometric (calculations beyond the standard trig-related buttons, such as the hyperbolic cotangent), and User. Again, you can add your own here — more on that below.
If you frequently use scientific and engineering constants, there are plenty of those, too. Tap the 42 button — because what more important constant is there? — and you get quick access to known parameters in seven basic categories: astronomical (for example, the radius of the earth and standard gravity), atomic (e.g., the Rydberg Constant), electromagnetic (e.g., elementary charge), mathematical (e.g., pi, e, and the golden ratio), physicochemical (e.g., Avogadro Constant), universal (e.g., the speed of light), and user (those you've added manually, though you can also add your own constant to any of the other categories). Choose any constant, and it's entered in the main display.
Speaking of the display, PCalc's "LCD" is highly configurable: You can choose the number of lines (with separate settings for horizontal and vertical orientations), and choose what each line is used for (for example, for particular registers or stack levels) — you can even two-finger swipe up or down to view fewer or more lines, respectively, on the fly. You can also change the background color and the font and color of the digits. You can copy results from the display for pasting elsewhere, and you can paste numbers copied elsewhere to use them in your calculations. And PCalc supports undo and redo via dedicated buttons, screen swipes, or (for undo) a gentle shake of the device.
You can customize PCalc's overall appearance via thirteen different interface themes and a number of key layouts: three vertical layouts and three horizontal layouts on the iPad, and eight vertical layouts and ten horizontal layouts on the iPhone. (The iPad's larger screen accommodates more keys, so there's less need for different key layouts — the iPhone layouts each contain a different subset of keys.) The app can even automatically switch to a darker theme at night, and there's a nice Basic key layout that hides most of the extras for everyday use. (On the iPad, I use Basic for my vertical layout, and the default scientific layout for horizontal orientation.)
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