There are any number of spreadsheet apps available for the iPad, but recently the market changed dramatically when Microsoft released the full Office suite for iPad, including a version of Excel. While you can argue that Excel is many years late to the iPad party (and I wouldn't disagree), the iPad version of Excel is a solid entrant, and instantly changes the landscape for competitive apps.
I've spent quite a bit of time with Excel for iPad, and it's telling that my main complaints have little to do with the actual program: Microsoft has done an excellent job at transferring the power and capabilities of Excel for OS X into Excel for iPad. The program loads quickly, contains more of Excel's core functionality than I would have expected, and the transition from keyboard/mouse to full touch interface has been handled very nicely.
For a company with a history of not doing well with touch (i.e. Office apps on Surface in Windows 8), the interface in Excel for iPad is surprisingly well thought out. The window features six small buttons across the top of the screen, along with five words that are actually tabs on the yes-there's-a-ribbon.
But unlike desktop Excel, where I often find the ribbon too big and too cluttered, the ribbon on Excel for iPad is small and tidy. Each tab (Home, Insert, Formulas, Review, and View) contains only one row of text and icons, and the icons are reasonably sized. The default view opens with the Home tab active, which contains often-used formatting options, such as font face, color, and style; font fill and borders; text alignment; and cell formatting and style drop-downs.
The other ribbons are relatively self-explanatory, except perhaps Review, which is used to scan through comments on worksheet cells. Ironically, Excel for iPad can't actually create cell notes, so the Review ribbon is only used to browse through the existing comments. (You can, however, delete comments; you just can't create them.)
While working on a worksheet, touch actions work as you'd expect them to — two finger drags scroll, and pinch gestures zoom and unzoom. A single tap will select a cell, while a double-tap will open the cell for entry (and display the keyboard). When entering data in cells, you can also use a numeric keyboard, which greatly speeds entering formulas and numbers. External keyboards are also supported, and worked well in my testing — there's even a cheat sheet in the app's help (somewhat hidden in the menu used for saving and duplicating workbooks) that shows you how to navigate and work with the app using an external keyboard.
Cell references can be added to formulas by tapping (and tap-dragging) on other cells as you're working on the formula, further simplifying formula entry.
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