Chances are, you’re fully aware that the number-placement logic game Sudoku is massively popular. But if you’ve been in a cave the past few years and need evidence, look no further than the 18 (and counting) versions of Sudoku apps available in the iTunes App Store, ranging in price from 59p to £5.99.

If you’re a fan of Sudoku, which do you buy? Unfortunately, the current state of the App Store doesn’t allow for demo versions of software, leaving you to take a wild guess based on a short description and a few screenshots. But we’re here to help: I tested every iPhone Sudoku app and picked out those worthy of your iPhone’s screen.

Each of the three versions that made it through my screening process, and are covered here, satisfy the following requirements:
* Uses a standard 9-by-9-cell grid of numbers with nine square (3-by-3-cell) regions. (Some Sudoku variants break the standard 9-by-9 grid into non-square geometric areas, or use colors or images instead of numbers.)
* Includes puzzles with only a single solution.
* Provides a way to note (or “pencil in”) the possible values for each square as you solve a puzzle. (These markings are often called notations.)
* Offers good usability: easy-to-use input methods, clear controls, and readable graphics. (For example, a couple Sudoku apps that didn’t make the cut satisfied most criteria, but their methods for making notations obscured other parts of the puzzle.)
* Includes puzzles for multiple skill levels.

Unfortunately, none of the better Sudoku games available for the iPhone and iPod touch exclusively use symmetrical puzzles - those in which the pre-filled boxes (called givens) in opposing regions mirror each other. Although not technically a requirement, many purists don’t consider non-symmetrical puzzles to be “true” Sudoku. I tend to agree, but I didn't hold a lack symmetry against the candidates. However, I’ve noted in the summaries below if an app uses symmetrical puzzles.

If you’ve never played Sudoku before, I recommend checking out Wikipedia’s page on the game; these reviews assume you know the basics.

Big Bang Sudoku

The guys at Freeverse have a reputation for producing campy games with attractive visuals, and Big Bang Sudoku is no exception. The shiny, Chiclet-like cells float over a moving background of stars in space, and one of Freeverse’s quirky mascots occasionally pops up to provide feedback on your progress.

You enter notations and cell values using a row of numbers across the bottom of the screen, although the process is different from the one used by Sudoku Vol. 1: First you tap the number you want to enter (1 through 9), then you tap the cell(s) into which you want enter that number. To enter notations, you tap the pencil button at the bottom of the screen to switch to notation mode, then follow the same procedure. (When in notation mode, the pencil button glows orange and each number in the row appears in “notation position,” as shown in the screenshot to the left.)

Although this entry system is simple and understandable, I found it to be a bit of a hassle for entering multiple notations in a single cell; if I wanted, for example, to note that a particular cell could be a 3, 5, or 8, I had to tap 3, then the cell, then 5, then the cell, then 8, and then the cell again. With the input method used by the other two apps covered here, I could simply tap the cell and then tap 3, 5, and 8. On the other hand, if you’re the sort who prefers to first note all cells that could accept a 1, then to note all possible 2s, and so on, this approach is more efficient.

Other features include an option to show all incorrect moves (you can toggle this display on and off, via the Options menu, to quickly view your mistakes); the ability to mute sound; and a game timer. There’s also a stats screen that shows the percentage of puzzles you’ve played that you’ve solved.

Big Bang Sudoku’s games are non-symmetrical.

Compatible with any iPhone or iPod touch running iPhone 2.0 software.

## OUR VERDICT

While Big Bang Sudoku doesn’t offer nearly as many options and features as Hudson's Sudoku Vol 1, it’s perhaps simpler to use, and it includes over 10,000 puzzles across four difficulty levels (Easy, Medium, Hard, and Diabolical); you can choose your difficulty level at the beginning of each game.