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.

Sudoku Vol. 1

Perhaps the best Sudoku app for beginners (and lazy puzzle-solvers), Hudson Entertainment’s Sudoku Vol. 1 offers a tutorial mode that explains Sudoku and walks you through the solving of a puzzle. It also provides a number of visual tools for helping you solve puzzles.

For example, one of my favourite “helper” features is called Borders: when you tap on a cell, a gold outline (shown to the right) surrounds the selected cell’s host region and the row and column containing the cell, making it easier to determine which numbers that cell can and cannot contain. (You can turn off this feature if you don't want it.)

In addition, if you double-tap any given cell or solved cell, all other given and solved incidences of that cell’s number are highlighted; this is useful for quickly seeing, for example, which regions still need the number 2. A Hint button, which can be used three times for each puzzle, fills in a random unsolved cell. Finally, when all nine occurrences of a number have been entered, that number is greyed out on the onscreen keypad.

You use this 9-digit keypad to enter notations and cell values. To make notations in a cell, you tap the cell, then tap the notation button, and then tap (on the keypad) the possible values for the cell. To solve a cell, you tap the cell, then tap the solve button, then tap the desired number. (The keypad changes color to indicate whether you’re in notation or solve mode.)

There’s also a dedicated erase button, as well as Undo and Redo buttons - the latter two unique to Sudoku Vol. 1. Overall, Sudoku Vol. 1’s controls are excellent, and although the overall appearance of the game isn’t as attractive as that of the other two games here, it’s clear and effective. My only beef is that no matter which font you choose, the numbers used for notations can be difficult to read.

Sudoku Vol. 1 also provides a number of useful customization options. You can choose from six different fonts for numbers (including Japanese Kanji characters) and from six different backgrounds, and the music and sound effects can be separately muted. Other features include a game timer and an online ranking system. But my favourite feature is that the game auto-rotates between portrait and landscape mode, automatically shifting the controls from the bottom (in the former) to the side (in the latter). Unfortunately, the game’s menus for accessing settings are viewable only in portrait mode.

My only major complaint about Sudoku Vol. 1, and one that keeps it from getting a higher rating, is that the game offers only 50 levels, compared to 10,000 for the other two versions covered here. And while the games vary in difficulty, you can’t choose higher levels immediately; you need to solve easier levels to unlock more-difficult ones.

Sudoku Vol. 1’s games are non-symmetrical.

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

## Sudoku Vol. 1: Specs

• Requires an iPhone or iPod touch running 2.x software.
• Requires an iPhone or iPod touch running 2.x software.

## OUR VERDICT

Sudoku Vol. 1 is the most full-featured iPhone Sudoku game out there, and provides more assistance for learning the game; it’s too bad it doesn’t include more unique puzzles.