How to complete a Sudoku (16/01/2013)

Ye Olde Method(e)

Now, I know I am a bit behind the curve on this one, however, today I learnt what (for me) was a new method of completing Sudoku

Original Method:

I would scan the puzzle row by row (horizontally, then vertically) looking to complete the 'threes' (e.g. three of a number - horizontally, then vertically) where possible.

Once I had completed this; I would then look for the most completed horizontal/vertical line or box of 3x3 and see if I could complete it (generally, the puzzle would allow you to complete one line/box early on).

I would then go back to scanning the puzzle (horizontally, then vertically) and repeat over and over until the puzzle was complete.

However, this only worked all the while I could find boxes where only one number could be entered. Once I ran out of these (more likely on the more complex puzzles) I would become stuck and, likely, stop.

'I can see clearly now...'

New Method:

However, today I discovered a way to complete any puzzle.

Once you have looped through the puzzle a few times using the above method; go line by line (horizontally, then vertically) and box by box and look at each empty box.

Consider which of the available numbers could go in each empty box and make a small note of those numbers in the top right hand corner of each box.

You will discover:

1. Some boxes only have one possible number that can go in that box;
2. That number can then be removed as a possible input from the other blank boxes in that row (horizontally/vertically) and in that 3x3 box; and then
3. This may leave you with other boxes which can take only one number.

As you work your way through, crossing off numbers horizontally/vertically/per box you will reveal more and more single numbers for your blank boxes. Soon you will have completed your puzzle! 

And there you have a nice, straightforward, way to complete Sudoku!

This is my blog, my attempt to document one new thing I have learnt everyday. We learn so much and it is easy to overlook just how much we do learn. Any of the products or websites mentioned in my blog are ones I have found useful. I have no link to these websites (unless otherwise stated) and do not receive any benefit from linking to them. I am not an expert in any of the products I mention so you will have to do your own research and reach your own conclusions.