I’ve just begun doing sudoku puzzles, and am still doing the “easy” ones. Apparently, I’ve just made my first mistake. The number that “must” occupy a given square when looking at the row is different than the number that “must” occupy it when looking at the column. Obviously I’ve made a mistake somewhere, but how can I backtrack and find the mistake, short of erasing everything and starting over?
Not sure it’s going to be much help in this situation, but I find when I do su doku that the most common cause of a mistake is putting a number in that “obviously” shouldn’t go there (for example, entering a “4” in a column when there is already a “4” in that column). When this happens, it is sometimes possible to find the original error by erasing the last number you put in, then the one before that, etc (assuming you can remember the approximate order). Sometimes you come across an “obvious” error fairly quickly by doing this, and can rescue the puzzle.
However, it is probably more common for this not to be possible. When I make a mistake in su doku, if I can’t find the error (or I find multiple errors) after a few seconds of backtracking, I just give up. There are millions of possible su doku puzzles, so in my view there’s no point in losing sleep over messing one up - I just go on to the next one and take more care!
It can be difficult to find where you’ve gone awry - I found that if I ran up against a duplicated number the source of the error either came to me quickly or it didn’t. If the latter then I’d just abandon it like Dead Cat says.
Unless you’re some sort of natural genius at these things, you’ll probably need an annotation system that shows you the possible numbers for each square as you go along. It’s not necessary at the easier levels, but it seems essential for the harder Sudokus. This was usually the source of mistakes for me - I’d use dots to annotate the boxes and one would get smudged out etc.
I white out the given numbers when they don’t agree with what I have written in the empty squares.
I do alot of annotation outside of the square to denote that a number could go in a column (annotated above or below) in a certain box or to the right or left of a box if that number belongs in that row. Sometimes I will catch mistakes much quicker this way, but sometimes I erase the annotations to avoid accidentally duplicating that number in the box. Also, I will place an annotation in a corner of an already placed to denote that the annotated number could go into the other 3 squares adjacent to that corner. I seem to have sped up my solving speed in the range of 6-15 minutes on a 3x3.
When you start doing 4x4s, then you will need to learn an annotation system.
Another option is to compare what you have, to the solved puzzle…erase the wrongly placed numbers and then either table it for later solving, or just go on from there without trying to memorize the key.
one way might be to look at the rows and columns, and find one where you’ve put in the same number twice. Then you know that the problem starts in that row column.
I annotate inside the squares to indicate when there are only two choices for that square. I use a dot; top left =1, center top=2 top right =3 center left = 4 one dot in the middle for 5, etc. It’s pretty easy to see and doesn’t mess with writing in the numbers too much.