## Solving puzzles quicker?

Everything about Sudoku that doesn't fit in one of the other sections

### Solving puzzles quicker?

This is a typical "Easy" puzzle appearing in today's Melbourne "Herald- Sun"
Code: Select all
` *-----------* |.94|.25|.3.| |...|4..|285| |..1|3..|6..| |---+---+---| |1..|..7|..9| |2.6|.8.|7.3| |4..|5..|..6| |---+---+---| |..3|..2|9..| |915|..6|...| |.8.|73.|45.| *-----------*`

My normal "system" to solve an easy puzzle like this is to ignore pencilmarks and look for "box-line reductions" eg. noting 6's already exist in boxes 3 and 6 which places a 6 in box 9 in cell r7c8. With 6's now in boxes 8 and 9 I can now place a 6 in box 7 at r9c1. Continuing in this fashion and looking for naked singles it took me about ten minutes to complete this puzzle.

I can look back with some satisfaction remembering it took me about an hour (or was it two hours ?) when I first started sudoku about eight months ago. It's not surprising that I now find these puzzles somewhat "boring" and prefer to tackle more difficult puzzles.

What intrigues me is that I have read some posts where people can solve "Easy" puzzles in about two to three minutes and, for the life of me, I'm amazed how this can be done Whilst there are already eight naked singles in this puzzle (Simple Sudoku reveals this) these singles aren't as obvious by visual inspection, at least not by me, without individual checking and this of course takes time.

I'd be interested to know if there are "short-cuts" which other people use to solve these puzzles quicker or perhaps it's a case of me accepting a "lower brain cell count" due to not getting younger.

Cec
Cec

Posts: 1039
Joined: 16 June 2005

It sounds like I discovered Sudoko at about the same time as you. Right now I'm just starting to use some of the advanced solving techniques on some really tough puzzles and they seem to work - check out some of the threads in the advanced solving techniques forum - they'll make your eyes bleed!!!!

Anyway, to answer your question about any short-cuts - one I use is 'reading the line'. If you quickly scan a line, or a row or a box you can very quickly see what numbers are missing. In fairly easy puzzles there will only be a few numbers missing and it will be a quick process just by scanning through the puzzle to work out the missing numbers. For example in Row 9 of your puzzle you can quickly see that 1,2,6 and 9 are missing. Quick scanning (whilst chanting the numbers in your head!!! ) will show straight away that only the 6 can be in r9c1, it then follows that r9c3 has to be the 2, r9c9 the 1 and r9c6 the 9 and the row is complete.
moggymidge

Posts: 15
Joined: 18 February 2006

Hi Cec.

I don't normally go for speed but thought I'd try it on your puzzle to see how well I could do, 3m 27secs.

Basically, besides singles the only thing I could see that could be described as a short cut would be locked candidates.

First I went through the grid looking for singles, starting with 1, then 2 through to 9 keeping an eye out for locked candidates to help place a number.

Cec, look at the number 2.

Code: Select all
`*-----------*  |.94|.25|.3.|  |...|4..|285|  |..1|3..|6..|  |---+---+---|  |1..|..7|..9|  |2.6|.8.|7.3|  |4..|5..|..6|  |---+---+---|  |..3|..2|9..|  |915|..6|...|  |.8.|73.|45.|  *-----------*`

The only place for a 2 in box 1 is r3c2
Working downwards (the 2 in box 4 is already in place)
The only place for 2 in box 7 is r9c3

Working across the bottom (the 2 in box 8 is already in place)
Because of the 2 in r2c7
the 2 in box 9 can go into r8c89
but because the 2 is locked in box 6 in c8
the only place for the 2 in box 9 is r8c9.

You could, using singles to place the 2 here, but it all depends on what is spotted first, singles or locked candidates.

Cec look to box 1 and the number 5, using locked candidates see if you can place the 5 in box 1.

MCC
MCC

Posts: 1275
Joined: 08 June 2005

### Solving puzzles quicker

Thanks moggiemidge. I like your tip about "reading the line" and your clear explanation using row 9 as an example. I'll try it out next time. Yes, it's probably time that I looked at these advanced techniques also.

Thanks also MCC. 3m27s - Struth! I must have more than brain cell deficiency. Now on a serious note, I can understand your explanations but 'spotting' them quickly seems to be my problem.

I found it easy to place the 5 in r3c1(box1) but again the "suggestion" helps to make these placements quickly. I'll try concentrating more on placing singles as you suggest when starting the puzzle rather than my "system" of looking almost exclusively at "box-line reductions" which I used to place the 5 in box1 rather than seeing this as "locked candidates" or am I missing something here?

Cec
Cec

Posts: 1039
Joined: 16 June 2005

### Re: Solving puzzles quicker?

cecbevwr wrote:This is a typical "Easy" puzzle appearing in today's Melbourne "Herald- Sun"
Code: Select all
` *-----------* |.94|.25|.3.| |...|4..|285| |..1|3..|6..| |---+---+---| |1..|..7|..9| |2.6|.8.|7.3| |4..|5..|..6| |---+---+---| |..3|..2|9..| |915|..6|...| |.8.|73.|45.| *-----------*`

My normal "system" to solve an easy puzzle like this is to ignore pencilmarks and look for "box-line reductions" eg. noting 6's already exist in boxes 3 and 6 which places a 6 in box 9 in cell r7c8. With 6's now in boxes 8 and 9 I can now place a 6 in box 7 at r9c1.

Hi Cec,

Just to clarify,

What you're referring to as Box-line reductions are actually Hidden singles, I myself go for these before going to look for naked singles !!!!

On these forums & elsewhere box-line reduction/interaction/elimination is something else.

Tarek

tarek

Posts: 2755
Joined: 05 January 2006

### Re: Solving puzzles quicker?

tarek wrote:"..What you're referring to as Box-line reductions are actually Hidden singles...On these forums & elsewhere box-line reduction/interaction/elimination is something else""

Interesting! I didn't think this was so but after looking again at the SadMan link you are right. I am now thinking that the "something else" you mention is "locked candidates" which seems to agree with MCC's terminology for this elimination procedure. Thanks tarek for your input.

Cec
Cec

Posts: 1039
Joined: 16 June 2005