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Str8ts Strategies
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Most of the Str8ts puzzle is about identifying the next cells that can be deduced from the clues in the black and white cells. On the easier puzzles this can be eye-balled - that is by scanning the rows, columns and the 'compartments' you should be able to find several opportunities to solve a cell. On the harder puzzles you might need to make notes in some cells to remind you of the the options.

Both the Str8ts Player and the Str8ts Solver have the ability to type in small number notes. In the solver these are always displayed but they can be edited as well.

For the purposes of outlining the strategies for Str8ts I am going to follow the list on the solver.

Solving Sequence

Start solving a Str8ts by working the smaller compartments ( 1, 2, 3 or 4 spaces) first. Using all the rules and clues and strategies listed below, the smaller compartments are simply easier to solve. Once solved, these smaller compartments start to fill in portions of the longer straights, and the filled in numbers remove themselves in rows and columns as possible numbers from the larger straights. By adding additional numbers to the puzzle we essentially are turning larger straights ( 5, 6, 7, 8 and 9 spaces ) back into manageable compartments that will be easier to solve. Especially if the following hands on strategies are used.

Compartment Check
This is the meat of Str8ts. A compartment is a set of white cells in a row or column bounded by the edge of the puzzle or a black cell. They will be between two and nine cells long. We know from the rules that all the numbers in a compartment must be a 'straight' - no gaps in other words - but the order will be unknown until you complete the straight. Given four cells with 5x8x, as in the example, we can know that to fill the gap we must use 6 and 7.

If you are making notes or using the solver you can immediately eliminate 1,2,3,4 and 9 from those cells since they can never reach the clues we know about.

Fortunately we have another clue in this example. The 6 tells us the rightmost X can't be a 6 so it must be a 7. That allows us to put the 6 between the 5 and 8.
 A 'Compartment' of 5,6,7 and 8
A 'Compartment' of 5,6,7 and 8
When you are concidering a stright you need to check the rows and columns for any useful numbers. If they exist elsewhere then they cant belong to any cell the number can 'see' in the compartment.

Black cell clues are interesting since they can cut the options in half. Say you had a black cell with '5' and it intersected a comparment such that it saw all the cells in the compartment. Let us also say the compartment is four cells long. Now, because 5 is slap in the middle this compartment of four can only be 1/2/3/4 or 6/7/8/9. If you have a known you can decide which. So black cells, along with existing clues and solved cells will help pin down the remaining options.

In the solver, all the known numbers and the black cells clues are taken into consideration, as is the length of each compartment and the remaining possibles are computed. Usually this will find one or serveral cells for which there is only one remaining number. As a human, pencil and paper solver, you will not be able to pick out these opportunities as quickly but that's the fun of this puzzle, searching for the right combination of clues to get the next cell.

Stranded Digit
I have isolated this strategy from Compartment Check as it is worth looking for in it's own right. The Stranded Digit is easily understood - take the example on the right. Looking at the second row starting with the black clue 5 it cuts through a set of possible numbers for the two-cell on that row. Because the 5 is not possible (the clue) The 4's have become isolated from the 6,7,8 and 9s. That means we can discard 4 as a candidate. Similarly in the third row the 8 has isolated the 9 from the green cell. It leaves only the 7 and that is the solution.

Another way to look at it is this. Write out all the remaining numbers available in a compartment. If there is a gap - then only the numbers before or the numbers after the gap can be the solution to all the cells. Does this help you remove some candidates?
 Standed Digits
Standed Digits
Lets take this small section and see how it plays out:

Stranded Digit


The 7 forces the 6 to thr right. Above the 7 the 9 is too far away from 7 to be used - so 8 must fit there. 6 is disallowed because of the black cell clue above. And finally we can insert 7 into the last cell (marked in green).
Naked Pairs

These is some cross over from Sudoku in this puzzle. Since rows and columns have the same rules as Sudoku they have some of the same strategies. The simplest is Naked Pairs. If two cells on the same row or column have the same two candidates - such as 1/6 and 1/6 - then we can be sure that 1 and 6 will appear in both those cells. We just can't be certain which way round yet. But that is extremely useful. If 1 and 6 are bound to be on those cells then any other 1 or 6 can be removed from that row or column.

The Naked Pair acts like a single cell clue - but make sure you apply it to only the row or column common to both cells. The two cells need not be next to each other or in the same compartment.

In the example we have three Naked Pairs - all aligned on different rows. The top row is the Pair 2/3. This attacks the cells at the start of the row (highlighted in red). Likewise the 7/8 on the second row effects the far righthand cell. And finally the 2/4 on the third row gives us a solution - marked in green.
Naked Triples

This is the same strategy as Naked Pairs except we are considering three cells, not two. For example, if 1/3/6 was present in three cells we know that 1,3 and 6 must be found in those cells – we just can’t know which way round at this point. So any other 1, 3 or 6 in the row or column can be discounted.

The trick with Naked Triples is that we don’t need all three numbers to appear in all three cells. Consider three cells with these number:

[1,3], [1,6] and [3,6]

Think of any combination from these pairs and you will always use up all three numbers in those cells. The rule really is – “If any three cells contain at most three numbers – then those numbers can be removed from the rest of the row or column”.


There are bound to be further strategies and I don't pretend to have exhausted them all on this one page. Some will be borrowed from Sudoku - I can think of Hidden Pairs and Triples, for example, and some will be unique to Str8ts.

If you find a new strategy and want to share it with others we'd be delighted to explain it here and credit you with the discovery.

Happy solving!

Andrew Stuart


Article created on 22-November-2008. Views: 19652
This page was last modified on 25-November-2008, at 09:09.
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