This is the simplest technique in Direct Elimination Techniques. All you have to do is to "read" the puzzle without any logical analysis involved. This is because its condition, if satisfied, is very obvious. As a result, it can only be used to handle extremely simple puzzles.
Focus on Row B in the figure above. It is clear that all cells have been placed with numbers except [B3]. According to sudoku rule, that is, no digit can occur more than once in a column, a row or a block. Since all other cells in Row B has a value, the number to be placed in [B3] can only be the number that has not occurred yet in Row B, which is 3. So we can safely set [B3] = 3.
This is the use of Sole Position Technique in a row. It can also be used in columns and blocks. So there exist the following circumstances:
No matter which condition is met, we can quickly assign the blank cell of the unit (column, row or block) with the number that has not occurred yet in that unit.
An example of using Sole Position Technique in a column is as follows:
In Column 7, only [F7] is blank and digit 8 has not occurred yet, so we set [F7] = 8.
The same happens to a block:
In Block at [D7], only [E7] is not assigned and value 5 is missing, so we can just place 5 into [E7].
When you just begin to solve a puzzle, you may have few chances to use Sole Position Technique. At the final stage, with more and more cells assigned with values, however, the chances increase greatly.
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