74: Su Doku
Title text: This one is from the Red Belt collection, of 'medium' difficulty
Su Doku (Japanese for "Single number", and now usually written as "Sudoku") is a type of number puzzle, in which the player must place digits in a matrix playfield in such a way that no digit appears twice in a horizontal and vertical row, and in a region of nine digits in the said matrix. The most common arrangement is a 9x9 grid subdivided into nine 3x3 grids, each of which must contain the digits 1-9. The number and combination of pre-filled squares determines the difficulty of the puzzle. The image text refers to the "Red Belt"-collection, which is a series of extremely difficult puzzles.
In this comic, Randall presents us with a binary Sudoku puzzle. A normal Sudoku is "decimal" like our normal counting system (ten digits) counting from one to nine. Some Sudoku puzzles use the hexidecimal system with 16 digits (0-9 and A-F) and a 16x16 grid for more difficulty. The joke is that binary system has only two digits (0 and 1), and therefore binary Sudoku puzzles would be infinitely easy and thus pointless. There really are only two possible puzzles in a 4x4 grid. The puzzle in the comic would be completed by filling 0 in the top-left and 1 in the bottom-left empty box. The only other possible grid would have the 0s and 1s swapped. This fulfills the criteria of having no repeated digits in any row or column, although digits would repeat in the 4x4 grid. Presumably binary Sudoku has no sub-regions.
The image text appears to reference a series of published Sudoku puzzle books called the "Martial Arts Sudoku" series. The difficulty of each book is denoted by a belt colour which itself references the fact that in many Martial Arts, participants are awarded coloured belts when they reach certain skill levels, with each colour representing a certain skill level. It appears Judo was the first to use this system. A black belt is the stereotypical "highest" belt, although this is not always in fact the case, depending on the Martial Arts discipline. In the Sudoku series, red belt was level 8 (of 9) just below black belt. Thus the red belt reference therefore implies that this puzzle was found in the second-highest level of binary Sudoku books.
The puzzle is labeled as "medium" difficulty in the title text because it has two of the squares filled. In a 2x2 binary Sudoku puzzle, only one square is required to solve the puzzle. By labeling this puzzle as "medium" difficulty, it implies that a puzzle with three squares filled would be labeled "easy," and a puzzle with only one square filled would be labeled "hard," with the irony being that none of the puzzles could possibly be considered difficult.
- [A square divided into 2x2 squares, the top-right one has an 1 in it, the bottom-right one has a 0, the two left ones are empty]
- Binary Su Doku