1a1b

1a1b (often written as 1A1B) is a score notation used in bulls-and-cows style guessing games to communicate feedback about a guessed code. In this system, the letter A denotes a digit that is correct and in the correct position, while B denotes a correct digit that is in the code but in the wrong position. The string 1A1B therefore means one digit is in the right place and one other digit is present but misplaced.

The notation is commonly used in games where players try to deduce a secret sequence of digits (often four digits). After each guess, the judge responds with a string such as xAyB, where x is the number of bulls (A) and y is the number of cows (B). Correct counting is important when digits repeat, and standard implementations ensure that each digit is counted at most once between bulls and cows.

Example: with a secret 1234 and a guess 1325, the feedback is 1A2B. The first digit '1' is in the correct position (A). The digits '3' and '2' are present in the secret but in different positions (two Bs). The digit '5' is not in the secret.

Origins and usage: the A/B feedback convention derives from the Bulls and Cows puzzle and has become common in programming challenges, educational materials, and online puzzle communities. It is also used in variants of Mastermind that employ digits rather than colored pegs. The lowercase variant 1a1b is simply a stylistic or typographical variation of the same scoring concept.

See also: Bulls and Cows, Mastermind, number-guessing puzzles.