2A2B

2A2B is a shorthand notation used in the Bulls and Cows puzzle and in many programming challenges to describe feedback after a guess. The notation xAyB means that x digits are correct and in the correct position (A), and y digits are correct but in the wrong position (B).

In the common 4-digit version, both the secret and the guess have the same length. A is counted by directly comparing corresponding positions; B is counted for digits that appear in both numbers but are not already counted as A, with duplicates handled so that a digit is not counted more times than it occurs in the secret.

Variants vary in length and rules about repeated digits. Some problems allow repeated digits; others require all digits to be unique. Some versions use letters instead of digits, or cast the puzzle as a string-based game.

Example: secret 1234 and guess 1432 yield 2A2B, because digits 1 and 3 are in the correct positions, while digits 2 and 4 are present but in each other’s positions.

Origin and usage: Bulls and Cows is a historical code-breaking game with roots in European puzzles; the A/B notation was adopted in modern programming challenges in the early 2000s as a concise feedback format. 2A2B has become a common target outcome in algorithmic challenges and interview questions for counting matches efficiently.

Implementation notes: a typical approach uses two passes—first to count A by direct comparison, then to count B using frequency counts or maps for digits not matched in A, ensuring each digit is counted only as many times as it appears in the secret.