This is a simulation of the game Cows and Bulls with three digit numbers I am trying to get the number of cows and bulls between two numbers. One of which is generated by the computer and the other is guessed by the user. I have parsed the two numbers I have so that now I have two lists with three elements each and each element is one of the digits in the number. So: 237 will give the list [2,3,7]. And I make sure that the relative indices are maintained.the general pattern is:(hundreds, tens, units). And these two lists are stored in the two lists: machine and person. ALGORITHM 1 So, I wrote the following code, The most intuitive algorithm: cows and bulls are initialized to 0 before the start of this loop. for x in person: if x in machine: if machine.index(x) == person.index(x): bulls += 1 print x,' in correct place' else: print x,' in wrong place' cows += 1 And I started testing this with different type of numbers guessed by the computer. Quite randomly, I decided on 277. And I guessed 447. Here, I got the first clue that this algorithm may not work. I got 1 cow and 0 bulls. Whereas I should have got 1 bull and 1 cow. This is a table of outputs with the first algorithm: Guess Output Expected Output 447 0 bull, 1 cow 1 bull, 1 cow 477 2 bulls, 0 cows 2 bulls, 0 cows 777 0 bulls, 3 cows 2 bulls, 0 cows So obviously this algorithm was not working when there are repeated digits in the number randomly selected by the computer. I tried to understand why these errors are taking place, But I could not. I have tried a lot but I just could not see any mistake in the algorithm(probably because I wrote it!) ALGORITHM 2 On thinking about this for a few days I tried this: cows and bulls are initialized to 0 before the start of this loop. for x in range(3): for y in range(3): if x == y and machine[x] == person[y]: bulls += 1 if not (x == y) and machine[x] == person[y]: cows += 1 I was more hopeful about this one. But when I tested this, this is what I got: Guess Output Expected Output 447 1 bull, 1 cow 1 bull, 1 cow 477 2 bulls, 2 cows 2 bulls, 0 cows 777 2 bulls, 4 cows 2 bulls, 0 cows The mistake I am making is quite clear here, I understood that the numbers were being counted again and again. i.e.: 277 versus 477 When you count for bulls then the 2 bulls come up and thats alright. But when you count for cows: the 7 in 277 at units place is matched with the 7 in 477 in tens place and thus a cow is generated. the 7 in 277 at tens place is matched with the 7 in 477 in units place and thus a cow is generated.' Here the matching is exactly right as I have written the code as per that. But this is not what I want. And I have no idea whatsoever on what to do after this. Furthermore... I would like to stress that both the algorithms work perfectly, if there are no repeated digits in the number selected by the computer. Please help me with this issue. P.S.: I have been thinking about this for over a week, But I could not post a question earlier as my account was blocked(from asking questions) because I asked a foolish question. And did not delete it even though I got 2 downvotes immediately after posting the question.