MASTERMIND

First look for exact matches.

Correct answer is:  1234
Guess: 3333

12@4
33@3

Count digits in both correct answer and guess

0:
1: 1
2: 1
3:      3
4: 4
5:
6:
7:
8:
9:

Add up the minimums per digit and that is the number of correct digits in incorrect spots.

Correct Answer:  1234
Guess:  4135

First find exact answers:  We have one correct digit in the correct place.  Remove these

12@4
41@5

Compute number of each digit in correct answer and in guess

0:
1: 1  1
2: 1
3:
4: 1  1
5:    1
6:
7:
8:
9:

Compute min of each digit

0: 0
1: 1
2: 0
3: 0
4: 1
5: 0
6: 0
7: 0
8: 0
9: 0

Sum of the minimums is 2.  There are 2 correct digits in incorrect locations.