Related below is a situation where for certain combinations of the variables one does not care what the value of the function becomes (either 0 or 1).
For these can't happen and don't care situations the Karnaugh map entry is X indicating that the particular cell can be taken either as 0 or 1.
A binary coded decimal number has values ranging from 0000 to 1001 (decimal 0 to 9) the values 1010 to 1111 (decimal 10 to 15) never occurs. Let the logic network have inputs A, B, C, D where A is connected to the most significant digit of the binary coded decimal number and D to the least significant. The output from the logic network will be:
Z = f(A, B, C, D) = (0011,0101,0110,0111,1001) = (3,5,6,7,9).
With can't happen conditions: (1010, 1011, 1100, 1101, 1110,
1111) = (10, 11, 12, 13, 14, 15)
and the don't care conditions: (0001)
Entering this on a Karnaugh map:
The required function is therefore: Z = f(A, B, C, D) = BC + D
Check if the above function is right.