if (condition 1) then
do the most important thing
else if (condition 2) then
do another thing
.....
else if(condition n) then
do the less important thing
end if
A 4-bit priority encoder (also sometimes called a priority decoder). This circuit basically converts the 4-bit input into a binary representation. If the input n is active, all lower inputs (n-1 .. 0) are ignored:
x3 x2 x1 x0 y1 y0
------------------
1 X X X 1 1
0 1 X X 1 0
0 0 1 X 0 1
0 0 0 X 0 0
priority encoders are commonly for interrupt controllers, to select the most critical out of multiple interrupt requests.
Imagine you have a microprocessor with only one external INT. Now you want to handle several external INT, so you put all your INT on the priority encoder inputs and through an OR gate to the µP INT pin.
When an INT occur, you just read the output of the priority encoder in order to know with INT is active.