Which
Which[
, 
, 
, 
, ... ] evaluates each of the 
in turn, returning the value of the 
corresponding to the first one that yields True.
Example: Which[1==2, x, 1==1, y]
.
If any of the 
evaluated by Which give neither True nor False, then a Which object containing these remaining elements is returned unevaluated.
You can make Which return a "default value" by taking the last 
to be True.
If all the 
evaluate to False, Which returns Null.
See The Mathematica Book on the web: Section 2.5.8.
See also: Switch, If.
Further Examples
