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Mathematica® Teacher's Edition (2002)

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Which

FilledSmallSquare Which[, , , , ... ] evaluates each of the in turn, returning the value of the corresponding to the first one that yields True.

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