PositionIndex
PositionIndex[list]
gives an association between unique elements in list and the positions at which they occur.
PositionIndex[assoc]
gives an association whose keys are the distinct values in assoc, and whose values are lists of the keys with which they are associated.
Examples
Wolfram Research (2014), PositionIndex, Wolfram Language function, https://reference.wolfram.com/language/ref/PositionIndex.html.
Text
Wolfram Research (2014), PositionIndex, Wolfram Language function, https://reference.wolfram.com/language/ref/PositionIndex.html.
CMS
Wolfram Language. 2014. "PositionIndex." Wolfram Language & System Documentation Center. Wolfram Research. https://reference.wolfram.com/language/ref/PositionIndex.html.
APA
Wolfram Language. (2014). PositionIndex. Wolfram Language & System Documentation Center. Retrieved from https://reference.wolfram.com/language/ref/PositionIndex.html