OrderingBy
OrderingBy[list,f]
gives the positions in list at which each successive element of SortBy[list,f] appears.
OrderingBy[list,f,n]
gives the positions in list at which the first n elements of SortBy[list,f] appear.
OrderingBy[list,f,-n]
gives the positions of the last n elements of SortBy[list,f].
OrderingBy[list,f,n,p]
gives positions in list of elements of SortBy[list,f,p].
Details

- list[[OrderingBy[list,f]]] is the same as SortBy[list,f].
- OrderingBy[list,f,seq] is equivalent to Take[OrderingBy[list,f],seq].
- OrderingBy[list,f,All,p] gives the position at which all elements of list appear in SortBy[list,f,p].
- OrderingBy can be used on expressions with any head, not only List.
Examples
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Wolfram Research (2019), OrderingBy, Wolfram Language function, https://reference.wolfram.com/language/ref/OrderingBy.html.
CMS
Wolfram Language. 2019. "OrderingBy." Wolfram Language & System Documentation Center. Wolfram Research. https://reference.wolfram.com/language/ref/OrderingBy.html.
APA
Wolfram Language. (2019). OrderingBy. Wolfram Language & System Documentation Center. Retrieved from https://reference.wolfram.com/language/ref/OrderingBy.html