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based on an earlier version of the Wolfram Language.
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Position[expr, pattern]
gives a list of the positions at which objects matching pattern appear in expr.
Position[expr, pattern, levelspec]
finds only objects that appear on levels specified by levelspec.
Position[expr, pattern, levelspec, n]
gives the positions of the first n objects found.
  • Position[expr, pattern] tests all the subparts of expr in turn to try and find ones that match pattern.
  • A part specification {} returned by Position represents the whole of expr.
  • Position[list, pattern, {1}, Heads->False] finds positions only of objects that appear as complete elements of list.
  • Position uses standard level specifications:
nlevels 1 through n
Infinitylevels 1 through Infinity
{n}level n only
{n1,n2}levels n1 through n2
  • A positive level n consists of all parts of expr specified by n indices.
  • A negative level -n consists of all parts of expr with depth n.
  • Level 0 corresponds to the whole expression.
  • With the default option setting Heads->True, Position includes heads of expressions, and their parts.
  • Position traverses expressions in depth-first order, yielding lists of indices in lexicographic order.
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