gives the position of the first element in expr that matches pattern, or Missing["NotFound"] if no such element is found.

gives default if no element matching pattern is found.

finds only objects that appear on levels specified by levelspec.

represents an operator form of FirstPosition that can be applied to an expression.

Details and OptionsDetails and Options

  • In FirstPosition[expr,pattern,default], default is evaluated only if no element matching pattern is found.
  • The expression expr can be a list, an association, or any other type of expression. In the case of an association, FirstPosition gives a key.
  • The default level specification for FirstPosition is {0,Infinity}, with Heads->True.
  • A part specification returned by FirstPosition represents the whole of expr.
  • FirstPosition[list,pattern,default,{1},Heads->False] finds positions only of objects that appear as complete elements of list.
  • FirstPosition uses standard level specifications:
  • nlevels through n
    Infinitylevels through Infinity
    {n}level n only
    {n1,n2}levels through
  • 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 corresponds to the whole expression.
  • With the default option setting Heads->True, FirstPosition includes heads of expressions and their parts.
  • FirstPosition traverses expressions in depthfirst order.
  • FirstPosition[pattern][expr] is equivalent to FirstPosition[expr,pattern].
Introduced in 2014
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