`FirstPosition`¶

Returns the position of the first occurrence of an expression.

$ wo 'FirstPosition[{1, 2, 3, 2, 1}, 2]'
{2}

It reports the first position Position finds, heads included — the head of an expression sits at position {0}, so it is reached before any element:

$ wo 'FirstPosition[{1, a, 2}, _Symbol]'
{0}

$ wo 'FirstPosition[{1, 2}, _Integer]'
{1}

The whole expression is position {}, and an association element is located by its key:

$ wo 'FirstPosition[{1, a}, _List]'
{}

$ wo 'ToString[FirstPosition[<|"a" -> 1, "b" -> 2|>, 2], InputForm]'
{Key["b"]}

Nothing found gives the default:

$ wo 'FirstPosition[{1, 2, 3}, 5]'
Missing[NotFound]

$ wo 'FirstPosition[{1, 2, 3}, 5, none]'
none

FirstPosition¶

`FirstPosition`¶