FirstPosition

The position of the first occurrence of value, searching every level.

✓ checked against that system · ! checked, with disagreements (hover a chip)Wolfram LanguageFirstPosition!↗

FirstPosition(collection, value)the position of the first occurrence of value, searching every level rather than just the top one.

Typed signatures

Type	Package	Applies
(any, any) -> list<integer>	combinatorics	always

Domain: Collections

Details

• Unlike IndexOf, which only looks at the top level, FirstPosition descends into nested expressions of any head — depth-first, outer to inner, left to right — and matches heads too, at index $0$: the first $Power$ in $x^2 + y^2$ is at $\{1, 0\}$. It is Position's first answer.
• The empty $List$ when the value isn't found anywhere.

Examples

Basic #

The first occurrence of $b$, nested inside the first sublist

live ↗

In$\mathrm{FirstPosition}(\bigl\lbrack\bigl\lbrack a, a, b\bigr\rbrack, \bigl\lbrack b, a, a\bigr\rbrack, \bigl\lbrack a, b, a\bigr\rbrack\bigr\rbrack, b)$Out$\bigl\lbrack1, 3\bigr\rbrack$

The empty list when the value isn't found anywhere

live ↗

In$\mathrm{FirstPosition}(\bigl\lbrack a, \bigl\lbrack a, c\bigr\rbrack\bigr\rbrack, b)$Out$\bigl\lbrack\bigr\rbrack$

Only the earliest of several matches is reported, as a position list

live ↗

In$\mathrm{FirstPosition}(\bigl\lbrack a, b, a, a, b, c, b\bigr\rbrack, b)$Out$\bigl\lbrack2\bigr\rbrack$

live ↗

In$\mathrm{FirstPosition}(\bigl\lbrack x, y, z\bigr\rbrack, b)$Out$\bigl\lbrack\bigr\rbrack$

Scope #

Searches nested lists and returns the full multi-level position

live ↗

In$\mathrm{FirstPosition}(\bigl\lbrack\bigl\lbrack1, 2\bigr\rbrack, \bigl\lbrack2, 3\bigr\rbrack, \bigl\lbrack3, 1\bigr\rbrack\bigr\rbrack, 3)$Out$\bigl\lbrack2, 2\bigr\rbrack$

live ↗

In$\mathrm{FirstPosition}(x^2+y^2, \mathrm{Power})$Out$\bigl\lbrack1, 0\bigr\rbrack$

Properties #

A match nested inside an earlier sublist beats a plain match later at the top level: depth-first, outer to inner, left to right

live ↗

In$\mathrm{FirstPosition}(\bigl\lbrack a, \bigl\lbrack a, b\bigr\rbrack, b\bigr\rbrack, b)$Out$\bigl\lbrack2, 2\bigr\rbrack$

Repeated matches later in the list are ignored once one is found at the front

live ↗

In$\mathrm{FirstPosition}(\bigl\lbrack a, b, b, a, a, c\bigr\rbrack, a)$Out$\bigl\lbrack1\bigr\rbrack$

About

Examples	8
Checked against	Wolfram Language 6 agree2 differ
Crosswalk	Wolfram Language

See also: IndexOf, Position, At