IndexOf

The position of the first occurrence of value in the collection.

IndexOf(collection, value)the position of the first occurrence of value.

Typed signatures

Type	Package	Applies
(indexed_collection<any>, any) -> integer	compute-engine	always

Domain: Collections

Details

• Returns 0 when the value isn't present, since 0 is never a valid position.
• Round-trips with At: $At(c, IndexOf(c, v)) = v$ whenever v occurs in c.
• Reports only the first occurrence, as a plain index. See Position for every occurrence.

Examples

Basic #

live ↗

In$\mathrm{IndexOf}(\bigl\lbrack1, 2, 3\bigr\rbrack, 2)$Out$2$

0 signals that the value isn't present, since 0 is never a valid position

live ↗

In$\mathrm{IndexOf}(\bigl\lbrack1, 2, 3\bigr\rbrack, 9)$Out$0$

Only the first of several occurrences is reported

live ↗

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

Scope #

The value can itself be a list

live ↗

In$\mathrm{IndexOf}(\bigl\lbrack\bigl\lbrack1, 2\bigr\rbrack, \bigl\lbrack3, 4\bigr\rbrack\bigr\rbrack, \bigl\lbrack3, 4\bigr\rbrack)$Out$2$

Properties #

$At(c, IndexOf(c, v)) = v$ whenever v occurs in c. See At

live ↗

In$\bigl\lbrack10, 20, 30\bigr\rbrack[\mathrm{IndexOf}(\bigl\lbrack10, 20, 30\bigr\rbrack, 20)]=20$Out$\mathrm{True}$

Possible issues #

IndexOf reports only the first occurrence; see Position for every one

live ↗

In$\mathrm{IndexOf}(\bigl\lbrack1, 2, 3, 2\bigr\rbrack, 2)$Out$2$

IndexOf stays at the top level, where this value doesn't occur at all

live ↗

In$\mathrm{IndexOf}(\bigl\lbrack\bigl\lbrack a, a, b\bigr\rbrack, \bigl\lbrack b, a, a\bigr\rbrack, \bigl\lbrack a, b, a\bigr\rbrack\bigr\rbrack, b)$Out$0$

About

Examples	7
Checked against	Wolfram Language 7 agree

See also: At, Count, Position, FirstPosition