Riffle

Interleave a separator (or a second list) between the elements of a list.

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

Riffle(list, x)$x$ between every consecutive pair of elements; if $x$ is itself a $List$, its elements are used in turn (and, once list and x run the same length, a trailing one too).

Riffle(list, x, n)a separator every $n$ elements, rather than between every pair.

Typed signatures

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

Domain: Collections

Examples

Basic #

$x$ between consecutive elements

live ↗

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

Interleave two lists of the same length

live ↗

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

Inserts $x$ between every adjacent pair

live ↗

In$\mathrm{Riffle}(\bigl\lbrack1, 2, 3, 4, 5, 6, 7, 8, 9\bigr\rbrack, x)$Out$\bigl\lbrack1, x, 2, x, 3, x, 4, x, 5, x, 6, x, 7, x, 8, x, 9\bigr\rbrack$

Inserts $x$ after every third element

live ↗

In$\mathrm{Riffle}(\bigl\lbrack1, 2, 3, 4, 5, 6, 7, 8, 9\bigr\rbrack, x, 3)$Out$\bigl\lbrack1, 2, x, 3, 4, x, 5, 6, x, 7, 8, x, 9\bigr\rbrack$

A list of fillers alternates between them

live ↗

In$\mathrm{Riffle}(\bigl\lbrack1, 2, 3, 4, 5, 6, 7, 8, 9\bigr\rbrack, \bigl\lbrack x, y\bigr\rbrack)$Out$\bigl\lbrack1, x, 2, y, 3, x, 4, y, 5, x, 6, y, 7, x, 8, y, 9\bigr\rbrack$

Scope #

A list one shorter goes strictly between the elements

live ↗

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

Every 3rd element of the result is x

live ↗

In$\mathrm{Riffle}(\bigl\lbrack1, 2, 3, 4, 5, 6, 7\bigr\rbrack, x, 3)$Out$\bigl\lbrack1, 2, x, 3, 4, x, 5, 6, x, 7\bigr\rbrack$

A single element has nothing to separate

live ↗

In$\mathrm{Riffle}(\bigl\lbrack a\bigr\rbrack, x)$Out$\bigl\lbrack a\bigr\rbrack$

With equal lengths, the lists interleave and the last filler is appended

live ↗

In$\mathrm{Riffle}(\bigl\lbrack a, b, c, d\bigr\rbrack, \bigl\lbrack x, y, z, w\bigr\rbrack)$Out$\bigl\lbrack a, x, b, y, c, z, d, w\bigr\rbrack$

Applications #

Zeros between consecutive integers

live ↗

In$\mathrm{Riffle}(1..10, 0)$Out$\bigl\lbrack1, 0, 2, 0, 3, 0, 4, 0, 5, 0, 6, 0, 7, 0, 8, 0, 9, 0, 10\bigr\rbrack$

Interleaves the integers with their negatives

live ↗

In$\mathrm{Riffle}(1..10, -(1..10))$Out$\bigl\lbrack1, -1, 2, -2, 3, -3, 4, -4, 5, -5, 6, -6, 7, -7, 8, -8, 9, -9, 10, -10\bigr\rbrack$

Properties #

Like joining with a separator: none at the ends

live ↗

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

About

Examples	12
Checked against	Wolfram Language 12 agree
Crosswalk	Wolfram Language

See also: Join, Partition