PadLeft

Pad a list on the left to length n, or truncate it from the left.

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

PadLeft(list)a ragged array (a list of lists) padded with 0s to a full matrix.

PadLeft(list, n, x?)padded on the left with $x$ (default 0) to length $n$; a shorter $n$ drops elements from the left instead.

Typed signatures

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

Domain: Collections

Examples

Basic #

Padded with zeros on the left

live ↗

In$\mathrm{PadLeft}(\bigl\lbrack1, 2, 3\bigr\rbrack, 5)$Out$\bigl\lbrack0, 0, 1, 2, 3\bigr\rbrack$

A padding element

live ↗

In$\mathrm{PadLeft}(\bigl\lbrack1, 2, 3\bigr\rbrack, 5, x)$Out$\bigl\lbrack x, x, 1, 2, 3\bigr\rbrack$

Pads on the left with zeros to the requested length

live ↗

In$\mathrm{PadLeft}(\bigl\lbrack a, b, c\bigr\rbrack, 10)$Out$\bigl\lbrack0, 0, 0, 0, 0, 0, 0, a, b, c\bigr\rbrack$

A custom padding element

live ↗

In$\mathrm{PadLeft}(\bigl\lbrack a, b, c\bigr\rbrack, 10, x)$Out$\bigl\lbrack x, x, x, x, x, x, x, a, b, c\bigr\rbrack$

A padding list repeats cyclically, aligned to the right-hand end

live ↗

In$\mathrm{PadLeft}(\bigl\lbrack a, b, c\bigr\rbrack, 10, \bigl\lbrack x, y, z\bigr\rbrack)$Out$\bigl\lbrack z, x, y, z, x, y, z, a, b, c\bigr\rbrack$

Scope #

A shorter length drops elements from the left

live ↗

In$\mathrm{PadLeft}(\bigl\lbrack1, 2, 3, 4, 5\bigr\rbrack, 3)$Out$\bigl\lbrack3, 4, 5\bigr\rbrack$

With no length, a ragged array is padded to a full matrix

live ↗

In$\mathrm{PadLeft}(\bigl\lbrack\bigl\lbrack1\bigr\rbrack, \bigl\lbrack2, 3\bigr\rbrack\bigr\rbrack)$Out$\bigl\lbrack\bigl\lbrack0, 1\bigr\rbrack, \bigl\lbrack2, 3\bigr\rbrack\bigr\rbrack$

A negative length pads on the right instead

live ↗

In$\mathrm{PadLeft}(\bigl\lbrack a, b, c\bigr\rbrack, -10)$Out$\bigl\lbrack a, b, c, 0, 0, 0, 0, 0, 0, 0\bigr\rbrack$

Possible issues #

A nested padding list is used whole as the padding element

live ↗

In$\mathrm{PadLeft}(\bigl\lbrack a, b, c\bigr\rbrack, 7, \bigl\lbrack\bigl\lbrack u, v\bigr\rbrack\bigr\rbrack)$Out$\bigl\lbrack\bigl\lbrack u, v\bigr\rbrack, \bigl\lbrack u, v\bigr\rbrack, \bigl\lbrack u, v\bigr\rbrack, \bigl\lbrack u, v\bigr\rbrack, a, b, c\bigr\rbrack$

The padding cycle is aligned to the data, so it reads $v,u,v,u$ before the list

live ↗

In$\mathrm{PadLeft}(\bigl\lbrack a, b, c\bigr\rbrack, 7, \bigl\lbrack u, v\bigr\rbrack)$Out$\bigl\lbrack v, u, v, u, a, b, c\bigr\rbrack$

About

Examples	10
Checked against	Wolfram Language 10 agree
Crosswalk	Wolfram Language

See also: PadRight, Take