GatherEvery / List Layer

Create sublists by selecting every nth element starting from a specified offset. Similar to Polars' list.gather_every(). Returns a list column containing regularly spaced elements from each input list.

Example transformation:

lists	n	offset	result
[1, 2, 3, 4, 5]	2	0	[1, 3, 5]
[6, 7, 8]	1	1	[7, 8]
[9, 10, 11, 12]	3	0	[9, 12]
[]	2	0	[]

Common applications:

Downsampling time series data
Regular sampling from sequences
Creating strided subsequences
Periodic data extraction
Pattern analysis at regular intervals

Note: If n or offset is larger than list length, returns empty list. Supports both fixed values and column references for n and offset.

Table

Select

column

The source list column to gather from. Examples:

Time series: [t1, t2, t3, t4, t5]
Measurements: [m1, m2, m3, m4]
Sequences: [a, b, c, d, e, f] Lists can have different lengths

NthElement

oneof

FixedN

u32

Take every nth element. Examples:

1: Take every element
2: Take every second element
3: Take every third element Must be greater than 0

SelectN

column

Column containing n values. Each row can have different sampling intervals

Offset

oneof

FixedOffset

u32

Starting position for gathering. Examples:

0: Start from first element
1: Start from second element
2: Start from third element

SelectOffset

column

Column containing offset values. Each row can have different starting positions

AsColumn

name

Name for the new column. If not provided, the system generates a unique name. If AsColumn matches an existing column, the existing column is replaced. The name should follow valid column naming conventions.