# cvsegments

##### Generate segments for cross-validation

The function generates a list of segments for cross-validation. It can generate random, consecutive and interleaved segments, and supports keeping replicates in the same segment.

- Keywords
- models

##### Usage

```
cvsegments(N, k, length.seg = ceiling(N / k), nrep = 1,
type = c("random", "consecutive", "interleaved"))
```

##### Arguments

- N
Integer. The number of rows in the data set.

- k
Integer. The number of segments to return.

- length.seg
Integer. The length of the segments. If given, it overrides

`k`

.- nrep
Integer. The number of (consecutive) rows that are replicates of the same object. Replicates will always be kept in the same segment.

- type
One of

`"random"`

,`"consecutive"`

and`"interleaved"`

. The type of segments to generate. Default is`"random"`

.

##### Details

If `length.seg`

is specified, it is used to calculate the number of
segments to generate. Otherwise `k`

must be specified. If
\(k*length.seg \ne N\), the
\(k*length.seg - N\) last segments will contain only
\(length.seg - 1\) indices.

If `type`

is `"random"`

, the indices are allocated to
segments in random order. If it is `"consecutive"`

, the first
segment will contain the first \(length.seg\) indices, and so on.
If `type`

is `"interleaved"`

, the first segment will contain
the indices \(1, length.seg+1, 2*lenght.seg+1, \ldots,
(k-1)*length.seg+1\), and so on.

If \(nrep > \), it is assumed that each `nrep`

consecutive rows
are replicates (repeated measurements) of the same object, and care is
taken that replicates are never put in different segments.

Warning: If `k`

does not divide `N`

, a specified
`length.seg`

does not divide `N`

, or `nrep`

does not
divide `length.seg`

, the number of segments and/or the segment
length will be adjusted as needed. Warnings are printed for some of
these cases, and one should always inspect the resulting segments to
make sure they are as expected.

##### Value

A list of vectors. Each vector contains the indices for one segment.
The attribute `"incomplete"`

contains the number of incomplete
segments, and the attribute `"type"`

contains the type of segments.

##### Examples

```
# NOT RUN {
## Segments for 10-fold randomised cross-validation:
cvsegments(100, 10)
## Segments with four objects, taken consecutive:
cvsegments(60, length.seg = 4, type = "cons")
## Incomplete segments
segs <- cvsegments(50, length.seg = 3)
attr(segs, "incomplete")
## Leave-one-out cross-validation:
cvsegments(100, 100)
## Leave-one-out with variable/unknown data set size n:
n <- 50
cvsegments(n, length.seg = 1)
## Data set with replicates
cvsegments(100, 25, nrep = 2)
## Note that rows 1 and 2 are in the same segment, rows 3 and 4 in the
## same segment, and so on.
# }
```

*Documentation reproduced from package pls, version 2.7-2, License: GPL-2*