collapse (version 1.7.6)

seqid: Generate Group-Id from Integer Sequences

Description

seqid can be used to group sequences of integers in a vector, e.g. seqid(c(1:3, 5:7)) becomes c(rep(1,3), rep(2,3)). It also supports increments > 1, unordered sequences, and missing values in the sequence.

Some applications are to facilitate identification of, and grouped operations on, (irregular) time series and panels.

Usage

seqid(x, o = NULL, del = 1L, start = 1L, na.skip = FALSE,
      skip.seq = FALSE, check.o = TRUE)

Arguments

x

a factor or integer vector. Numeric vectors will be converted to integer i.e. rounded downwards.

o

an (optional) integer ordering vector specifying the order by which to pass through x.

del

integer. The integer deliminating two consecutive points in a sequence. del = 1 lets seqid track sequences of the form c(1,2,3,..), del = 2 tracks sequences c(1,3,5,..) etc.

start

integer. The starting value of the resulting sequence id. Default is starting from 1. For C++ programmers, starting from 0 could be a better choice.

na.skip

logical. Skip missing values in the sequence. The default behavior is skipping such that seqid(c(1, NA, 2)) is regarded as one sequence and coded as c(1, NA, 1).

skip.seq

logical. If na.skip = TRUE, this changes the behavior such that missing values are viewed as part of the sequence, i.e. seqid(c(1, NA, 3)) is regarded as one sequence and coded as c(1, NA, 1).

check.o

logical. Programmers option: FALSE prevents checking that each element of o is in the range [1, length(x)], it only checks the length of o. This gives some extra speed, but will terminate R if any element of o is too large or too small.

Value

An integer vector of class 'qG'. See qG.

Details

seqid was created primarily as a workaround to deal with problems of computing lagged values, differences and growth rates on irregularly spaced time series and panels before collapse version 1.5.0 (#26). Now flag, fdiff and fgrowth natively support irregular data so this workaround is superfluous, except for iterated differencing which is not yet supported with irregular data.

The theory of the workaround was to express an irregular time series or panel series as a regular panel series with a group-id created such that the time-periods within each group are consecutive. seqid makes this very easy: For an irregular panel with some gaps or repeated values in the time variable, an appropriate id variable can be generated using settransform(data, newid = seqid(time, radixorder(id, time))). Lags can then be computed using L(data, 1, ~newid, ~time) etc.

In general, for any regularly spaced panel the identity given by identical(groupid(id, order(id, time)), seqid(time, order(id, time))) should hold.

Regularly spaced panels with gaps in time (such as a panel-survey with measurements every 2 years) can be handled either by seqid(…, del = gap) or, in most cases, simply by converting the time variable to factor using qF, which will make observations consecutive.

There are potentially other more analytical applications for seqid…

For the opposite operation of creating a new time-variable that is consecutive in each group, see data.table::rowid.

See Also

groupid, qG, Fast Grouping and Ordering, Collapse Overview

Examples

# NOT RUN {
## This creates an irregularly spaced panel, with a gap in time for id = 2
data <- data.frame(id = rep(1:3, each = 4),
                   time = c(1:4, 1:2, 4:5, 1:4),
                   value = rnorm(12))
data

## This gave a gaps in time error previous to collapse 1.5.0
L(data, 1, value ~ id, ~time)

## Generating new id variable (here seqid(time) would suffice as data is sorted)
settransform(data, newid = seqid(time, order(id, time)))
data

## Lag the panel this way
L(data, 1, value ~ newid, ~time)
# }
# NOT RUN {
## A different possibility: Creating a consecutive time variable
settransform(data, newtime = data.table::rowid(id))
data
L(data, 1, value ~ id, ~newtime)
# }
# NOT RUN {
## With sorted data, the time variable can also just be omitted..
L(data, 1, value ~ id)

# }