# Panel Operations

`DiffinDiffsBase.PanelStructure`

— Type`PanelStructure{R<:Signed, IP<:AbstractVector, TP<:AbstractVector}`

Panel data structure defined by unique combinations of unit ids and time periods. It contains the information required for certain operations such as `lag`

and `diff`

. See also `setpanel`

.

**Fields**

`refs::Vector{R}`

: reference values that allow obtaining time gaps by taking differences.`invrefs::Dict{R, Int}`

: inverse map from`refs`

to indices.`idpool::IP`

: unique unit ids.`timepool::TP`

: sorted unique time periods.`laginds::Dict{Int, Vector{Int}}`

: a map from lag distances to vectors of indices of lagged values.

`Base.diff`

— Method`diff(panel::PanelStructure, v::AbstractArray; kwargs...)`

Return the differences of `v`

within observations for each unit in `panel`

. By default, it calculates the first differences. See also `diff!`

.

**Keywords**

`order::Integer=1`

: the order of differences to be taken.`l::Integer=1`

: the time interval between each pair of observations.`default=missing`

: default values for indices where the differences do not exist.

`DiffinDiffsBase.aligntime`

— Method```
aligntime(col::AbstractArray, time::ScaledArrOrSub)
aligntime(col::AbstractArray, time::RotatingTimeArray)
aligntime(data, colname::Union{Symbol,Integer}, timename::Union{Symbol,Integer})
```

Convert a column of time values `col`

to a `ScaledArray`

with a `pool`

that has the same first element and step size as the `pool`

from the `ScaledArray`

`time`

. If `time`

is a `RotatingTimeArray`

with the `time`

field being a `ScaledArray`

, the returned array is also a `RotatingTimeArray`

with the `time`

field being the converted `ScaledArray`

. Alternative, the arrays may be specified with a `Tables.jl`

-compatible `data`

table and column indices `colname`

and `timename`

. See also `settime`

.

This is useful for representing all discretized time periods with the same scale so that the underlying reference values returned by `DataAPI.refarray`

can be directly comparable across the columns.

`DiffinDiffsBase.cellrows`

— Method`cellrows(cols::VecColumnTable, refrows::IdDict)`

A utility function for processing the object `refrows`

returned by `findcell`

. Unique row values from `cols`

corresponding to the keys in `refrows`

are sorted lexicographically and stored as rows in a new `VecColumnTable`

. Groups of row indices from the values of `refrows`

are permuted to match the order of row values and collected in a `Vector`

.

**Returns**

`cells::VecColumnTable`

: unique row values from columns in`cols`

.`rows::Vector{Vector{Int}}`

: row indices for each combination.

`DiffinDiffsBase.diff!`

— Method`diff!(dest::AbstractArray, panel::PanelStructure, v::AbstractArray; kwargs...)`

Take the differences of `v`

within observations for each unit in `panel`

and store the result in `dest`

. By default, it calculates the first differences. See also `diff`

.

**Keywords**

`order::Integer=1`

: the order of differences to be taken.`l::Integer=1`

: the time interval between each pair of observations.`default=missing`

: default values for indices where the differences do not exist.

`DiffinDiffsBase.findcell`

— Method```
findcell(cols::VecColumnTable)
findcell(names, data, esample=Colon())
```

Group the row indices of a collection of data columns so that the combination of row values from these columns are the same within each group.

Instead of directly providing the relevant portions of columns as `VecColumnTable`

``, one may specify the`

names`of columns from`

data`of any`

Tables.jl`-compatible table type over selected rows indicated by`

esample`. Note that unless`

esample`covers all rows of`

data`, the row indices are those for the subsample selected based on`

esample`rather than those for the full`

data`.

**Returns**

`IdDict{Tuple, Vector{Int}}`

: a map from unique row values to row indices.

`DiffinDiffsBase.findlag!`

— Function`findlag!(panel::PanelStructure, l::Integer=1)`

Construct a vector of indices of the `l`

th lagged values for all id-time combinations of `panel`

and save the result in `panel.laginds`

. If a lagged value does not exist, its index is filled with 0. See also `ilag!`

.

`DiffinDiffsBase.findlead!`

— Function`findlead!(panel::PanelStructure, l::Integer=1)`

Construct a vector of indices of the `l`

th lead values for all id-time combinations of `panel`

and save the result in `panel.laginds`

. If a lead value does not exist, its index is filled with 0. See also `ilead!`

.

`DiffinDiffsBase.ilag!`

— Function`DiffinDiffsBase.ilead!`

— Function`DiffinDiffsBase.setpanel`

— Method```
setpanel(data, idname, timename; step, reftype, rotation)
setpanel(id::AbstractArray, time::AbstractArray; step, reftype, rotation)
```

Declare a `PanelStructure`

which is required for certain operations such as `lag`

and `diff`

. Unit IDs and time values can be provided either as a table containing the relevant columns or as arrays. `timestep`

must be specified unless the `time`

array is a `ScaledArray`

that is returned by `settime`

.

**Arguments**

`data`

: a`Tables.jl`

-compatible data table.`idname::Union{Symbol,Integer}`

: the name of the column in`data`

that contains unit IDs.`timename::Union{Symbol,Integer}`

: the name of the column in`data`

that contains time values.`id::AbstractArray`

: the array containing unit IDs (only needed for the alternative method).`time::AbstractArray`

: the array containing time values (only needed for the alternative method).

**Keywords**

`step=nothing`

: the length of each time interval; try`step=one(eltype(time))`

if not specified.`reftype::Type{<:Signed}=Int32`

: the element type of the reference values for`PanelStructure`

.`rotation=nothing`

: rotation groups in a rotating sampling design; use`RotatingTimeValue`

s as reference values.

If the underlying data used to create the `PanelStructure`

are modified. The changes will not be reflected in the existing instances of `PanelStructure`

. A new instance needs to be created with `setpanel`

.

`DiffinDiffsBase.settime`

— Function`settime(time::AbstractArray, step; start, stop, reftype, rotation)`

Convert a column of time values to a `ScaledArray`

for representing discretized time periods of uniform length. If `rotation`

is specified (time values belong to multiple rotation groups), a `RotatingTimeArray`

is returned with the `time`

field being a `ScaledArray`

. The returned array ensures well-defined time intervals for operations involving relative time (such as `lag`

and `diff`

). See also `aligntime`

.

**Arguments**

`time::AbstractArray`

: the array containing time values.`step=nothing`

: the length of each time interval; try`step=one(eltype(time))`

if not specified.

**Keywords**

`start=nothing`

: the first element of the`pool`

of the returned`ScaledArray`

.`stop=nothing`

: the last element of the`pool`

of the returned`ScaledArray`

.`reftype::Type{<:Signed}=Int32`

: the element type of the reference values for the`ScaledArray`

.`rotation=nothing`

: rotation groups in a rotating sampling design.

`ShiftedArrays.lag`

— Function`ShiftedArrays.lead`

— Function`DiffinDiffsBase.RotatingTimeArray`

— Type`RotatingTimeArray{T<:RotatingTimeValue,N,C,I} <: AbstractArray{T,N}`

Array type for `RotatingTimeValue`

s that stores the field values `rotation`

and `time`

in two arrays for efficiency. The two arrays that hold the field values for all elements can be accessed as properties.

`DiffinDiffsBase.RotatingTimeArray`

— Method`RotatingTimeArray(rotation::AbstractArray, time::AbstractArray)`

Construct a `RotatingTimeValue`

from arrays of `rotation`

and `time`

.

`DiffinDiffsBase.RotatingTimeValue`

— Type`RotatingTimeValue{R, T}`

A wrapper around a time value for distinguishing potentially different rotation group it could belong to in a rotating sampling design. See also `rotatingtime`

and `settime`

.

**Fields**

`rotation::R`

: a rotation group in a rotating sampling design.`time::T`

: a time value belonged to the rotation group.

`DiffinDiffsBase.rotatingtime`

— Method`rotatingtime(rotation, time)`

Construct `RotatingTimeValue`

s from `rotation`

and `time`

. This method simply broadcasts the default constructor over the arguments.