ffirst_flast function

Fast (Grouped) First and Last Value for Matrix-Like Objects

ffirst and flast are S3 generic functions that (column-wise) returns the first and last values in x, (optionally) grouped by g. The TRA argument can further be used to transform x using its (groupwise) first and last values.

ffirst(x, ...)
flast(x, ...)

## Default S3 method:
ffirst(x, g = NULL, TRA = NULL, na.rm = .op[["na.rm"]],
       use.g.names = TRUE, ...)
## Default S3 method:
flast(x, g = NULL, TRA = NULL, na.rm = .op[["na.rm"]],
      use.g.names = TRUE, ...)

## S3 method for class 'matrix'
ffirst(x, g = NULL, TRA = NULL, na.rm = .op[["na.rm"]],
       use.g.names = TRUE, drop = TRUE, ...)
## S3 method for class 'matrix'
flast(x, g = NULL, TRA = NULL, na.rm = .op[["na.rm"]],
      use.g.names = TRUE, drop = TRUE, ...)

## S3 method for class 'data.frame'
ffirst(x, g = NULL, TRA = NULL, na.rm = .op[["na.rm"]],
       use.g.names = TRUE, drop = TRUE, ...)
## S3 method for class 'data.frame'
flast(x, g = NULL, TRA = NULL, na.rm = .op[["na.rm"]],
      use.g.names = TRUE, drop = TRUE, ...)

## S3 method for class 'grouped_df'
ffirst(x, TRA = NULL, na.rm = .op[["na.rm"]],
       use.g.names = FALSE, keep.group_vars = TRUE, ...)
## S3 method for class 'grouped_df'
flast(x, TRA = NULL, na.rm = .op[["na.rm"]],
      use.g.names = FALSE, keep.group_vars = TRUE, ...)

Arguments

• x: a vector, matrix, data frame or grouped data frame (class 'grouped_df').
• g: a factor, GRP object, atomic vector (internally converted to factor) or a list of vectors / factors (internally converted to a GRP object) used to group x.
• TRA: an integer or quoted operator indicating the transformation to perform: 0 - "na" | 1 - "fill" | 2 - "replace" | 3 - "-" | 4 - "-+" | 5 - "/" | 6 - "%" | 7 - "+" | 8 - "*" | 9 - "%%" | 10 - "-%%". See TRA.
• na.rm: logical. TRUE skips missing values and returns the first / last non-missing value i.e. if the first (1) / last (n) value is NA, take the second (2) / second-to-last (n-1) value etc..
• use.g.names: logical. Make group-names and add to the result as names (default method) or row-names (matrix and data frame methods). No row-names are generated for data.table's.
• drop: matrix and data.frame method: Logical. TRUE drops dimensions and returns an atomic vector if g = NULL and TRA = NULL.
• keep.group_vars: grouped_df method: Logical. FALSE removes grouping variables after computation.
• ...: arguments to be passed to or from other methods. If TRA is used, passing set = TRUE will transform data by reference and return the result invisibly.

Returns

ffirst returns the first value in x, grouped by g, or (if TRA is used) x transformed by its first value, grouped by g. Similarly flast returns the last value in x, ...

Note

Both functions are significantly faster if na.rm = FALSE, particularly ffirst which can take direct advantage of the 'group.starts' elements in GRP objects.

See Also

Fast Statistical Functions , Collapse Overview

Examples

## default vector method
ffirst(airquality$Ozone)                   # Simple first value
ffirst(airquality$Ozone, airquality$Month) # Grouped first value
ffirst(airquality$Ozone, airquality$Month,
       na.rm = FALSE)                      # Grouped first, but without skipping initial NA's

## data.frame method
ffirst(airquality)
ffirst(airquality, airquality$Month)
ffirst(airquality, airquality$Month, na.rm = FALSE) # Again first Ozone measurement in month 6 is NA

## matrix method
aqm <- qM(airquality)
ffirst(aqm)
ffirst(aqm, airquality$Month) # etc..
 
## method for grouped data frames - created with dplyr::group_by or fgroup_by
library(dplyr)
airquality |> group_by(Month) |> ffirst()
airquality |> group_by(Month) |> select(Ozone) |> ffirst(na.rm = FALSE)

# Note: All examples generalize to flast.