Modifying dplyr::lag function

Question

I am trying to use the lag function from the dplyr package. However when I give a lag > 0 I want the missing values to be replaced by the first value in x. How can we achieve this

library(dplyr)
x<-c(1,2,3,4)
z<-lag(x,2)
z
## [1] NA NA  1  2

@DavidArenburg - yes it does: > stats::lag(1:10, 2) [1] NA NA 1 2 3 4 5 6 7 8 — Tim
– Tim, Commented Feb 18, 2015 at 8:55
@DavidArenburg you are right... so isn't stats::lag calling the stats-lag?! — Tim
– Tim, Commented Feb 18, 2015 at 9:01
@Tim it seems like some bug in dplyrs lag, this seems related — David Arenburg
– David Arenburg, Commented Feb 18, 2015 at 9:07

shadow · Accepted Answer · 2015-02-18 10:46:46Z

5

Since you are using the lag function dplyr, there is an argument default. So you can specify that you want x[1] to be the default.

lag(x, 2, default=x[1])

answered Feb 18, 2015 at 10:46

shadow

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David Arenburg · Accepted Answer · 2015-02-18 09:04:15Z

3

Here's a modified function mylag:

mylag <- function(x, k = 1, ...)
  replace(lag(x, k, ...), seq(k), x[1])

x <- 1:4
mylag(x, k = 2)
# [1] 1 1 1 2

edited Feb 18, 2015 at 9:04

David Arenburg

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answered Feb 18, 2015 at 8:44

Sven Hohenstein

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2 Comments

user3710546 Over a year ago

It gives me [1] 1 1 3 4.

David Arenburg Over a year ago

@Pascal this is because Sven is also using lag from the dplyr package. welcome to Hadlywerse...

agenis - code en bois · Accepted Answer · 2016-03-01 15:17:44Z

0

May I suggest adapting the function so that it works both ways: for lag and lead (positive AND negative lags).

shift = function(x, lag, fill=FALSE) {
  require(dplyr)
  switch(sign(lag)/2+1.5, 
         lead( x, n=abs(lag), default=switch(fill+1, NA, tail(x, 1))  ), 
         lag(  x, n=abs(lag), default=switch(fill+1, NA, head(x, 1))  )
  )
}

It has a "fill" argument that automatically fills with first of last value depending on the sign of the lag.

> shift(1:10, -1)
#### [1]  2  3  4  5  6  7  8  9 10 NA
> shift(1:10, +1, fill=TRUE)
#### [1] 1 1 2 3 4 5 6 7 8 9

answered Mar 1, 2016 at 15:17

agenis - code en bois

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