Apply na.locf to multiple datasets - r

Question

I have multiple datasets (Eg: data01, data02..). In all these datasets, I want to apply na.locf to var1, and create a new variable 'var2' from the locf applied 'var1'. I tried using the following code:
L=list(data01,data02)
for (i in L){i$var2 <- na.locf(i$var1)}
However, when I try to read the locf column using code:
head(data01$var2)
The result given is NULL.

Answer 1

There are a few problems:
in the question i is a copy of each data frame so L is not changed. Index into L to ensure that it is the data frame in L that is changed.
use na.locf0 or equivalently na.locf(..., na.rm = FALSE) to ensure that the output is the same length as the input
the data01 and data02 in L are copies of data01 and data02 and modifying one does not modify the other. That is why you get NULL.
Using the built-in BOD data frame to construct sample input:
library(zoo)
# construct sample input
BOD1 <- BOD2 <- BOD
BOD1$Time[c(1, 3)] <- BOD2$Time[c(3, 5)] <- NA
L <- list(BOD1, BOD2)
for(i in seq_along(L)) L[[i]]$Time2 <- na.locf0(L[[i]]$Time)
giving:
str(L)
List of 2
$ :'data.frame': 6 obs. of 3 variables:
..$ Time : num [1:6] NA 2 NA 4 5 7
..$ demand: num [1:6] 8.3 10.3 19 16 15.6 19.8
..$ Time2 : num [1:6] NA 2 2 4 5 7
..- attr(*, "reference")= chr "A1.4, p. 270"
$ :'data.frame': 6 obs. of 3 variables:
..$ Time : num [1:6] 1 2 NA 4 NA 7
..$ demand: num [1:6] 8.3 10.3 19 16 15.6 19.8
..$ Time2 : num [1:6] 1 2 2 4 4 7
..- attr(*, "reference")= chr "A1.4, p. 270"
Any of these would also work and instead of modifying L produce a new list:
L2 <- lapply(L, function(x) { x$Time2 <- na.locf0(x$Time); x })
L3 <- lapply(L, transform, Time2 = na.locf0(Time))
If your aim is to modify BOD1 and BOD2 as opposed to creating a list with the modified BOD1 and BOD2 then the following would do that (although it is usually better to organize objects in a list if you intend to iterate over them) rather than leave them loose in the global environment.
nms <- c("BOD1", "BOD2")
for(nm in nms) assign(nm, transform(get(nm), Time2 = na.locf0(Time)))
or
nms <- c("BOD1", "BOD2")
for(nm in nms) .GlobalEnv[[nm]]$Time2 <- na.locf0(.GlobalEnv[[nm]]$Time2)
or other variations.