R Code Programming: filling missing column values down first and then up on a stacked sorted table

Question

I have a dataframe with 3 columns, but the 3rd column has some missing values which need to be filled in with the following logic.

The desired result is that there are n

Answer 1

This is essentially a "last observation carried forward" task

library(zoo)
levels(df$part)[1] <- NA

df$part <- with(df,
  ave(part, item, FUN=function(x) {
    x <- na.locf(x,na.rm=FALSE)
    x[is.na(x)] <- na.omit(x)[1]
    x
  })
)

#   item week part
#1     A    1    X
#2     A    2    X
#3     A    3    X
#4     A    4    X
#5     A    5    Y
#6     A    6    Y
#7     A    7    Y
#8     B    1    Z
#9     B    2    Z
#10    B    3    Z
#11    D   10    T
#12    D   11    T
#13    D   12    U
#14    D   13    U
#15    D   14    U

all.equal(df,df2, check.names=FALSE)
#[1] TRUE