I'm aware that the question is awkward. If I could phrase it better I'd probably find the solution in an other thread.
I have this data structure:
df <- data.frame(group = c("X", "F", "F", "F", "F", "C", "C"),
subgroup = c(NA, "camel", "horse", "dog", "cat", "orange", "banana"))
and would like to turn it into this:
data.frame(group = c("X", "F", "camel", "horse", "dog", "cat", "C", "orange", "banana"))
which is surprisingly confusing. Also, I would prefer not using a loop.
I updated the example to clarify that solutions that depend on sorting unfortunately do not do the trick.
Here an (edited) answer with new data.
Using data.table is going to help a lot. The idea is to split the df into groups and lapply() to each group what we need. Whe have to take care of some things meanwhile.
library(data.table)
# set as data.table
setDT(df)
# to mantain the ordering, you need to put as factor the group.
# the levels are going to give the ordering infos to split
df[,':='(group = factor(group, levels =unique(df$group)))]
# here the split function, splitting df int a list
df_list <-split(df, df$group, sorted =F)
# now you lapply to each element what you need
df_list <-lapply(df_list, function(x) data.frame(group = unique(c(as.character(x$group),x$subgroup))))
# put into a data.table and remove NAs
rbindlist(df_list)[!is.na(df_onecol$group)]
group
1: X
2: F
3: camel
4: horse
5: dog
6: cat
7: C
8: orange
9: banana