Group by in data.table in R which only keep non NA values from columns

Question

I am new to R.

I want to group by a data.table and keep only non NA values from columns.

My table is look like:

c1   c2   c3   c4
1    A    NA   NA
1    NA   B    NA
1    NA   NA   C
2    A1   NA   NA
2    NA   B1   NA
2    NA   NA   C1

I want to have a result:

c1   c2   c3   c4
1    A    B    C
2    A1   B1   C1

Hope anyone can help!

Answer 1

Try

library(data.table)
setDT(df1)[, lapply(.SD, na.omit) , by = c1]
#    c1 c2 c3 c4
#1:  1  A  B  C
#2:  2 A1 B1 C1

Or

setDT(df)[, lapply(.SD, function(x) x[!is.na(x)]) , by = c1]