问题
I have a datatable where each column represents a time series, and I want to grab the last NA value per time series in a column-ordered manner. In my particular use case my data looks like this:
a b c
1 2 5
1 -17 9
NA 11 4
NA 57 NA
63 NA NA
So out of this I would like to extract:
a b c
63 57 4
How can I accomplish this? So far I only see answers addressing the converse situation of extracting the last non-NA per row rather than per column.
回答1:
If the dataset is data.table, loop through the Subset of Data.table (.SD), subset the non-NA element (x[!is.na(x)]) and extract the last element among those with tail.
df1[, lapply(.SD, function(x) tail(x[!is.na(x)],1))]
# a b c
#1: 63 57 4
回答2:
For someone wishing to use only base R.
sapply(df, function(x) x[max(which(!is.na(x)))])
where
df <- data.frame(a = c(1, 1, NA, NA, 63),
b = c(2, -17, 11, 57, NA),
c = c(5, 9, 4, NA, NA))
来源：https://stackoverflow.com/questions/39934159/get-value-of-last-non-na-row-per-column-in-data-table