Sorting Column of dates in R

Question

I have a dataframe with a date column. These dates represent the date that a particular poll result was actually taken. However, the website takes these results and adds them to a table not necessarily on the date of the poll taking. So for example:

• 20/01/2018
• 21/01/2018
• 20/01/2018
• 19/01/2018

so the date at the top (20/01/2018) came in after the ones below. But the poll below says 21st and thats the date that the poll was taken so the earliest date that the one above could have been added is the 21st thus the list becomes;

• 21/01/2018
• 21/01/2018
• 20/01/2018
• 19/01/2018

and now my column is sorted. I need to do this for like 50 variables! Suggestions? I want to sort my dates column such that if i go from bottom to top of the column if a date has a later date below it, then that date becomes that later date too. enter image description here

Answer 1

Maybe there is a prettier way, but this should give the desired output:

data$Date <- as.POSIXct(rev(cummax(rev(as.numeric(data$Date)))), origin = "1970-01-01")

The idea is that you want a rolling maximum from the bottom up, for example once the 2018-01-02 was reached, the rows above can not have a date that is "smaller" than the 2018-01-02. This is done by the cummax function. It carries the maximum date reached and overwrites earlier/smaller dates. Since you want it to go from the bottom up, you have to reverse your date column via rev and then reverse it back after your call of cummax. Because cummax only works for numeric input I transformed your date column to numeric and back to date in the end.