R中的特定日期格式转换问题

Question

Basically I want to know why as.Date(200322,format="%Y%W") gives me NA . While we are at it, I would appreciate any advice on a data structure for repeated cross-section (aka pseudo-panel) in R. I did get aggregate() to (sort of) work, but it is not flexible enough - it misses data on columns when I omit the missed values, for example. Specifically, I have a survey that is repeated weekly for a couple of years with a bunch of similar questions answers to which I would like to combine, average, condition and plot in both dimensions. Getting the date conversion right should presumably help me towards my goal with zoo package or something similar.

Any input is appreciated.

Update: thanks for string suggestion, but as you can see in your own example, %W part doesn't work - it only identifies the year while setting the current day while I need to set a specific week (and leave the day blank).

Answer 1

Use a string as first argument in as.Date() and select a specific weekday (format %w , value 0-6). There are seven possible dates in each week, therefore strptime needs more information to select a unique date. Otherwise the current day and month are returned.

> as.Date(paste("200947", "0", sep="-"), format="%Y%W-%w")
[1] "2009-11-22"