thanks Jeff and Gabor,
appreciate you spending time on this,
you both use similar ideas - add/subtract days/weeks from a day that exists
combining all i learned i would go with something like this:
## step 1) find 1st sunday of year Y
d11 <- as.Date( sprintf( "%04d 1 1"
Replace the week in the date with week 2, say -- a week in which
nothing will go wrong and then add or subtract the appropriate number
of weeks.
d <- c('2016 00 Sun', '2017 53 Sun', '2017 53 Mon') # test data
as.Date(sub(" .. ", "02", d), "%Y %U %a") +
7 * (as.numeric(sub(" (..)
You cannot obtain a predictable result by sending invalid time
representation data to strptime... you have to work with valid time
representations.
See sample approach below:
weekEnds <- function( DF ) {
d1_1 <- as.Date( sprintf( "%04d 1 1"
hi,
thanks for replying,
it has taken some time to understand
i have year+week and i need to find the 1st day and the last day of that
week
i can decide when week starts
for example these 3 examples:
df <- data.frame(id = 1:3, year = c(2018, 2018, 2018), week=c(0,1,52))
## now run for all 3
Er, my mistake, you are using %U not %W... but now I am really confused,
because the first Sunday is trivial with %U/%u.
Can you clarify what your actual upstream input is? Is it an invalid date
string as you say below, or is it year number?
On October 16, 2018 10:22:10 AM PDT, Jeff Newmiller
If the date in your character representation does not exist then there is no
requirement for a POSIX function to give any reliable answer... including NA.
Using 00 as the week number won't always work.
The first week/weekday combination that is guaranteed to exist by POSIX is 1/1
(first
it is simpler than i thought
first day of given week is the last day minus 6days
in other words:
d1 = as.Date('2018 00 Sat',format="%Y %U %a") - 6
d2 = as.Date('2018 00 Sun',format="%Y %U %a")
are the same as long both are not NA
therefore to get the one that is not NA one can do
max(
hi,
to turn year and week into the date one can do the following:
as.Date('2018 05 Sun', "%Y %W %a")
however, when we want the Sunday (1st day of week) of the 1st week of 2018
we get NA because 1/1/2018 was on Monday
as.Date('2018 00 Mon',format="%Y %U %a")
## 2018-01-01
as.Date('2018 00
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