How to convert a date as year/week to the first day of the week via POSIXct?

Question

I want to convert

myDate=as.character("2017/02")

which is in the format year/week to a date that is the first day of this week. So I tried:

print(as.POSIXct(myDate,format="%Y/%U"))

However, this gives me

[1] "2017-05-03 CEST"

which is certainly not the first day of the second week in 2017.

Question : How do I need to change as.POSIXct(myDate,format="%Y/%U") in order to make it work as described above?

Answer 1

A year and a week is not a proper date. A day would need to be associated with the week. For example:

myDate=as.character("2017/02")    
as.POSIXct(paste(myDate, "0"),format="%Y/%U %w")

#[1] "2017-01-08 EST"

this assumes the first day of the week is Sunday. If you prefer a Monday then see the %u option.

Answer 2

Here a possible approach:

myDate=as.character("2017/02")

Creation of the calendar (yyyy/dd/mm) of indicated year (Eg 2017)

year_date<-seq.Date(as.Date(paste0(substr(myDate,1,4),"/01/01")),as.Date(paste0(substr(myDate,1,4),"/12/31")),by=1)

Using library ISOweek , this code finds the first day of each week

library("ISOweek")
first_day<-cumsum(ISOweekday(year_date)==1)

Extraction of the first day of the indicated week

year_date[which(first_day==as.numeric(unlist(strsplit(myDate,split="/"))[2]))[1]]
[1] "2017-01-09"

NB : in this example weeks start on monday