Calculate previous date

[ply]

I have list of days when wages are taken
e.g. wages_days <- "Tuesday,Thursday"

I want to compare wages between the two days

for example if we run wages today 17/05/2022 which is a tuesday I want to know the previous date in the list which will of course be Thursday 12/05/2022

So if we know the date which is todays date which we can calculate is a tuesday using R how do we find the date for the previous thursday

the list could change
ie e.g. wages_days <- "Monday,Tuesday,Thursday"

Thanks

[dvetsch75]

Can you clarify a bit what exactly you are trying to do? After reading your question, I am a bit unsure which of the following problems you are trying to solve:

• How many days are between a given weekday and a prior second given weekday (so if you give a function a "Friday" and a "Tuesday", the function would return 3)?
• You have one vector of dates, and you want to figure out what weekday each date is on?

Or something else? Maybe a Reprex would help?

[ply]

Sorry for my very poor explanation

These could be any days

wages_days <- "Monday,Wednesday, Friday"

1. Check todays date is one of the days in the list ie a Monday,Wednesday or Thursday

2. If not print error

3. If it is then get the previous date based on the list

     e.g. In aboves case if todays date is a Wednesday we would want the date of the Monday
              if todays date is a Monday we would want the date of the previous Friday

Does that make more sense ?

[nirgrahamuk]

my_calc <- function(wages_days, today) {
  (wd_v <- unlist(strsplit(wages_days, ",|, ")))
  (lwd <- length(wd_v))
  (todayw <- weekdays(today))

  (prev <- ((which(todayw == wd_v) + 1) %% lwd) + 1)
  
  if (identical(prev, numeric(0))) {
    stop("invalid date to lookup")
  }

  (prev_v <- wd_v[prev])

  (back_6 <- seq.Date(from = today - 7, 
                      length.out = 6,
                      by = 1))
  (back_6w <- weekdays(back_6))
  back_loc <- which(back_6w == prev_v)
  
  list(
    date = back_6[ back_loc],
    wd   = back_6w[back_loc]
  )
}

my_calc(
  "Monday,Wednesday, Friday",
  Sys.Date() + 0 # move around today by adding or subtracting
)

[ply]

Thanks for the hard work on this..

But I dont want in your example the Week before but the previous day in the list ie if Today is a monday in your example I would want the prev date to be the Friday the week before

[nirgrahamuk]

I think if today wqs monday my code gives the previous friday. By adding the weekbefore you mean it should be more than 7 days apart ? I.e. two fridays ago ?

[nirgrahamuk]

@nirgrahamuk

list(
    date = back_6[ back_loc],
    wd   = back_6w[back_loc]
  )

If you always want a firther 7 days back just add -7 at the end
I.e

list(
    date = back_6[ back_loc] -7,
    wd   = back_6w[back_loc]
  )