如何解决此错误“条件的长度> 1，并且仅使用第一个元素”？

Question

I am writing a function to find a date x months back form a current date. I keep getting an error/warning.

DateBack <- function(CurrentYear, CurrentMonth, MonthsBack){
if(MonthsBack< 13){
    if(MonthsBack>=CurrentMonth){
        month<-12+CurrentMonth-MonthsBack
        datet<-paste(CurrentYear-1,month, sep="-")
    } 
    else{
        month<-CurrentMonth-MonthsBack
        datet<-paste(CurrentYear-1, month, sep="-")
    }
} 
else{
    Years <- trunc(MonthsBack/12,0)
    if((MonthsBack-12*Years)>=CurrentMonth){
        month<-12+CurrentMonth-MonthsBack
    } 
    else{
        month<-CurrentMonth-MonthsBack
    }
    datet<-paste(CurrentYear-Years, month, sep="-")
}
return(datet)
}

CurrentYear<- c(2000, 2000, 2003, 2004)
CurrentMonth<-c(1, 2, 6, 12) 

df<- data.frame(CurrentYear, CurrentMonth)

df$MonthsBack <- DateBack(df$CurrentYear,df$CurrentMonth,1)

Warning message: In if (MonthsBack >= CurrentMonth) { : the condition has length > 1 and only the first element will be used

How would I go about correcting this error?

Answer 1

I second CarlWitthoft comment too, although there does not seem to be an easy answer through date functions.

The issue in the code is the conditional statement which is not vectorized. You can implement it with no IF statement in a fully vectorized manner:

DateBack = function (Y,M, monthsBack) {
   YM = (Y*12 + M - 1) - monthsBack
   newY = YM %/% 12
   newM = YM %% 12 + 1
   return( paste( newY, newM, sep="-"))
 }

DateBack( c(2000,2000,2003,2004),c(1,2,6,12),1)
[1] "1999-12" "2000-1"  "2003-5"  "2004-11"