Ifelse statements for a dataframe in R - r

Question

I am hoping that someone can help me figure out how to write an if-else statement to work on my dataset. I have data on tree growth rates by year. I need to calculate whether growth rates decreased by >50% from one year to the next. I am having trouble applying an ifelse statement to calculate my final field. I am relatively new to R, so my code is probably not very efficient, but here is an example of what I have so far:
For an example dataset,
test<-data.frame(year=c("1990","1991","1992","1993"),value=c(50,25,20,5))
year value
1 1990 50
2 1991 25
3 1992 20
4 1993 5
I then calculate the difference between the current year and previous year's growth ("value"):
test[-1,"diff"]<-test[-1,"value"]-test[-nrow(test),"value"]
year value diff
1 1990 50 NA
2 1991 25 -25
3 1992 20 -5
4 1993 5 -15
and then calculate what 50% of each years' growth would be:
test$chg<-test$value * 0.5
year value diff chg
1 1990 50 NA 25.0
2 1991 25 -25 12.5
3 1992 20 -5 10.0
4 1993 5 -15 2.5
I am then trying to use an ifelse statement to calculate a field "abrupt" that would be "1" when the decline from one year to the next is greater than 50%. This is the code I am trying to use, but I'm not sure how to properly reference the "chg" field from the previous year, because I am getting an error (copied below):
test$abrupt<-ifelse(test$diff<0 && abs(test$diff)>=test[-nrow(test),"chg"],1,0)
Warning message:
In abs(test$diff) >= test[-nrow(test), "chg"] :
longer object length is not a multiple of shorter object length
> test
year value diff chg abrupt
1 1990 50 NA 25.0 NA
2 1991 25 -25 12.5 NA
3 1992 20 -5 10.0 NA
4 1993 5 -15 2.5 NA
A test of a similar ifelse statement worked when I just assigned a few numbers, but I'm not sure how to get this to work in the context of a datframe. Here is an example of it working on just a few values:
prevyear<-50
curryear<-25
chg<-prevyear*0.5
> chg
[1] 25
> diff<-curryear-prevyear
> diff
[1] -25
> abrupt<-ifelse(diff<0 && abs(diff)>= chg,1,0)
> abrupt
[1] 1
If anyone could help me figure out how to apply a similar ifelse statement to my dataframe I would greatly appreciate it! Thank you for any help you can provide.
thank you,
Katie

Answer 1

It's throwing a warning because the two vectors compared abs(test$diff) >= test[-nrow(test),"chg"] have different lengths. Also, for logical and, you are using && (which gives only one TRUE or FALSE) when you should be using & (which is vectorized: it operates elementwise over two vectors and returns a vector of the same length). Try this:
test$abrupt<-ifelse(test$diff<0 & abs(test$diff)>=test$chg,1,0)

Answer 2

I would change where you're putting chg so that it lines up with the diff you want to compare it to:
test$chg[2:nrow(test)] <- test$value[1:(nrow(test)-1)] * 0.5
Then, correct your logical operator like Blue Magister said:
test$abrupt<-ifelse(test$diff<0 & abs(test$diff)>=test$chg,1,0)
and you have your results:
year value diff chg abrupt
1 1990 50 NA NA NA
2 1991 25 -25 25.0 1
3 1992 20 -5 12.5 0
4 1993 5 -15 10.0 1
Also, you may find the function diff helpful: rather than doing this:
test[-1,"value"]-test[-nrow(test),"value"]
you can just do
diff(test$value)