R中的ifelse行为

Question

First of all I declare that I checked all the related question on ifelse , but without any success. I am trying to use ifelse to add a factor (mating period) in wildlife species according to the months. An example of the DB is below:

> DB <- data.frame(ID = 1:6, Month = 1:12)
colnames(DB) <- c("ID","Month")

What I have tried is:

Period<-with(DB,ifelse(Month==c(2,3,4,5,6,7,8,9,10),"M","R")))

but the result doesn't match the condition:

(Period)[1] "R" "R" "R" "R" "R" "R" "R" "R" "R" "R" "R" "R"

and there is also a warning:

Warning message:
In Month == c(2, 3, 4, 5, 6, 7, 8, 9, 10) :
longer object length is not a multiple of shorter object length`

Surprisingly, when I launch the script on the real DB two things happen:

• In the new column Period some M appears
• No warning comes out

I am now really confused on how ifelse works. I hope someone could clarify how this function works and how properly type the script to obtain the correct result.

Answer 1

Use %in% as in:

Period<-with(DB,ifelse(Month %in% c(2,3,4,5,6,7,8,9,10),"M","R"))
Period
 [1] "R" "M" "M" "M" "M" "M" "M" "M" "M" "M" "R" "R"

Answer 2

Your comparison is of the wrong form. ifelse goes through one element at a time making the comparison whereas Month == 2:10 will pairwise compare the elements and return a vector indicating whether each pairwise comparison is true or false.

You want to use %in% which returns true or false depending on whether an element is in a vector:

Period<-with(DB,ifelse(Month %in% 2:10,"M","R")))