using 'ifelse' in R: variable taking static value - r

Question

I am trying to create new variable in a dataset based on the value of an indicator. The following is the code for the same:
prac_data <- head(iris,10)
COPY_IND='Y' ##declaring the indicator to be 'Y'
prac_data <- prac_data %>% mutate(New_Var=ifelse(COPY_IND=='Y', Sepal.Length, 'N'))
I get the following output:
Sepal.Length Sepal.Width Petal.Length Petal.Width Species New_Var
1 5.1 3.5 1.4 0.2 setosa 5.1
2 4.9 3.0 1.4 0.2 setosa 5.1
3 4.7 3.2 1.3 0.2 setosa 5.1
4 4.6 3.1 1.5 0.2 setosa 5.1
5 5.0 3.6 1.4 0.2 setosa 5.1
6 5.4 3.9 1.7 0.4 setosa 5.1
7 4.6 3.4 1.4 0.3 setosa 5.1
8 5.0 3.4 1.5 0.2 setosa 5.1
9 4.4 2.9 1.4 0.2 setosa 5.1
10 4.9 3.1 1.5 0.1 setosa 5.1
I actually want to copy the variable 'Sepal.Length' in the 'New_Var' for every observation if indicator(COPY_IND) is Yes('Y').
If I do the the following, I get the desired response:
if (COPY_IND=='Y')
{
prac_data$New_Var <- prac_data$Sepal.Length
} else {prac_data$New_Var <- 'N'}
I just want to understand why R treats both 'if-else' approaches differently?
Is there another better elegant way to the same?
Thanks in advance!!

Answer 1

Actually, this might be easier to read as an answer.
From ifelse() help: "ifelse returns a value with the same shape as test which is filled with elements selected from either yes or no depending on whether the element of test is TRUE or FALSE".
Your test is just a single value, so ifelse() returns a single value, either Sepal.Length[1] or N, which is then duplicated across the whole column.
You need rowwise() on your way: prac_data <- prac_data %>% rowwise() %>% mutate(New_Var = ifelse(COPY_IND=='Y', Sepal.Length, 'N'))

Answer 2

COPY_IND is always "Y" in your case, then the code could be simplified to prac_data$New_Var = prac_data$Sepal.Length. Even if you want to use ifelse statement row-wisely, it is better to follow the instructions in the help document
Further note that if(test) yes else no is much more efficient and often much preferable to ifelse(test, yes, no) whenever test is a simple true/false result, i.e., when length(test) == 1.
I guess the desired COPY_IND should be one column of the data frame/vector rather than a single fixed value. In this case, you code generate the right answer, e.g. keep the first five number:
library(dplyr)
prac_data <- head(iris,10)
prac_data$COPY_IND=c(rep('Y',5),rep('N',5))
#COPY_IND=c(rep('Y',5),rep('N',5)) works too
prac_data <- prac_data %>% mutate(New_Var=ifelse(COPY_IND=='Y', Sepal.Length, 'N'))
generates the right column.