How to apply multiple if statements in R? - r

Question

I have a data frame (df) that lists the countries associated with every site
Site Country
Site1 USA
Site2 Vietnam
Site3 Spain
Site4 Germany
Site5 China
I want to attach a column, where for each country I associate its corresponding continent. I wrote a simple if loop to do this:
df$Continent <- NA
if(df$Country == "USA" |df$Country == "Canada" |df$Country == "Mexico")
{df$Continent <- "North America"}
if(df$Country == "Spain" |df$Country == "France" |df$Country == "Germany")
{df$Continent <- "Europe"}
## .. etc
summary(df)
However, each time I run it the df, I find that it assigns North America to all the countries. I understand that this may sound trivial, but does it make a difference if I use if statments everywhere and not else or if else? Any suggestions for correcting this?

Answer 1

Build a lookup table and merge() it with the data.
For example:
lookup <- data.frame(Country = c("USA", "Canada", "Mexico",
"Spain", "France", "Germany",
"Vietnam", "China"),
Continent = rep(c("North America", "Europe", "Asia"),
times = c(3,3,2)))
Using your snippet of data as data frame df, we can add Continent via merge() (a join in database terminology):
> merge(df, lookup, sort = FALSE, all.x = TRUE)
Country Site Continent
1 USA Site1 North America
2 Vietnam Site2 Asia
3 Spain Site3 Europe
4 Germany Site4 Europe
5 China Site5 Asia

Answer 2

If you're working with a factor you can also do some nonsense with levels, or levels<- to be exact:
`levels<-`(dat$Country, list(
`North America` = c("USA","Canada","Mexico"),
`Europe` = c("Spain","France","Germany"),
`Asia` = c("Vietnam","China")
))
#[1] North America Asia Europe Europe Asia
#Levels: North America Europe Asia

Answer 3

I like ifelse() for things like this. You could use it with the %in% operator like this:
df$Continent <- ifelse(df$Country %in% c("USA", "Canada", "Mexico"),
"North America", df$Continent)
df$Continent <- ifelse(df$Country %in% c("Spain", "France", "Germany"),
"Europe", df$Continent)
df
Site Country Continent
1 Site1 USA North America
2 Site2 Vietnam <NA>
3 Site3 Spain Europe
4 Site4 Germany Europe
5 Site5 China <NA>