Select "europe" from df - r

Question

my df2:
League freq
18 England 108
27 Italy 79
20 Germany 74
43 Spain 64
19 France 49
39 Russia 34
31 Mexico 27
47 Turkey 24
32 Netherlands 23
37 Portugal 21
49 United States 18
29 Japan 16
25 Iran 15
7 Brazil 13
22 Greece 13
14 Costa 11
45 Switzerland 11
5 Belgium 10
17 Ecuador 10
23 Honduras 10
42 South Korea 9
2 Argentina 8
48 Ukraine 7
3 Australia 6
11 Chile 6
12 China 6
15 Croatia 6
35 Norway 6
41 Scotland 6
34 Nigeria 5
I try to select europe.
europe <- subset(df2, nrow(x=18, 27, 20) select=c(1, 2))
What is the most effective way to select europe, africa, Asia ... from df2?

Answer 1

You either need to identify which countries are on which continents by hand, or you might be able to scrape this information from somewhere:
(basic strategy from Scraping html tables into R data frames using the XML package)
library(XML)
theurl <- "http://en.wikipedia.org/wiki/List_of_European_countries_by_area"
tables <- readHTMLTable(theurl)
library(stringr)
europe_names <- str_extract(as.character(tables[[1]]$Country),"[[:alpha:] ]+")
head(sort(europe_names))
## [1] "Albania" "Andorra" "Austria" "Azerbaijan" "Belarus"
## [6] "Belgium"
## there's also a 'Total' entry in here but it's probably harmless ...
subset(df2,League %in% europe_names)
Of course you'd have to figure this out again for Asia, America, etc.

Answer 2

So here's a slightly different approach from #BenBolker's, using the countrycode package.
library(countrycode)
cdb <- countrycode_data # database of countries
df2[toupper(df2$League) %in% cdb[cdb$continent=="Europe",]$country.name,]
# League freq
# 27 Italy 79
# 20 Germany 74
# 43 Spain 64
# 19 France 49
# 32 Netherlands 23
# 37 Portugal 21
# 22 Greece 13
# 45 Switzerland 11
# 5 Belgium 10
# 48 Ukraine 7
# 15 Croatia 6
# 35 Norway 6
One problem you're going to have is that "England" is not a country in any database (rather, "United Kingdom"), so you'll have to deal with that as a special case.
Also, this database considers the "Americas" as a continent.
df2[toupper(df2$League) %in% cdb[cdb$continent=="Americas",]$country.name,]
so to get just South America you have to use the region field:
df2[toupper(df2$League) %in% cdb[cdb$region=="South America",]$country.name,]
# League freq
# 7 Brazil 13
# 17 Ecuador 10
# 2 Argentina 8
# 11 Chile 6