Calculate ratio of values within one column - r

Question

I have created a simple data frame with simulated GDP data for Costa Rica and the US, using the following code
gdp_test <- read.table(text = "Country, Year, GDP
costa_rica 1979 200
costa_rica 1980 210
costa_rica 1981 250
usa 1979 350
usa 1980 375
usa 1981 421", header=T)
gdp_test <- as.data.frame(gdp_test)
The output is as follows
Country. Year. GDP
1 costa_rica 1979 200
2 costa_rica 1980 210
3 costa_rica 1981 250
4 usa 1979 350
5 usa 1980 375
6 usa 1981 421
What I would like to do is to create a new variable consisting of the ratio of each country's GDP, for each year, to the usa gdp for that same year (obviously the ratio wouldl be 1 for the usa every year).
Any ideas of how to do it? It is an easy task in Excel, but I have found no way of doing it withing R
I have not been able to write any code that would do the task

Answer 1

That might do the trick, using tidyverse.
if(no_NA) {
Remove last pipe line
}
:)
gdp_test %>%filter(Country.=="usa") %>% group_by(Year.) %>% select(-Country.) %>%
left_join(gdp_test,by="Year.") %>%
rename(GDPus=GDP.x,GDP=GDP.y) %>%
mutate(ratio=GDP/GDPus) %>% ungroup() %>%
mutate(ratio=ifelse(ratio==1,NA,ratio))

Answer 2

Here is a very clumsy way of getting the job done. I am sure there are much better ways of doing it. Help would be enormously appreciated.
gdp_test <- read.table(text = "Country, Year, GDP
costa_rica 1979 200
costa_rica 1980 210
costa_rica 1981 250
usa 1979 350
usa 1980 375
usa 1981 421", header=T)
gdp_test <- as.data.frame(gdp_test) %>%
mutate(ID=row_number(),)
gdp_usa <- gdp_test$GDP[4:6]
usa <- as.data.frame(c(gdp_usa,gdp_usa)) %>%
mutate(ID=row_number(),)
gdp <-full_join(gdp_test,usa, by = "ID")
gdp <- gdp %>% mutate(ratio = GDP/gdp_usa)