R：使用另一个数据框的映射在一个数据框中创建一个新列

Question

I have the following data frames.

> temp
  x1    x2
1  1 INDIA
2  2 INDIA
3  3    US
4  4    US

> PortfolioIndices
  Country   Index CCY
1   INDIA   CNX50 INR
2      US   SP500 USD
3      UK FTSE100 GBP

I want to add one more column to temp with currencies corresponding to the respective countries in x2 column using the mapping from PortfolioIndices data frame. Something like this should be the output

> temp
  x1    x2  x3
1  1 INDIA INR
2  2 INDIA INR
3  3    US USD
4  4    US USD

I don't want to use for loop as the actual data could be very large and using a for loop in that case would be very inefficient. Is there a better way to achieve the given output?

Thanks in advance.

Answer 1

You can use merge :

> merge(temp, PortfolioIndices, by.x = "x2", by.y = "Country")
     x2 x1 Index CCY
1 INDIA  1 CNX50 INR
2 INDIA  2 CNX50 INR
3    US  3 SP500 USD
4    US  4 SP500 USD

Answer 2

Here is a data.table approach, recomended for big tables.

require(data.table)

# I imagine you have your data in data.frames.

temp <- data.frame(x1 = c(1:4), x2 = c("INDIA", "INDIA", "US", "US"))

PortfolioIndices <- data.frame(Country = c("INDIA", "US", "UK"),
                            Index = c("CNX50", "SP500", "FTSE100"),
                            CCY = c("INR", "USD", "GBP"))                           

# Coerce your data to data.table objects (they are still data.frames) and use the J() 
# function    

temp <- as.data.table(temp)
PortfolioIndices <- as.data.table(PortfolioIndices)
setkey(temp, x2)
setkey(PortfolioIndices, Country)

PortfolioIndices[temp, list(x1,Index,CCY),]

#   Country x1 Index CCY
# 1:   INDIA  1 CNX50 INR
# 2:   INDIA  2 CNX50 INR
# 3:      US  3 SP500 USD
# 4:      US  4 SP500 USD