subset dataframe variables through part of names - r

Question

Suppose I have a data frame that contains these series and something else.
Where Ru and Uk are country codes.
Date CPI.Ru CPI.g.Ru CPI.s.Ru CPI.Uk CPI.g.Uk CPI.s.Uk
Q4-1990 61.4 66.4 67.5 72.2 68.2 32.4
Q1-1991 61.3 67.0 68.0 72.6 68.8 33.2
Q2-1991 61.4 67.5 68.1 73.2 69.5 35.1
Q3-1991 61.7 68.7 68.9 73.7 70.6 35.9
Q4-1991 62.3 68.4 69.3 74.3 71.9 38.2
Q1-1992 62.3 69.7 69.6 74.7 72.9 39.2
Q2-1992 62.1 70.3 70.0 75.3 73.7 40.6
Q3-1992 62.2 71.4 70.5 75.3 74.1 41.2
Q4-1992 62.5 71.1 70.9 75.7 74.3 44.0
I want to subset dataframe by country and then do something with this series.
For example I want to divide CPI index for each country by its first element.
How can I do it in cycle or maybe with apply function?
countries <- c("Ru","Uk")
for (i in countries)
{dataFrameName$CPI.{i} <- dfName$CPI.{i}/dfName$CPI.{i}[1]}
What should I write instead of {i}?

Answer 1

$ only accept fixed column names. To select columns based on an expression you can instead use double brackets:
countries <- c("Ru", "Uk")
for (i in countries){
x <- paste0("CPI.", i)
dfName[[x]] <- dfName[[x]]/dfName[[x]][1]
}

Answer 2

This is not a loop, but if your data is always of the same form for each country, so that each country has 3 columns, and you always want to operate on the first column per country, you could try this:
sub <- df[,seq(2,ncol(df), 3)] #create a subsetted data.frame containing the CPI index per country
apply(sub, 2, function(x) x/x[1]) #then use apply to operate on each column
# CPI.Ru CPI.Uk
# [1,] 1.0000000 1.000000
# [2,] 0.9983713 1.005540
# [3,] 1.0000000 1.013850
# [4,] 1.0048860 1.020776
# [5,] 1.0146580 1.029086
# [6,] 1.0146580 1.034626
# [7,] 1.0114007 1.042936
# [8,] 1.0130293 1.042936
# [9,] 1.0179153 1.048476