regular expression match numbers in R

Question

I want to compare Postal Codes encoded by 4 numbers which are factor labels in R. The problem is that in 1 format the fourth digit of these codes always is 0 and in another it can vary, so I have 1010 but also 1012, 1015, but they all mean the same. Therefore, I want to read the full postal codes and replace them by the 'standard code' with 0 as fourth digit.

v1 <- c(,"1011", "1015", "1017","1207","1209")
v2 <- c("1010","1200")

How can I read all factor levels as in the vector v1 above and systematically replace the last digit to look like vector v2 using regular expressions?

Answer 1

You will get the first three elements of a vector using:

    Three.str <- substr(v1, 1,3)

So, you can change all the factors starting with "101" using:

    v1[Three.str=="101"] <- "1010"

and so on with the corresponding codification you want to use.