R: How to add a zero into the middle of a string

Question

I have a dataset with locations defined by alphanumeric codes like this:

locations<-c('25N35W1', '25N36W1', '25N6W1')
presence<-c(0, 1, 0)

df<-cbind.data.frame(locations, presence)

Most of the codes have 7 digits, like the first two above, but a few of them only have six digits. I would like to add a zero after the third digit to all codes that have six digits, while leaving the seven-digit codes as they are. Does anybody know how to do this? Thanks!

Answer 1

With paste0 and substr

ind = which(nchar(locations) == 6)
locations[ind] = paste0(substr(locations[ind], 1, 3), "0", substr(locations[ind], 4, 6))

Answer 2

We can use the following regular expression to insert the 0. This has an additional advantage that it checks that the rest of your location code is in the right format:

df$locations <- sub('(?i)(\\d{2}[a-z])(\\d[a-z]\\d)', '\\10\\2', df$locations)

Output:

> df
  locations presence
1   25N35W1        0
2   25N36W1        1
3   25N06W1        0