Question

Suppose I have a dataset:

test = data.frame(x=c(1:11), y=as.character(c(1:11)))

where the collumn ‘y’ is made up of characters/strings.
Now, I would like to change the strings containing two characters (i.e. test[10,2] & test[11,2]) so that those strings start with a character “0”. The result of this would be “010” & “011”, whereas the other strings (with only one character) remain the same.
To me, the logical solution would be:

test[nchar(test[,2])==2,2] = paste(c("0", test[nchar(test[,2])==2,2]), collapse="")

Indeed, only test[10,2] & test[11,2] are affected. The odd thing is though, that the result is test[10,2] = “01011”, and test[11,2] = “01011”. This means that all strings having two characters are pasted together with a preceding “0”. This definitely is not what I would like to see.

What should I do to add only one character to a string in a dataset when certain conditions (of length) are met?
Your answer would be greatly appreciated.

Answer 1

use

paste("0", test[nchar(test[,2])==2,2], sep="")

so for example

> test[nchar(test[,2])==2,2] = paste("0", test[nchar(test[,2])==2,2], sep="")
> test
    x   y
1   1   1
2   2   2
3   3   3
4   4   4
5   5   5
6   6   6
7   7   7
8   8   8
9   9   9
10 10 010
11 11 011

collapse and sep have different properties

> paste(1,c(2:3),collapse=',')
[1] "1 2,1 3"
> paste(1,c(2:3),sep=',')
[1] "1,2" "1,3"