Using gsub for a specific occurrence in a string in R?

Question

I have two strings:

mystring1 <- c("hello i am a cat.  just kidding, i'm not a cat i'm a cat.  dogs are the best animal.  not cats!")

mystring2 <- c("hello i am a cat.  just kidding, i'm not a cat i'm a cat.  but i have a cat friend that is a cat.")

I want to change the third occurrence of the word cat in both strings to dog.

Ideally, string1 and string2 would read:

mystring1
[1] "hello i am a cat.  just kidding, i'm not a cat i'm a dog.  dogs are the best animal.  not cats!"

mystring2
[1] "hello i am a cat.  just kidding, i'm not a cat i'm a dog.  but i have a cat friend that is a cat."

What is the best way of doing this? Up until now I have only used gsub to replace characters but I don't know if this can be used to replace specific occurrences of a character.

Answer 1

You could use

mystring1 <- c("hello i am a cat.  just kidding, i'm not a cat i'm a cat.  dogs are the best animal.  not cats!")
mystring2 <- c("hello i am a cat.  just kidding, i'm not a cat i'm a cat.  but i have a cat friend that is a cat who knows a cat knowing a cat.")

sub("((cat.*?){2})\\bcat\\b", "\\1dog", mystring1, perl=TRUE)

which gives

> sub("((cat.*?){2})\\bcat\\b", "\\1dog", c(mystring1, mystring2), perl=TRUE)
[1] "hello i am a cat.  just kidding, i'm not a cat i'm a dog.  dogs are the best animal.  not cats!"                                
[2] "hello i am a cat.  just kidding, i'm not a cat i'm a dog.  but i have a cat friend that is a cat who knows a cat knowing a cat."

Answer 2

You can use gsubfn

library(gsubfn)
p <- proto(fun = function(this, x) if(count == 3) 'dog' else x)
gsubfn('cat', p, c(mystring1, mystring2))

# [1] "hello i am a cat.  just kidding, i'm not a cat i'm a dog.  dogs are the best animal.  not cats!"  
# [2] "hello i am a cat.  just kidding, i'm not a cat i'm a dog.  but i have a cat friend that is a cat."

Or, if it needs to be surrounded by word boundaries,

gsubfn('\\bcat\\b', p, c(mystring1, mystring2), perl = TRUE)

# [1] "hello i am a cat.  just kidding, i'm not a cat i'm a dog.  dogs are the best animal.  not cats!"  
# [2] "hello i am a cat.  just kidding, i'm not a cat i'm a dog.  but i have a cat friend that is a cat."