Generate unique alphanumeric IDs

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I have a data frame and I want to add to it a column that contains not duplicated alphanumeric values.

Firstly, I adapted a function that I found on a blog. (<a href="https://ryouready.wordpress.com/2008/12/18/generate-random-string-name/" rel="nofollow">https://ryouready.wordpress.com/2008/12/18/generate-random-string-name/</a>)

idGenerator <- function(n, lengthId) {

  alphaNum <- c(0:9, letters, LETTERS)

  if (n > length(alphaNum)^lengthId) {
    return("Error! n > perms : Infinite loop")
  }

  idList <- rep(NULL, n)

  for (i in 1:n) {
    idList[i] <- paste(sample(alphaNum, 
                                    lengthId, replace = TRUE), collapse = "")
    while(idList[i] %in% idList[-i]) {
      idList[i] <- paste(sample(alphaNum, 
                                lengthId, replace = TRUE), collapse = "")
    }
  }

  return(idList)
}

My problem is that my dataframe has about 250k rows so with n = 250k this function is just running for ever.
I know that with n = 250k, if I increase the length of the id string (

lengthId

) the odds to get the same string are unrealistic so the

while

loop is such a waste of ressources but I really need to be sure that will not happen, I mean "sure" with control structures.

So I found a more efficient way to do it, instead of calling a

while

and checking all the vector for each

i

in the loop, I check if there is duplicated in the final vector :

idGenerator <- function(n, lengthId) {

  alphaNum <- c(0:9, letters, LETTERS)

  if (n > length(alphaNum)^lengthId) {
    return("Error! n > perms : Infinite loop")
  }

  idList <- 1:n

  for (i in 1:n) {
    idList[i] <- paste(sample(alphaNum, 
                              lengthId, replace = TRUE), collapse = "")
  }

  while(any(duplicated(idList))) {
    idList[which(duplicated(idList))] <- paste(sample(alphaNum, lengthId, 
                                                replace = TRUE), collapse = "")
  }

  return(idList)
}

It's very slow if the

while

must run a lot of times => When n is very close to the number of permutations.

> system.time(idGenerator(62^2, 2))
    utilisateur     système     écoulé 
    8.00            0.00        8.02 

 > system.time(idGenerator(62^3, 3))

 Timing stopped at: 584.35 16.66 602.46

But it's quite acceptable for a long id string :

> system.time(idGenerator(250000, 12))
    utilisateur     système     écoulé 
    3.2             0.0         3.2

However it's still 3sec+ to create a column so I'm looking for a faster way.
I know that the loop isn't so good and I should prefer vectorization but I'm not realy a master of code optimization. So if you have any ideas, thank you in advance.