Add "0" to the front of numbers to make n-digit string

prepend.zero.to.numbers  <- function(x, n = NULL, pad.char = "0")
{ 
    ## Purpose: Add "0" to the front of numbers to make n-digit string. 
    ## Arguments:
    ##   x: a vector of integers/characters
    ##   n: Number of total digits after padding with zero. Default to
    ##      using the largest number to calculate "n".
    ##   pad.char: what character to pad.  Default to "0". 
    ## Return: a character vector with nuumbers padded with zero's in the head.
    ## Author: Feiming Chen, Date: 19 Jun 2017, 13:19
    ## ________________________________________________

    x1 <- as.character(x)
    y <- nchar(x1)
    if (is.null(n)) n <- nchar(as.character(max(x)))
    y1 <- n - y                         # how long is the padding for each number
    pad.vector <- sapply(y1, function(w) paste(rep(pad.char, w), collapse = ""))
    paste0(pad.vector, x1)
}
if (F) {                                # Unit Test
    x <- c(2, 14, 156, 1892)
    prepend.zero.to.numbers(x)          # "0002" "0014" "0156" "1892"
    prepend.zero.to.numbers(x, pad.char = "X") # "XXX2" "XX14" "X156" "1892"
    prepend.zero.to.numbers(x, n = 6)          # "000002" "000014" "000156" "001892"
    y <- c("A", "BC", "DEF")
    prepend.zero.to.numbers(y, 3) # "00A" "0BC" "DEF"
}