repeating vector of letters

Question

Is there a function to create a repeating list of letters in R?

something like

letters[1:30]
[1] \"a\" \"b\" \"c\" \"d\" \"e\" \"f\" \"g\" \"h\" \"i         


        

Answer 1

Working solution

A function to produce Excel-style column names, i.e.

# A, B, ..., Z, AA, AB, ..., AZ, BA, BB, ..., ..., ZZ, AAA, ...

letterwrap <- function(n, depth = 1) {
    args <- lapply(1:depth, FUN = function(x) return(LETTERS))
    x <- do.call(expand.grid, args = list(args, stringsAsFactors = F))
    x <- x[, rev(names(x)), drop = F]
    x <- do.call(paste0, x)
    if (n <= length(x)) return(x[1:n])
    return(c(x, letterwrap(n - length(x), depth = depth + 1)))
}

letterwrap(26^2 + 52) # through AAZ

---

Botched attempt

Initially I thought this would best be done cleverly by converting to base 26, but that doesn't work. The issue is that Excel column names aren't base 26, which took me a long time to realize. The catch is 0: if you try to map a letter (like A) to 0, you've got a problem when you want to distinguish between A and AA and AAA...

Another way to illustrate the problem is in "digits". In base 10, there are 10 single-digit numbers (0-9), then 90 double-digit numbers (10:99), 900 three-digit numbers... generalizing to 10^d - 10^(d - 1) numbers with d digits for d > 1. However, in Excel column names there are 26 single-letter names, 26^2 double-letter names, 26^3 triple-letter names, with no subtraction.

I'll leave this code as a warning to others:

## Converts a number to base 26, returns a vector for each "digit"
b26 <- function(n) {
    stopifnot(n >= 0)
    if (n <= 1) return(n)
    n26 <- rep(NA, ceiling(log(n, base = 26)))
    for (i in seq_along(n26)) {
        n26[i] <- (n %% 26)
        n <- n %/% 26
    }
    return(rev(n26))
}

## Returns the name of nth value in the sequence
## A, B, C, ..., Z, AA, AB, AC, ..., AZ, BA, ...
letterwrap1 <- function(n, lower = FALSE) {
    let <- if (lower) letters else LETTERS
    base26 <- b26(n)
    base26[base26 == 0] <- 26
    paste(let[base26], collapse = "")
}

## Vectorized version of letterwrap
letter_col_names <- Vectorize(letterwrap, vectorize.args="n")

> letter_col_names(1:4)
[1] "A" "B" "C" "D"

> letter_col_names(25:30)
[1] "Y"  "Z"  "AA" "AB" "AC" "AD"

# Looks pretty good
# Until we get here:
> letter_col_names(50:54)
[1] "AX" "AY" "BZ" "BA" "BB"