Question

My normal search foo is failing me. I’m trying to find an R function that returns ALL of the factors of an integer. There are at least 2 packages with factorize() functions: gmp and conf.design, however these functions return only prime factors. I’d like a function that returns all factors.

Obviously searching for this is made difficult since R has a construct called factors which puts a lot of noise in the search.

Answer 1

To follow up on my comment (thanks to @Ramnath for my typo), the brute force method seems to work reasonably well here on my 64 bit 8 gig machine:

FUN <- function(x) {
    x <- as.integer(x)
    div <- seq_len(abs(x))
    factors <- div[x %% div == 0L]
    factors <- list(neg = -factors, pos = factors)
    return(factors)
}

A few examples:

> FUN(100)
$neg
[1]   -1   -2   -4   -5  -10  -20  -25  -50 -100

$pos
[1]   1   2   4   5  10  20  25  50 100

> FUN(-42)
$neg
[1]  -1  -2  -3  -6  -7 -14 -21 -42

$pos
[1]  1  2  3  6  7 14 21 42

#and big number

> system.time(FUN(1e8))
   user  system elapsed 
   1.95    0.18    2.14