Program for Armstrong number in r

Question

A Number is called Armstrong Number if the sum of the cube of each digit of that number is equals to the Number itself.

Example:

• 153 = 1 + 5^

Answer 1

(a <- rowSums(matrix(as.numeric(unlist(strsplit(as.character(100:999),''))),nrow=900,byrow=TRUE)^3))[a==100:999]
[1] 153 370 371 407

Answer 2

A variation on a theme...

I have in my R snippets a function from Greg Snow. I'll see if I can dig up a link later. Here's the original answer to a somewhat similar question. It's called "digits" and serves to split up a number into digits without using strsplit.

digits <- function(x) {
  if(length(x) > 1 ) {
    lapply(x, digits)
  } else {
    n <- nchar(x)
    rev( x %/% 10^seq(0, length.out=n) %% 10 )
  }
}

Using that function, we can do something like:

A <- 100:999
A[sapply(digits(A), function(y) sum(y^3)) == A]
# [1] 153 370 371 407

It is, unfortunately, the slowest of the three functions here :(

Answer 3

A quick and dirty solution using ?strsplit:

armstrong <- function(x) {
  tmp <- strsplit(as.character(x), split="")  
  cubic <- sapply(tmp, function(y)sum(as.numeric(y)^3))
  return(cubic == x)
}

E.g.:

armstrong(c(153, 142))
# [1] TRUE FALSE

# find all 3 digit numbers:
s <- 100:999
s[armstrong(s)]
# [1] 153 370 371 407
# @CarlWitthoft: wikipedia was right ;)