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katrinleinweber's solution

to Prime Factors in the R Track

Published at Jul 13 2018 · 3 comments
Instructions
Test suite
Solution

Compute the prime factors of a given natural number.

A prime number is only evenly divisible by itself and 1.

Note that 1 is not a prime number.

Example

What are the prime factors of 60?

  • Our first divisor is 2. 2 goes into 60, leaving 30.
  • 2 goes into 30, leaving 15.
    • 2 doesn't go cleanly into 15. So let's move on to our next divisor, 3.
  • 3 goes cleanly into 15, leaving 5.
    • 3 does not go cleanly into 5. The next possible factor is 4.
    • 4 does not go cleanly into 5. The next possible factor is 5.
  • 5 does go cleanly into 5.
  • We're left only with 1, so now, we're done.

Our successful divisors in that computation represent the list of prime factors of 60: 2, 2, 3, and 5.

You can check this yourself:

  • 2 * 2 * 3 * 5
  • = 4 * 15
  • = 60
  • Success!

Installation

See this guide for instructions on how to setup your local R environment.

How to implement your solution

In each problem folder, there is a file named <exercise_name>.R containing a function that returns a NULL value. Place your implementation inside the body of the function.

How to run tests

Inside of RStudio, simply execute the test_<exercise_name>.R script. This can be conveniently done with testthat's auto_test function. Because exercism code and tests are in the same folder, use this same path for both code_path and test_path parameters. On the command-line, you can also run Rscript test_<exercise_name>.R.

Source

The Prime Factors Kata by Uncle Bob http://butunclebob.com/ArticleS.UncleBob.ThePrimeFactorsKata

Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.

test_prime-factors.R

source("./prime-factors.R")
suppressPackageStartupMessages({
  library(testthat)
})

context("prime factors")

test_that("no factors", {
  number <- 1
  expect_equal(prime_factors(number),
               c())
})

test_that("prime number", {
  number <- 2
  expect_equal(prime_factors(number),
               c(2))
})

test_that("square of a prime", {
  number <- 9
  expect_equal(prime_factors(number),
               c(3, 3))
})

test_that("cube of a prime", {
  number <- 8
  expect_equal(prime_factors(number),
               c(2, 2, 2))
})

test_that("product of primes and non-primes", {
  number <- 12
  expect_equal(prime_factors(number),
               c(2, 2, 3))
})

test_that("product of primes", {
  number <- 901255
  expect_equal(prime_factors(number),
               c(5, 17, 23, 461))
})
test_that("factors include a large prime", {
  number <- 93819012551
  expect_equal(prime_factors(number),
               c(11, 9539, 894119))
})

message("All tests passed for exercise: prime-factors")
prime_factors <- function(number) {

  # set start conditions
  factors <- c()
  f <- 2
  
  # loop through checking factors for even division and append
  # [ ] recursion possible as in collatz.R?
  while (number > 1) {
    if (number %% f == 0) {
      factors <- c(factors, f)
      number <- number / f
    } else {
      f <- f + 1
    }
  }
  
  factors
}

Community comments

Find this solution interesting? Ask the author a question to learn more.
Avatar of katrinleinweber

I wonder whether recursion is at all possible here, instead of a loop? I got it working for Collatz Conjecture scripts in R, Python and C, but not here...

Avatar of jejones3141

Recursion is possible in R... but it doesn't do tail call optimization. What happened when you tried it?

Avatar of katrinleinweber

Don't know anymore, sorry. Some errors that I found easier to circumvent by another approach in this case, than trying to apply what worked for Collatz :-(

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