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katrinleinweber's solution

to Luhn in the R Track

Published at Jul 13 2018 · 0 comments
Instructions
Test suite
Solution

Given a number determine whether or not it is valid per the Luhn formula.

The Luhn algorithm is a simple checksum formula used to validate a variety of identification numbers, such as credit card numbers and Canadian Social Insurance Numbers.

The task is to check if a given string is valid.

Validating a Number

Strings of length 1 or less are not valid. Spaces are allowed in the input, but they should be stripped before checking. All other non-digit characters are disallowed.

Example 1: valid credit card number

4539 1488 0343 6467

The first step of the Luhn algorithm is to double every second digit, starting from the right. We will be doubling

4_3_ 1_8_ 0_4_ 6_6_

If doubling the number results in a number greater than 9 then subtract 9 from the product. The results of our doubling:

8569 2478 0383 3437

Then sum all of the digits:

8+5+6+9+2+4+7+8+0+3+8+3+3+4+3+7 = 80

If the sum is evenly divisible by 10, then the number is valid. This number is valid!

Example 2: invalid credit card number

8273 1232 7352 0569

Double the second digits, starting from the right

7253 2262 5312 0539

Sum the digits

7+2+5+3+2+2+6+2+5+3+1+2+0+5+3+9 = 57

57 is not evenly divisible by 10, so this number is not valid.

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 Luhn Algorithm on Wikipedia http://en.wikipedia.org/wiki/Luhn_algorithm

Submitting Incomplete Solutions

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

test_luhn.R

source("./luhn.R")
library(testthat)

context("luhn")

test_that("single digit strings can not be valid", {
  input <- "1"
  expect_equal(is_valid(input), FALSE)
})

test_that("A single zero is invalid", {
  input <- "0"
  expect_equal(is_valid(input), FALSE)
})

test_that("a simple valid SIN that remains valid if reversed", {
  input <- "059"
  expect_equal(is_valid(input), TRUE)
})

test_that("a simple valid SIN that becomes invalid if reversed", {
  input <- "59"
  expect_equal(is_valid(input), TRUE)
})

test_that("valid Canadian SIN", {
  input <- "046 454 286"
  expect_equal(is_valid(input), TRUE)
})

test_that("invalid Canadian SIN", {
  input <- "046 454 287"
  expect_equal(is_valid(input), FALSE)
})

test_that("invalid credit card", {
  input <- "8273 1232 7352 0569"
  expect_equal(is_valid(input), FALSE)
})

test_that("valid strings with a non-digit added become invalid", {
  input <- "055a 444 285"
  expect_equal(is_valid(input), FALSE)
})

test_that("punctuation is not allowed", {
  input <- "055-444-285"
  expect_equal(is_valid(input), FALSE)
})

test_that("symbols are not allowed", {
  input <- "055£ 444$ 285"
  expect_equal(is_valid(input), FALSE)
})

test_that("single zero with space is invalid", {
  input <- " 0"
  expect_equal(is_valid(input), FALSE)
})

test_that("more than a single zero is valid", {
  input <- "0000 0"
  expect_equal(is_valid(input), TRUE)
})

test_that("another valid sin", {
  input <- "055 444 285"
  expect_equal(is_valid(input), TRUE)
})

test_that("nine doubled is nine", {
  input <- "091"
  expect_equal(is_valid(input), TRUE)
})

message("All tests passed for exercise: luhn")
library(magrittr)

is_valid <- function(input) {
  
  # convert to digits
  digits <- input %>%
    gsub(" ", "", x = .) %>%
    strsplit("") %>%
    unlist
  
  # reject invalid input (too small or non-digit characters)
  if (length(digits) <= 1
      | TRUE %in% stringi::stri_detect_regex(digits, "\\D"))
    return(FALSE)
  
  # prepare for Luhn's algorithm
  stigid <- digits %>%
    as.integer %>%
    rev
  
  # conduct Luhn's algorithm
  # [ ] more elegant
  for (d in seq(2, length(stigid), 2)) {
    stigid[d] <- 2 * stigid[d]
    if (stigid[d] > 9)
      stigid[d] <- stigid[d] - 9
  }
  
  # conforms to Luhn, if divisible by 10
  if (stigid %>% sum %% 10 == 0)
    return(TRUE)
  else
    return(FALSE)
}

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