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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 <- "055Â£ 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)
}

## Community comments

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### What can you learn from this solution?

A huge amount can be learned from reading other peopleâ€™s code. This is why we wanted to give exercism users the option of making their solutions public.

Here are some questions to help you reflect on this solution and learn the most from it.

• What compromises have been made?
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