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Published at Jul 13 2018
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Instructions

Test suite

Solution

Given a string of digits, calculate the largest product for a contiguous substring of digits of length n.

For example, for the input `'1027839564'`

, the largest product for a
series of 3 digits is 270 (9 * 5 * 6), and the largest product for a
series of 5 digits is 7560 (7 * 8 * 3 * 9 * 5).

Note that these series are only required to occupy *adjacent positions*
in the input; the digits need not be *numerically consecutive*.

For the input `'73167176531330624919225119674426574742355349194934'`

,
the largest product for a series of 6 digits is 23520.

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

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.

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`

.

A variation on Problem 8 at Project Euler http://projecteuler.net/problem=8

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

```
source("./largest-series-product.R")
library(testthat)
context("largest series product")
test_that("finds the largest product if span equals length", {
digits <- "29"
span <- 2
expect_equal(largest_series_product(digits, span), 18)
})
test_that("can find the largest product of 2 with numbers in order", {
digits <- "0123456789"
span <- 2
expect_equal(largest_series_product(digits, span), 72)
})
test_that("can find the largest product of 2", {
digits <- "576802143"
span <- 2
expect_equal(largest_series_product(digits, span), 48)
})
test_that("can find the largest product of 3 with numbers in order", {
digits <- "0123456789"
span <- 3
expect_equal(largest_series_product(digits, span), 504)
})
test_that("can find the largest product of 3", {
digits <- "1027839564"
span <- 3
expect_equal(largest_series_product(digits, span), 270)
})
test_that("can find the largest product of 5 with numbers in order", {
digits <- "0123456789"
span <- 5
expect_equal(largest_series_product(digits, span), 15120)
})
test_that("can get the largest product of a big number", {
digits <- "73167176531330624919225119674426574742355349194934"
span <- 6
expect_equal(largest_series_product(digits, span), 23520)
})
test_that("reports zero if the only digits are zero", {
digits <- "0000"
span <- 2
expect_equal(largest_series_product(digits, span), 0)
})
test_that("reports zero if all spans include zero", {
digits <- "99099"
span <- 3
expect_equal(largest_series_product(digits, span), 0)
})
test_that("rejects span longer than string length", {
digits <- "123"
span <- 4
expect_error(largest_series_product(digits, span))
})
# There may be some confusion about whether this should be 1 or error.
# The reasoning for it being 1 is this:
# There is one 0-character string contained in the empty string.
# That's the empty string itself.
# The empty product is 1 (the identity for multiplication).
# Therefore LSP("", 0) is 1.
# It's NOT the case that LSP("", 0) takes max of an empty list.
# So there is no error.
# Compare against LSP("123", 4):
# There are zero 4-character strings in "123".
# So LSP("123", 4) really DOES take the max of an empty list.
# So LSP("123", 4) errors and LSP("", 0) does NOT.
test_that("reports 1 for empty string and empty product (0 span)", {
digits <- ""
span <- 0
expect_equal(largest_series_product(digits, span), 1)
})
# As above, there is one 0-character string in "123".
# So again no error. It's the empty product, 1.
test_that("reports 1 for nonempty string and empty product (0 span)", {
digits <- "123"
span <- 0
expect_equal(largest_series_product(digits, span), 1)
})
test_that("rejects empty string and nonzero span", {
digits <- ""
span <- 1
expect_error(largest_series_product(digits, span))
})
test_that("rejects invalid character in digits", {
digits <- "1234a5"
span <- 2
expect_error(largest_series_product(digits, span))
})
test_that("rejects negative span", {
digits <- "12345"
span <- -1
expect_error(largest_series_product(digits, span))
})
message("All tests passed for exercise: largest-series-product")
```

```
library(magrittr)
largest_series_product <- function(digits, span){
# catch edge cases & invalid input
if (nchar(digits) < span)
stop("Span must not be longer then the digit string!")
else if (digits == "" | span == 0)
return(1)
else if (stringi::stri_detect_regex(digits, "[^0-9]"))
stop("There are non-digits in the digit string!")
# turn digit string into integer vector
list <- digits %>%
strsplit("") %>%
unlist %>%
as.numeric
# calculate number of steps
steps <- length(list) - span + 1
products <- c()
# walk along vector
for (s in 1:steps) {
products <- c(products,
prod(list[s:(s + span - 1)]))
}
max(products)
# [ ] speed up by ignoring steps with 0
}
```

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?
- Are there new concepts here that you could read more about to improve your understanding?

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