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## to Largest Series Product in the R Track

Published at Jul 13 2018 · 0 comments
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.

## 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

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

## Submitting Incomplete Solutions

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

### test_largest-series-product.R

``````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
}``````

## Community comments

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### What can you learn from this solution?

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