Add nth-prime exercise (#378)

config.json

@@ -848,6 +848,14 @@
         "practices": [],
         "prerequisites": [],
         "difficulty": 4
+      },
+      {
+        "slug": "nth-prime",
+        "name": "Nth Prime",
+        "uuid": "e61c707d-eb0d-4c37-8c0f-014db4504d3d",
+        "practices": [],
+        "prerequisites": [],
+        "difficulty": 5
       }
     ]
   },

exercises/practice/nth-prime/.docs/instructions.md

@@ -0,0 +1,7 @@
+# Instructions
+
+Given a number n, determine what the nth prime is.
+
+By listing the first six prime numbers: 2, 3, 5, 7, 11, and 13, we can see that the 6th prime is 13.
+
+If your language provides methods in the standard library to deal with prime numbers, pretend they don't exist and implement them yourself.

exercises/practice/nth-prime/.meta/config.json

@@ -0,0 +1,19 @@
+{
+  "authors": [
+    "erikschierboom"
+  ],
+  "files": {
+    "solution": [
+      "nth-prime.R"
+    ],
+    "test": [
+      "test_nth-prime.R"
+    ],
+    "example": [
+      ".meta/example.R"
+    ]
+  },
+  "blurb": "Given a number n, determine what the nth prime is.",
+  "source": "A variation on Problem 7 at Project Euler",
+  "source_url": "https://projecteuler.net/problem=7"
+}

exercises/practice/nth-prime/.meta/example.R

@@ -0,0 +1,23 @@
+prime <- function(n) {
+  stopifnot(n > 0)
+
+  primes <- c(2, 3)
+  current <- 6
+
+  while (length(primes) < n) {
+    limit <- sqrt(current + 1)
+    prime_check <- primes[primes <= limit]
+
+    if (all((current - 1) %% prime_check != 0)) {
+      primes <- c(primes, current - 1)
+    }
+
+    if (all((current + 1) %% prime_check != 0)) {
+      primes <- c(primes, current + 1)
+    }
+
+    current <- current + 6
+  }
+
+  primes[n]
+}

exercises/practice/nth-prime/.meta/tests.toml

@@ -0,0 +1,25 @@
+# This is an auto-generated file.
+#
+# Regenerating this file via `configlet sync` will:
+# - Recreate every `description` key/value pair
+# - Recreate every `reimplements` key/value pair, where they exist in problem-specifications
+# - Remove any `include = true` key/value pair (an omitted `include` key implies inclusion)
+# - Preserve any other key/value pair
+#
+# As user-added comments (using the # character) will be removed when this file
+# is regenerated, comments can be added via a `comment` key.
+
+[75c65189-8aef-471a-81de-0a90c728160c]
+description = "first prime"
+
+[2c38804c-295f-4701-b728-56dea34fd1a0]
+description = "second prime"
+
+[56692534-781e-4e8c-b1f9-3e82c1640259]
+description = "sixth prime"
+
+[fce1e979-0edb-412d-93aa-2c744e8f50ff]
+description = "big prime"
+
+[bd0a9eae-6df7-485b-a144-80e13c7d55b2]
+description = "there is no zeroth prime"

exercises/practice/nth-prime/nth-prime.R

@@ -0,0 +1,3 @@
+prime <- function(n) {
+
+}

exercises/practice/nth-prime/test_nth-prime.R

@@ -0,0 +1,22 @@
+source("./nth-prime.R")
+library(testthat)
+
+test_that("First prime", {
+  expect_equal(prime(1), 2)
+})
+
+test_that("Second prime", {
+  expect_equal(prime(2), 3)
+})
+
+test_that("Sixth prime", {
+  expect_equal(prime(6), 13)
+})
+
+test_that("Big prime", {
+  expect_equal(prime(10001), 104743)
+})
+
+test_that("There is no zeroth prime", {
+  expect_error(prime(0))
+})