Nth prime: add new practice exercise

config.json

@@ -485,6 +485,20 @@
           "loops",
           "math"
         ]
+      },
+      {
+        "slug": "nth-prime",
+        "name": "Nth Prime",
+        "uuid": "96b66be6-7e08-4ea6-8684-c21d3ce32900",
+        "practices": [],
+        "prerequisites": [],
+        "difficulty": 3,
+        "status": "active",
+        "topics": [
+          "control_flow_conditionals",
+          "loops",
+          "math"
+         ]
       }
     ]
   },

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 +1,17 @@
+{
+  "authors": ["colinleach"],
+  "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 +1,35 @@
+# Not the prettiest code, but it passes the tests reasonably quickly
+
+prime <- function(n) {
+  if (n < 1) stop("n must be a positive integer")
+
+  known <- vector("integer", n) # to store primes as we find them
+
+  is_prime <- function(candidate) {
+    sqrt_candidate <- sqrt(candidate)
+    for (item in known) {
+      if (item > sqrt_candidate) return(TRUE)
+      if (candidate %% item == 0) return(FALSE)
+      TRUE
+    }
+  }
+
+  # Early primes are added explicitly:
+  known[1] <- 2
+  known[2] <- 3
+
+  # Prime candidates are 6i +/- 1 : 5, 7, 11, 13...
+  # Python does this with `itertools.choice()`. Is there an equivalent in R?
+  known_count <- 2
+  number <- 6
+  repeat {
+    for (candidate in c(number - 1, number + 1)) {
+      if (known_count >= n) return(known[n]) # result!
+      if (is_prime(candidate)) {
+        known_count <- known_count + 1
+        known[known_count] <- candidate
+      }
+    }
+    number <- number + 6
+  }
+}

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 +1,32 @@
+source("./nth-prime.R")
+library(testthat)
+
+context("nth-prime")
+
+test_that("first prime", {
+  n <- 1
+  expect_equal(prime(n), 2)
+})
+
+test_that("second prime", {
+  n <- 2
+  expect_equal(prime(n), 3)
+})
+
+test_that("sixth prime", {
+  n <- 6
+  expect_equal(prime(n), 13)
+})
+
+test_that("big prime", {
+  n <- 10001
+  expect_equal(prime(n), 104743)
+})
+
+test_that("there is no zeroth prime", {
+  n <- 0
+  expect_error(prime(n))
+})
+
+
+message("All tests passed for exercise: nth-prime")