Add strain exercise (#366)

config.json

@@ -777,6 +777,14 @@
         "prerequisites": [],
         "difficulty": 3
       },
+      {
+        "slug": "strain",
+        "name": "Strain",
+        "uuid": "869fcbef-a5b3-4d79-bc30-e42eeb2fd319",
+        "practices": [],
+        "prerequisites": [],
+        "difficulty": 1
+      },
       {
         "slug": "proverb",
         "name": "Proverb",

exercises/practice/strain/.docs/instructions.md

@@ -0,0 +1,29 @@
+# Instructions
+
+Implement the `keep` and `discard` operation on collections.
+Given a collection and a predicate on the collection's elements, `keep` returns a new collection containing those elements where the predicate is true, while `discard` returns a new collection containing those elements where the predicate is false.
+
+For example, given the collection of numbers:
+
+- 1, 2, 3, 4, 5
+
+And the predicate:
+
+- is the number even?
+
+Then your keep operation should produce:
+
+- 2, 4
+
+While your discard operation should produce:
+
+- 1, 3, 5
+
+Note that the union of keep and discard is all the elements.
+
+The functions may be called `keep` and `discard`, or they may need different names in order to not clash with existing functions or concepts in your language.
+
+## Restrictions
+
+Keep your hands off that filter/reject/whatchamacallit functionality provided by your standard library!
+Solve this one yourself using other basic tools instead.

exercises/practice/strain/.meta/config.json

@@ -0,0 +1,19 @@
+{
+  "authors": [
+    "erikschierboom"
+  ],
+  "files": {
+    "solution": [
+      "strain.R"
+    ],
+    "test": [
+      "test_strain.R"
+    ],
+    "example": [
+      ".meta/example.R"
+    ]
+  },
+  "blurb": "Implement the `keep` and `discard` operation on collections.",
+  "source": "Conversation with James Edward Gray II",
+  "source_url": "http://graysoftinc.com/"
+}

exercises/practice/strain/.meta/example.R

@@ -0,0 +1,17 @@
+keep <- function(input, fun) {
+  if (is.null(input)) return(c())
+
+  kept <- vector(mode = mode(input))
+
+  for (element in input) {
+    if (fun(element)) {
+      kept[[length(kept) + 1]] <- element
+    }
+  }
+
+  kept
+}
+
+discard <- function(input, fun) {
+  keep(input, Negate(fun))
+}

exercises/practice/strain/.meta/tests.toml

@@ -0,0 +1,52 @@
+# 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.
+
+[26af8c32-ba6a-4eb3-aa0a-ebd8f136e003]
+description = "keep on empty list returns empty list"
+
+[f535cb4d-e99b-472a-bd52-9fa0ffccf454]
+description = "keeps everything"
+
+[950b8e8e-f628-42a8-85e2-9b30f09cde38]
+description = "keeps nothing"
+
+[92694259-6e76-470c-af87-156bdf75018a]
+description = "keeps first and last"
+
+[938f7867-bfc7-449e-a21b-7b00cbb56994]
+description = "keeps neither first nor last"
+
+[8908e351-4437-4d2b-a0f7-770811e48816]
+description = "keeps strings"
+
+[2728036b-102a-4f1e-a3ef-eac6160d876a]
+description = "keeps lists"
+
+[ef16beb9-8d84-451a-996a-14e80607fce6]
+description = "discard on empty list returns empty list"
+
+[2f42f9bc-8e06-4afe-a222-051b5d8cd12a]
+description = "discards everything"
+
+[ca990fdd-08c2-4f95-aa50-e0f5e1d6802b]
+description = "discards nothing"
+
+[71595dae-d283-48ca-a52b-45fa96819d2f]
+description = "discards first and last"
+
+[ae141f79-f86d-4567-b407-919eaca0f3dd]
+description = "discards neither first nor last"
+
+[daf25b36-a59f-4f29-bcfe-302eb4e43609]
+description = "discards strings"
+
+[a38d03f9-95ad-4459-80d1-48e937e4acaf]
+description = "discards lists"

exercises/practice/strain/strain.R

@@ -0,0 +1,7 @@
+keep <- function(input, fun) {
+
+}
+
+discard <- function(input, fun) {
+
+}

exercises/practice/strain/test_strain.R

@@ -0,0 +1,90 @@
+source("./strain.R")
+library(testthat)
+
+test_that("Empty keep", {
+  input <- c()
+  expected <- c()
+  expect_equal(keep(input, \(x) x < 10), expected)
+})
+
+test_that("Keep everything", {
+  input <- c(1, 2, 3)
+  expected <- c(1, 2, 3)
+  expect_equal(keep(input, \(x) x < 10), expected)
+})
+
+test_that("Keep first and last", {
+  input <- c(1, 2, 3)
+  expected <- c(1, 3)
+  expect_equal(keep(input, \(x) x %% 2 != 0), expected)
+})
+
+test_that("Keep neither first nor last", {
+  input <- c(1, 2, 3, 4, 5)
+  expected <- c(2, 4)
+  expect_equal(keep(input, \(x) x %% 2 == 0), expected)
+})
+
+test_that("Keep strings", {
+  input <- c("apple", "zebra", "banana", "zombies", "cherimoya", "zelot")
+  expected <- c("zebra", "zombies", "zelot")
+  expect_equal(keep(input, \(x) substr(x, 1, 1) == "z"), expected)
+})
+
+test_that("Keep list of vectors", {
+  input <- list(
+    c(1, 2, 3),
+    c(5, 5, 5),
+    c(5, 1, 2),
+    c(2, 1, 2),
+    c(1, 5, 2),
+    c(2, 2, 1),
+    c(1, 2, 5)
+  )
+  expected <- list(c(5, 5, 5), c(5, 1, 2), c(1, 5, 2), c(1, 2, 5))
+  expect_equal(keep(input, \(x) 5 %in% x), expected)
+})
+
+test_that("Empty discard", {
+  input <- c()
+  expected <- c()
+  expect_equal(discard(input, \(x) x < 10), expected)
+})
+
+test_that("Discard nothing", {
+  input <- c(1, 2, 3)
+  expected <- c(1, 2, 3)
+  expect_equal(discard(input, \(x) x > 10), expected)
+})
+
+test_that("Discard first and last", {
+  input <- c(1, 2, 3)
+  expected <- c(2)
+  expect_equal(discard(input, \(x) x %% 2 != 0), expected)
+})
+
+test_that("Discard neither first nor last", {
+  input <- c(1, 2, 3, 4, 5)
+  expected <- c(1, 3, 5)
+  expect_equal(discard(input, \(x) x %% 2 == 0), expected)
+})
+
+test_that("Discard strings", {
+  input <- c("apple", "zebra", "banana", "zombies", "cherimoya", "zelot")
+  expected <- c("apple", "banana", "cherimoya")
+  expect_equal(discard(input, \(x) substr(x, 1, 1) == "z"), expected)
+})
+
+test_that("Discard list of vectors", {
+  input <- list(
+    c(1, 2, 3),
+    c(5, 5, 5),
+    c(5, 1, 2),
+    c(2, 1, 2),
+    c(1, 5, 2),
+    c(2, 2, 1),
+    c(1, 2, 5)
+  )
+  expected <- list(c(1, 2, 3), c(2, 1, 2), c(2, 2, 1))
+  expect_equal(discard(input, \(x) 5 %in% x), expected)
+})