Add concept exercise named-resistor-color (#446)

* add concept exercise named-resistor-color

* add prereqs for resistor-color series

* fix typo

* fix typos

* add general heading to hints.md

Author: depial

config.json

@@ -115,6 +115,18 @@
           "switch"
         ],
         "status": "wip"
+      },
+      {
+        "slug": "named-resistor-color",
+        "name": "Named Resistor Color",
+        "uuid": "f0c4e105-5db7-4e35-bfe1-694e9a1c199c",
+        "concepts": [
+          "names-attribute"
+        ],
+        "prerequisites": [
+          "vector-filtering"
+        ],
+        "status": "wip"
       }
     ],
     "practice": [
@@ -344,16 +356,24 @@
         "slug": "resistor-color",
         "name": "Resistor Color",
         "uuid": "8af70f6c-85e2-41de-b5a3-20aaacb32a26",
-        "practices": [],
-        "prerequisites": [],
+        "practices": [
+          "vector-filtering"
+        ],
+        "prerequisites": [
+          "vector-filtering"
+        ],
         "difficulty": 1
       },
       {
         "slug": "resistor-color-duo",
         "name": "Resistor Color Duo",
         "uuid": "a118a447-f41d-4904-8dfa-15f45ba3a990",
-        "practices": [],
-        "prerequisites": [],
+        "practices": [
+          "vector-filtering"
+        ],
+        "prerequisites": [
+          "vector-filtering"
+        ],
         "difficulty": 1
       },
       {

exercises/concept/named-resistor-color/.docs/hints.md

@@ -0,0 +1,29 @@
+# Hints
+
+## General
+
+Return values for the functions are of a specific type, so pay attention.
+
+## 1. Define a named vector
+
+- The necessary information for the names and values is in the instructions.
+- The way to construct a named vector is covered in detail in the introduction.
+
+## 2. Return the value associated with a band
+
+- You can use your `resistor_bands` named vector to accomplish this easily.
+- You need to return a numeric value alone.
+
+## 3. Return the value associated with two bands
+
+- Your `band_value` function can come in handy here.
+- Indexing hints for named vectors are in the introduction.
+- There are several ways to construct the number, most commonly via string manipulation or numerically.
+- The returned number must be a numeric value alone.
+
+## 4. Return the ohms associated with all three bands
+
+- Both `band_value` and `two_band_value` are useful here.
+- The first two values in the vector are for the two-band value, while the third is the multiplier.
+- Again, string manipulation or numerical means are possible to get the number.
+- The returned number must be a numeric value alone.

exercises/concept/named-resistor-color/.docs/instructions.md

@@ -0,0 +1,85 @@
+# Instructions
+
+If you want to build something using a Raspberry Pi, you'll probably use _resistors_.
+For this exercise, you need to know two things about them:
+
+- Each resistor has a resistance value.
+- Resistors are small - so small in fact that if you printed the resistance value on them, it would be hard to read.
+
+To get around this problem, manufacturers print color-coded bands onto the resistors to denote their resistance values.
+Each band has a position and a numeric value.
+
+The first 2 bands of a resistor have a simple encoding scheme: each color maps to a single number.
+
+The third band is a multiplier.
+
+In this exercise you are going to create a helpful program so that you don't have to remember the values of the bands.
+
+More information on the color encoding of resistors can be found in the [Electronic color code Wikipedia article][e-color-code].
+
+## 1. Define a named vector
+
+The colors are encoded as follows:
+
+- black: 0
+- brown: 1
+- red: 2
+- orange: 3
+- yellow: 4
+- green: 5
+- blue: 6
+- violet: 7
+- grey: 8
+- white: 9
+
+Create the named vector, `resistor_bands`, which should have the colors as the names and the numbers as the values.
+
+## 2. Return the value associated with a band
+
+First, you'll want to be able to find the single value for any colored band.
+
+Define a function `band_value(band)` which takes a `string` of a band color.
+The function returns the associated value for that color.
+
+```R
+band_value("green")
+# => [1] 5
+
+band_value("violet")
+# => [1] 7
+```
+
+## 3. Return the value associated with two bands
+
+The first two bands on a resistor combine to form a single number.
+If the first two bands have respective numbers `m` and `n`, they combine to make the single number `mn`.
+
+Define a function `two_band_value(bands)` which takes a `vector` of `string`s with two band colors.
+The function returns the associated value for the combination of those two colors.
+
+```R
+two_band_value(c("yellow", "red"))
+# => [1] 42
+
+two_band_value(c("green", "violet"))
+# => [1] 57
+```
+
+## 4. Return the ohms associated with all three bands
+
+Finally, you'll need to find the full ohm rating using the first two bands in concert with the third.
+To do that you'll need to find the two-band value and a multiplier.
+The multiplier can be thought of as how many zeros are tacked onto the two-band value.
+
+Define a function `ohms(bands)` which takes a `vector` of `string`s with three band colors.
+The function returns the associated ohm rating for the combination of the three colors.
+
+```R
+ohms(c("yellow", "red", "orange"))
+# => [1] 42000
+
+ohms(c("green", "violet", "yellow"))
+# => [1] 570000
+```
+
+[e-color-code]: https://en.wikipedia.org/wiki/Electronic_color_code

exercises/concept/named-resistor-color/.docs/introduction.md

@@ -0,0 +1,67 @@
+# Introduction
+
+Vectors can have named elements, which sometimes makes them more convenient to work with.
+
+## Creation
+
+There are three ways to add names to a vector.
+
+1) At vector creation time 
+
+```R
+> work_days <- c(Mon = TRUE, Tue = TRUE, Wed = TRUE, Thu = TRUE, Fri = TRUE, Sat = FALSE, Sun = FALSE)
+> work_days
+  Mon   Tue   Wed   Thu   Fri   Sat   Sun 
+ TRUE  TRUE  TRUE  TRUE  TRUE FALSE FALSE 
+```
+
+2) By assigning a character vector to `names()`
+
+```R
+> months <- c(31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31)
+> names(months) <- month.abb
+> months
+Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec 
+ 31  28  31  30  31  30  31  31  30  31  30  31 
+```
+
+3) With `setNames()`
+
+```R
+> months <- c(31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31)
+> setNames(months, month.name)
+  January  February     March     April       May      June      July    August September   October  November  December 
+       31        28        31        30        31        30        31        31        30        31        30        31 
+```
+
+## Working with names
+
+The `names()` function can retrieve names as well as set them.
+
+```R
+> names(months) <- month.abb
+> names(months)[1:3]
+[1] "Jan" "Feb" "Mar"
+```
+
+A name can be used in place of the position index, with quotes required in this case.
+
+```R
+> months[c("Jul", "Aug")]
+Jul Aug 
+ 31  31 
+```
+
+For such indexing to work correctly, it is best to ensure that names are unique and non-missing.
+However, R does not enforce uniqueness.
+
+The usual vector operations still work, and names usually will be preserved if that makes sense.
+
+```R
+> months[months == 30]
+Apr Jun Sep Nov 
+ 30  30  30  30 
+
+> sum(months)
+[1] 365  # no meaningful names possible
+```

exercises/concept/named-resistor-color/.meta/config.json

@@ -0,0 +1,17 @@
+{
+  "authors": [
+    "depial"
+  ],
+  "files": {
+    "solution": [
+      "named-resistor-color.R"
+    ],
+    "test": [
+      "test_named-resistor-color.R"
+    ],
+    "exemplar": [
+      ".meta/exemplar.R"
+    ]
+  },
+  "blurb": "Use named vectors to reason about resistor colors"
+}

exercises/concept/named-resistor-color/.meta/design.md

@@ -0,0 +1,24 @@
+# Design
+
+## Goal
+
+The goal of this exercise is to teach the student the use of `names-attribute` in R.
+
+## Learning objectives
+
+- Know the ways to create a named vector.
+- Know the how to access elements in a named vector via name.
+- Know how to unname an object.
+
+## Out of scope
+
+- Removing named objects from a named vector.
+- Lists, tibbles, dataframes, etc.
+
+## Concepts
+
+- `names-attribute`
+
+## Prerequisites
+
+- `vector-filtering`

exercises/concept/named-resistor-color/.meta/exemplar.R

@@ -0,0 +1,15 @@
+colors <- c("black", "brown", "red", "orange", "yellow", "green", "blue", "violet", "grey", "white")
+resistor_bands <- 0:9
+names(resistor_bands) <- colors
+
+band_value <- function(band) {
+  unname(resistor_bands[band])
+}
+
+two_band_value <- function(bands) {
+  sum(c(10, 1) * resistor_bands[bands])
+}
+
+ohms <- function(bands) {
+  two_band_value(bands[1:2]) * 10^band_value(bands[3])
+}

exercises/concept/named-resistor-color/named-resistor-color.R

@@ -0,0 +1,10 @@
+resistor_bands <- NULL
+
+band_value <- function(band) {
+}
+
+two_band_value <- function(bands) {
+}
+
+ohms <- function(bands) {
+}

exercises/concept/named-resistor-color/test_named-resistor-color.R

@@ -0,0 +1,43 @@
+source("./named-resistor-color.R")
+library(testthat)
+
+# 1. Define a named vector
+
+test_that("1. Define the named vector resistor_values", {
+  expect_length(resistor_bands, 10)
+  expected_names <- c("black", "brown",
+                      "red", "orange",
+                      "yellow", "green",
+                      "blue", "violet",
+                      "grey", "white")
+  expect_named(resistor_bands, expected_names)
+  expect_equal(order(resistor_bands), 1:10)
+})
+
+# 2. The `band_value` function
+
+test_that("2. band_value returns the correct value", {
+  expect_equal(band_value("black"), 0)
+  expect_equal(band_value("red"), 2)
+  expect_equal(band_value("green"), 5)
+  expect_equal(band_value("white"), 9)
+})
+
+# 3. The `two_band_value` function
+
+test_that("3. two_band_value returns the correct value", {
+  expect_equal(two_band_value(c("yellow", "red")), 42)
+  expect_equal(two_band_value(c("orange", "brown")), 31)
+  expect_equal(two_band_value(c("grey", "blue")), 86)
+  expect_equal(two_band_value(c("violet", "green")), 75)
+  expect_equal(two_band_value(c("black", "white")), 9)
+})
+
+# 4. The `ohms` function
+
+test_that("4. ohms returns the correct value", {
+  expect_equal(ohms(c("yellow", "red", "black")), 42)
+  expect_equal(ohms(c("green", "white", "red")), 5900)
+  expect_equal(ohms(c("red", "brown", "orange")), 21000)
+  expect_equal(ohms(c("blue", "orange", "green")), 6300000)
+})