[Darts] : Add Approaches (#3386)

* Initial docs for darts approaches.

* Initial rough draft of darts approaches.

* Removing .articles for now.

* Wrapped up approaches content details and removed performance info.

* Cleaned up typos and changed toss to throw.

exercises/practice/darts/.approaches/booleans-as-ints/content.md

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+# Using Boolean Values as Integers
+
+
+```python
+def score(x_coord, y_coord):
+    radius = (x_coord**2 + y_coord**2)
+    return (radius<=1)*5 + (radius<=25)*4 + (radius<=100)*1
+```
+
+
+In Python, the [Boolean values `True` and `False` are _subclasses_ of `int`][bools-as-ints] and can be interpreted as `0` (False) and `1` (True) in a mathematical context.
+This approach leverages that interpretation by checking which areas the throw falls into and multiplying each Boolean `int` by a scoring multiple.
+For example, a throw that lands on the 25 (_or 5 if using `math.sqrt(x**2 + y**2)`_) circle should have a score of 5:
+
+```python
+>>> (False)*5 + (True)*4 + (True)*1
+5
+```
+
+
+This makes for very compact code and has the added boost of not requiring any `loops` or additional data structures.
+However, it is considered bad form to rely on Boolean interpretation.
+Instead, the Python documentation recommends an explicit conversion to `int`:
+
+
+```python
+def score(x_coord, y_coord):
+    radius = (x_coord**2 + y_coord**2)
+    return int(radius<=1)*5 + int(radius<=25)*4 + int(radius<=100)*1
+```
+
+Beyond that recommendation, the terseness of this approach might be harder to reason about or decode — especially if a programmer is coming from a programming langauge that does not treat Boolean values as `ints`.
+Despite the "radius" variable name, it is also more difficult to relate the scoring "rings" of the Dartboard to the values being checked and calculated in the `return` statement.
+If using this code in a larger program, it would be strongly recommended that a docstring be provided to explain the Dartboard rings, scoring rules, and the corresponding scores.
+
+[bools-as-ints]: https://docs.python.org/3/library/stdtypes.html#boolean-type-bool

exercises/practice/darts/.approaches/booleans-as-ints/snippet.txt

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+def score(x_coord, y_coord):
+    radius = (x_coord**2 + y_coord**2)
+    return (radius<=1)*5 + (radius<=25)*4 +(radius<=100)*1

exercises/practice/darts/.approaches/config.json

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+{
+  "introduction": {
+    "authors": ["bethanyg"],
+    "contributors": []
+  },
+  "approaches": [
+    {
+      "uuid": "7d78f598-8b4c-4f7f-89e1-e8644e934a4c",
+      "slug": "if-statements",
+      "title": "Use If Statements",
+      "blurb": "Use if-statements to check scoring boundaries for a dart throw.",
+      "authors": ["bethanyg"]
+    },
+    {
+      "uuid": "f8f5533a-09d2-4b7b-9dec-90f268bfc03b",
+      "slug": "tuple-and-loop",
+      "title": "Use a Tuple & Loop through Scores",
+      "blurb": "Score the Dart throw by looping through a tuple of scores.",
+      "authors": ["bethanyg"]
+    },
+    {
+      "uuid": "a324f99e-15bb-43e0-9181-c1652094bc4f",
+      "slug": "match-case",
+      "title": "Use Structural Pattern Matching ('Match-Case')",
+      "blurb": "Use a Match-Case (Structural Pattern Matching) to score the dart throw.)",
+      "authors": ["bethanyg"]
+    },
+    {
+      "uuid": "966bd2dd-c4fd-430b-ad77-3a304dedd82e",
+      "slug": "dict-and-generator",
+      "title": "Use a Dictionary with a Generator Expression",
+      "blurb": "Use a generator expression looping over a scoring dictionary, getting the max score for the dart throw.",
+      "authors": ["bethanyg"]
+    },
+    {
+      "uuid": "5b087f50-31c5-4b84-9116-baafd3a30ed6",
+      "slug": "booleans-as-ints",
+      "title": "Use Boolean Values as Integers",
+      "blurb": "Use True and False as integer values to calculate the score of the dart throw.",
+      "authors": ["bethanyg"]
+    },
+    {
+      "uuid": "0b2dbcd3-f0ac-45f7-af75-3451751fd21f",
+      "slug": "dict-and-dict-get",
+      "title": "Use a Dictionary with dict.get",
+      "blurb": "Loop over a dictionary and retrieve score via dct.get.",
+      "authors": ["bethanyg"]
+    }
+  ]
+}

exercises/practice/darts/.approaches/dict-and-dict-get/content.md

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+# Using a Dictionary and `dict.get()`
+
+
+```python
+def score(x_coord, y_coord):
+    point = (x_coord**2 + y_coord**2)
+    scores = {
+        point <= 100: 1,
+        point <= 25: 5,
+        point <= 1: 10
+    }
+
+    return scores.get(True, 0)
+```
+
+At first glance, this approach looks similar to the [Booleans as Integers][approach-boolean-values-as-integers] approach, due to the Boolean evaluation used in the dictionary keys.
+However, this approach is **not** interpreting Booleans as integers and is instead exploiting three key properties of [dictionaries][dicts]:
+
+
+1.  [Keys must be hashable][hashable-keys] — in other words, keys have to be _unique_.
+2.  Insertion order is preserved (_as of `Python 3.7`_), and evaluation/iteration happens in insertion order.
+3.  Duplicate keys _overwrite_ existing keys.
+    If the first key is `True` and the third key is `True`, the _value_ from the third key will overwrite the value from the first key.
+
+Finally, the `return` line uses [`dict.get()`][dict-get] to `return` a default value of 0 when a throw is outside the existing circle radii.
+To see this in action, you can view this code on [Python Tutor][dict-get-python-tutor].
+
+
+Because of the listed dictionary qualities, **_order matters_**.
+This approach depends on the outermost scoring circle containing all smaller circles and that
+checks proceed from largest --> smallest circle.
+Iterating in the opposite direction will not resolve to the correct score.
+The following code variations do not pass the exercise tests:
+
+
+```python
+
+def score(x_coord, y_coord):
+    point = (x_coord**2 + y_coord**2)
+    scores = {
+        point <= 1: 10,
+        point <= 25: 5,
+        point <= 100: 1,
+    }
+
+    return scores.get(True, 0)
+
+    #OR#
+
+def score(x_coord, y_coord):
+    point = (x_coord**2 + y_coord**2)
+    scores = {
+        point <= 25: 5,
+        point <= 1: 10,
+        point <= 100: 1,
+    }
+
+    return scores.get(True, 0)
+
+```
+
+While this approach is a _very clever_ use of dictionary properties, it is likely to be very hard to reason about for those who are not deeply knowledgeable.
+Even those experienced in Python might take longer than usual to figure out what is happening in the code.
+Extensibility could also be error-prone due to needing a strict order for the `dict` keys.
+
+This approach offers no space or speed advantages over using `if-statements` or other strategies, so is not recommended for use beyond a learning context.
+
+[approach-boolean-values-as-integers]: https://exercism.org/tracks/python/exercises/darts/approaches/boolean-values-as-integers
+[dicts]: https://docs.python.org/3/library/stdtypes.html#mapping-types-dict
+[dict-get]: https://docs.python.org/3/library/stdtypes.html#dict.get
+[dict-get-python-tutor]: https://pythontutor.com/render.html#code=def%20score%28x_coord,%20y_coord%29%3A%0A%20%20%20%20point%20%3D%20%28x_coord**2%20%2B%20y_coord**2%29%0A%20%20%20%20scores%20%3D%20%7B%0A%20%20%20%20%20%20%20%20point%20%3C%3D%20100%3A%201,%0A%20%20%20%20%20%20%20%20point%20%3C%3D%2025%3A%205,%0A%20%20%20%20%20%20%20%20point%20%3C%3D%201%3A%2010%0A%20%20%20%20%7D%0A%20%20%20%20%0A%20%20%20%20return%20scores.get%28True,%200%29%0A%20%20%20%20%0Aprint%28score%281,3%29%29&cumulative=false&curInstr=0&heapPrimitives=nevernest&mode=display&origin=opt-frontend.js&py=311&rawInputLstJSON=%5B%5D&textReferences=false
+[hashable-keys]: https://www.pythonmorsels.com/what-are-hashable-objects/#dictionary-keys-must-be-hashable

exercises/practice/darts/.approaches/dict-and-dict-get/snippet.txt

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+def score(x_coord, y_coord):
+    point = (x_coord**2 + y_coord**2)
+    scores = {point <= 100: 1, point <= 25: 5, point <= 1: 10}
+
+    return scores.get(True, 0)

exercises/practice/darts/.approaches/dict-and-generator/content.md

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+# Use a Dictionary and a Generator Expression
+
+```python
+def score(x_coord, y_coord):
+    throw = x_coord**2 + y_coord**2
+    rules = {1: 10, 25: 5, 100: 1, 200: 0}
+
+    return max(point for distance, point in 
+               rules.items() if throw <= distance)
+```
+
+
+This approach is very similar to the  [tuple and loop][approach-tuple-and-loop] approach, but iterates over [`dict.items()`][dict-items] and writes the `loop` as a [`generator-expression`][generator-expression] inside `max()`.
+In cases where the scoring circles overlap, `max()` will return the maximum score available for the throw.
+The generator expression inside `max()` is the equivalent of using a `for-loop` and a variable to determine the max score:
+
+
+```python
+def score(x_coord, y_coord):
+    throw = x_coord**2 + y_coord**2
+    rules = {1: 10, 25: 5, 100: 1}
+    max_score = 0
+
+    for distance, point in rules.items():
+        if throw <= distance and point > max_score:
+            max_score = point
+    return max_score
+```
+
+
+A `list` or `tuple` can also be used in place of `max()`, but then requires an index to return the max score:
+
+```python
+def score(x_coord, y_coord):
+    throw = x_coord**2 + y_coord**2
+    rules = {1: 10, 25: 5, 100: 1, 200: 0}
+
+    return [point for distance, point in 
+               rules.items() if throw <= distance][0] #<-- have to specify index 0.
+
+#OR#
+
+def score(x_coord, y_coord):
+    throw = x_coord**2 + y_coord**2
+    rules = {1: 10, 25: 5, 100: 1, 200: 0}
+
+    return tuple(point for distance, point in 
+               rules.items() if throw <= distance)[0]
+```
+
+
+This solution can even be reduced to a "one-liner".
+However, this is not performant, and is difficult to read:
+
+```python
+def score(x_coord, y_coord):    
+    return max(point for distance, point in 
+               {1: 10, 25: 5, 100: 1, 200: 0}.items() if 
+               (x_coord**2 + y_coord**2) <= distance)
+```
+
+While all of these variations do pass the tests, they suffer from even more over-engineering/performance caution than the earlier tuple and loop approach (_although for the data in this problem, the performance hit is slight_).
+Additionally, the dictionary will take much more space in memory than using a `tuple` of tuples to hold scoring values.
+In some circumstances, these variations might also be harder to reason about for those not familiar with `generator-expressions` or `list comprehensions`.
+
+
+[approach-tuple-and-loop]:  https://exercism.org/tracks/python/exercises/darts/approaches/tuple-and-loop
+[dict-items]: https://docs.python.org/3/library/stdtypes.html#dict.items
+[generator-expression]: https://dbader.org/blog/python-generator-expressions

exercises/practice/darts/.approaches/dict-and-generator/snippet.txt

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+def score(x_coord, y_coord):
+    length = x_coord**2 + y_coord**2
+    rules = {1.0: 10, 25.0: 5, 100.0: 1, 200: 0}
+    score = max(point for
+                distance, point in
+                rules.items() if length <= distance)
+
+    return score

exercises/practice/darts/.approaches/if-statements/content.md

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+# Use `if-statements`
+
+
+```python
+import math
+
+# Checks scores from the center --> edge.
+def score(x_coord, y_coord):
+    distance = math.sqrt(x_coord**2 + y_coord**2)
+
+    if distance <= 1: return 10
+    if distance <= 5: return  5
+    if distance <= 10: return  1
+
+    return 0
+```
+
+This approach uses [concept:python/conditionals]() to check the boundaries for each scoring ring, returning the corresponding score.
+Calculating the euclidian distance is assigned to the variable "distance" to avoid having to re-calculate it for every if check.
+Because the `if-statements` are simple and readable, they're written on one line to shorten the function body.
+Zero is returned if no other check is true.
+
+
+To avoid importing the `math` module (_for a very very slight speedup_), (x**2 +y**2) can be calculated instead, and the scoring rings can be adjusted to 1, 25, and 100:
+
+
+```python
+# Checks scores from the center --> edge.
+def score(x_coord, y_coord):
+    distance = x_coord**2 + y_coord**2
+
+    if distance <= 1: return 10
+    if distance <= 25: return  5
+    if distance <= 100: return  1
+
+    return 0
+```
+
+
+# Variation 1: Check from Edge to Center Using Upper and Lower Bounds
+
+
+```python
+import math
+
+# Checks scores from the edge --> center
+def score(x_coord, y_coord):
+    distance = math.sqrt(x_coord**2 + y_coord**2)
+
+    if distance > 10: return 0
+    if 5 < distance <= 10: return 1
+    if 1 < distance <= 5: return 5
+
+    return 10
+```
+
+This variant checks from the edge moving inward, checking both a lower and upper bound due to the overlapping scoring circles in this direction.
+
+Scores for any of these solutions can also be assigned to a variable to avoid multiple `returns`, but this isn't really  necessary:
+
+```python
+# Checks scores from the edge --> center
+def score(x_coord, y_coord):
+    distance = x_coord**2 + y_coord**2
+    points = 10
+
+    if distance > 100: points = 0
+    if 25 < distance <= 100: points = 1
+    if 1 < distance <= 25: points = 5
+
+    return points
+```
+

exercises/practice/darts/.approaches/if-statements/snippet.txt

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+import math
+
+def score(x_coord, y_coord):
+    distance = math.sqrt(x_coord**2 + y_coord**2)
+    if distance <= 1: return 10
+    if distance <= 5: return  5
+    if distance <= 10: return  1
+    return 0