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[Wordy & Wordy Approaches]: Added 6 Additional Approaches & Modified …
…the Instruction Append for Wordy. (#3783) * Added 6 additional appraches and extended introduction for Wordy. * Corrected slug for regex with operator module approach.
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exercises/practice/wordy/.approaches/functools-reduce/content.md
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| # Functools.reduce for Calculation | ||
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| ```python | ||
| from operator import add, mul, sub | ||
| from operator import floordiv as div | ||
| from functools import reduce | ||
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| # Define a lookup table for mathematical operations | ||
| OPERATORS = {"plus": add, "minus": sub, "multiplied": mul, "divided": div} | ||
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| def answer(question): | ||
| # Check for basic validity right away, and fail out with error if not valid. | ||
| if not question.startswith( "What is") or "cubed" in question: | ||
| raise ValueError("unknown operation") | ||
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| # Using the built-in filter() to clean & split the question.. | ||
| list(filter(lambda x: | ||
| x not in ("What", "is", "by"), | ||
| question.strip("?").split())) | ||
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| # Separate candidate operators and numbers into two lists. | ||
| operations = question[1::2] | ||
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| # Convert candidate elements to int(), checking for "-". | ||
| # All other values are replaced with None. | ||
| digits = [int(element) if (element.isdigit() or element[1:].isdigit()) | ||
| else None for element in question[::2]] | ||
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| # If there is a mis-match between operators and numbers, toss error. | ||
| if len(digits)-1 != len(operations) or None in digits: | ||
| raise ValueError("syntax error") | ||
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| # Evaluate the expression from left to right using functools.reduce(). | ||
| # Look up each operation in the operation dictionary. | ||
| return reduce(lambda x, y: OPERATORS[operations.pop(0)](x, y), digits) | ||
| ``` | ||
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| This approach replaces the `while-loop` or `recursion` used in many solutions with a call to [`functools.reduce`][functools-reduce]. | ||
| It requires that the question be separated into candidate digits and candidate operators, which is accomplished here via [list-slicing][sequence-operations] (_for some additional information on working with `lists`, see [concept: lists](/tracks/python/concepts/lists)_). | ||
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| A nested call to `filter()` and `split()` within a `list` constructor is used to clean and process the question into an initial `list` of digit and operator strings. | ||
| However, this could easily be accomplished by either using [chained][method-chaining] string methods or a `list-comprehension`: | ||
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| ```python | ||
| # Alternative 1 is chaining various string methods together. | ||
| # The wrapping () invoke implicit concatenation for the chained functions | ||
| return (question.removeprefix("What is") | ||
| .removesuffix("?") | ||
| .replace("by", "") | ||
| .strip()).split() # <-- this split() turns the string into a list. | ||
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| # Alternative 2 to the nested calls to filter and split is to use a list-comprehension: | ||
| return [item for item in | ||
| question.strip("?").split() | ||
| if item not in ("What", "is", "by")] #<-- The [] of the comprehension invokes implicit concatenation. | ||
| ``` | ||
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| Since "valid" questions are all in the form of `digit-operator-digit` (_and so on_), it is safe to assume that every other element beginning at index 0 is a "number", and every other element beginning at index 1 is an operator. | ||
| By that definition, the operators `list` is 1 shorter in `len()` than the digits list. | ||
| Anything else (_or having None/an unknown operation in the operations list_) is a `ValueError("syntax error")`. | ||
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| The final call to `functools.reduce` essentially performs the same steps as the `while-loop` implementation, with the `lambda-expression` passing successive items of the digits `list` to the popped and looked-up operation from the operations `list` (_made [callable][callable] by adding `()`_), until it is reduced to one number and returned. | ||
| A `try-except` is not needed here because the error scenarios are already filtered out in the `if` check right before the call to `reduce()`. | ||
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| `functools.reduce` is certainly convenient, and makes the solution much shorter. | ||
| But it is also hard to understand what is happening if you have not worked with a reduce or foldl function in the past. | ||
| It could be argued that writing the code as a `while-loop` or recursive function is easier to reason about for non-functional programmers. | ||
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| ## Variation: 1: Use a Dictionary of `lambdas` instead of importing from operator | ||
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| The imports from operator can be swapped out for a dictionary of `lambda-expressions` (or calls to `dunder-methods`), if so desired. | ||
| The same cautions apply here as were discussed in the [lambdas in a dictionary][approach-lambdas-in-a-dictionary] approach: | ||
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| ```python | ||
| from functools import reduce | ||
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| # Define a lookup table for mathematical operations | ||
| OPERATORS = {"plus": lambda x, y: x + y, | ||
| "minus": lambda x, y: x - y, | ||
| "multiplied": lambda x, y: x * y, | ||
| "divided": lambda x, y: x / y} | ||
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| def answer(question): | ||
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| # Check for basic validity right away, and fail out with error if not valid. | ||
| if not question.startswith( "What is") or "cubed" in question: | ||
| raise ValueError("unknown operation") | ||
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| # Clean and split the question into a list for processing. | ||
| question = [item for item in | ||
| question.strip("?").split() if | ||
| item not in ("What", "is", "by")] | ||
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| # Separate candidate operators and numbers into two lists. | ||
| operations = question[1::2] | ||
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| # Convert candidate elements to int(), checking for "-". | ||
| # All other values are replaced with None. | ||
| digits = [int(element) if (element.isdigit() or element[1:].isdigit()) | ||
| else None for element in question[::2]] | ||
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| # If there is a mis-match between operators and numbers, toss error. | ||
| if len(digits)-1 != len(operations) or None in digits: | ||
| raise ValueError("syntax error") | ||
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| # Evaluate the expression from left to right using functools.reduce(). | ||
| # Look up each operation in the operation dictionary. | ||
| result = reduce(lambda x, y: OPERATORS[operations.pop(0)](x, y), digits) | ||
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| return result | ||
| ``` | ||
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| [approach-lambdas-in-a-dictionary]: https://exercise.org/tracks/python/exercises/wordy/approaches/lambdas-in-a-dictionary | ||
| [callable]: https://treyhunner.com/2019/04/is-it-a-class-or-a-function-its-a-callable/ | ||
| [functools-reduce]: https://docs.python.org/3/library/functools.html#functools.reduce | ||
| [method-chaining]: https://www.tutorialspoint.com/Explain-Python-class-method-chaining | ||
| [sequence-operations]: https://docs.python.org/3/library/stdtypes.html#common-sequence-operations |
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exercises/practice/wordy/.approaches/functools-reduce/snippet.txt
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| OPERATORS = {"plus": add, "minus": sub, "multiplied": mul, "divided": div} | ||
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| operations = question[1::2] | ||
| digits = [int(element) if (element.isdigit() or element[1:].isdigit()) | ||
| else None for element in question[::2]] | ||
| ... | ||
| return reduce(lambda x, y: OPERATORS[operations.pop(0)](x, y), digits) |
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