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| Original file line number | Diff line number | Diff line change |
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| # pycobytes[13] := Counting | ||
| <!-- #SQUARK live! | ||
| | dest = 13 | ||
| | title = Counting | ||
| | head = Counting | ||
| | index = 13 | ||
| | shard = functions / challenge | ||
| | date = 2025 December | ||
| --> | ||
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| > *If at first you do not succeed, call it version 1.0.* | ||
| Hey pips! | ||
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| When using a `for` loop, it’s often really useful to know which iteration number we’re on. | ||
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| ```py | ||
| >>> for i in range(3): | ||
| print(f"iteration {i}") | ||
| iteration 0 | ||
| iteration 1 | ||
| iteration 2 | ||
| ``` | ||
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| But if we’re iterating over an iterable, we don’t have a variable to count the iteration... | ||
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| ```py | ||
| >>> l = ["clear", "wing"] | ||
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| >>> for item in l: | ||
| print(f"unknown iteration: {item}") | ||
| unknown iteration: clear | ||
| unknown iteration: wing | ||
| ``` | ||
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| A pretty quick fix is to just make your own counting variable, and manually increment it each iteration. | ||
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| ```py | ||
| >>> i = 0 | ||
| >>> l = ["starving", "venom"] | ||
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| >>> for item in l: | ||
| print(f"iteration {i}: {item}") | ||
| i += 1 # manually increment | ||
| iteration 0: starving | ||
| iteration 1: venom | ||
| ``` | ||
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| > [!Tip] | ||
| > This approach is useful when you need to do other stuff with the increment variable – although to be honest, if you’ve reached that stage it may be indicative of other issues. | ||
| But with how often this comes up, you would think there’s an in-built solution. | ||
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| Well this is Python, so of course there is! | ||
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| ```py | ||
| >>> l = ["phantom", "knight"] | ||
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| >>> for i, item in enumerate(l): | ||
| print(f"iteration {i}: {item}") | ||
| iteration 0: phantom | ||
| iteration 1: knight | ||
| ``` | ||
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| The built-in `enumerate()` function works on any iterable. It pairs each item with its index to form an `(index, item)` tuple, so that when you iterate over it you can extract both the iteration index and item value. | ||
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| ```py | ||
| >>> l = ["blue", "eyes", "white", "dragon"] | ||
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| >>> list(enumerate(l)) | ||
| [(0, "blue"), | ||
| (1, "eyes"), | ||
| (2, "white"), | ||
| (3, "dragon")] | ||
| ``` | ||
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| You can use `enumerate()` on any iterable, including `str`, `tuple` and even `dict` objects. | ||
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| ```py | ||
| >>> list(enumerate("sup")) | ||
| [(0, "s"), | ||
| (1, "u"), | ||
| (2, "p")] | ||
| ``` | ||
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| Using this, we now have an extremely convenient way to count iterations while we’re looping – particularly in list comprehensions: | ||
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| ```py | ||
| >>> text = "Never Gonna Give You Up" | ||
| >>> out = [ | ||
| # capitalise every other character | ||
| char.upper() if i % 2 == 0 else char.lower() | ||
| for i, char in enumerate(text) | ||
| ] | ||
| >>> "".join(out) | ||
| NeVeR GoNnA GiVe yOu uP | ||
| ``` | ||
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| And fun fact, you can even pass in a second numerical argument to specify the starting index! | ||
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| ```py | ||
| >>> l = ["iTechnicals", "Sup"] | ||
| >>> [f"{i}: {player}" for i, player in enumerate(l, 1)] | ||
| ["1: iTechnicals", "2: Sup"] | ||
| ``` | ||
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| Keep in mind `enumerate()` doesn’t exactly return a `list`, so you can’t index it: | ||
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| ```py | ||
| >>> e = enumerate("phantasm") | ||
| >>> e[1] | ||
| Error: | ||
| ``` | ||
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| If you want the raw items, just convert the output to a `list` with the `list()` constructor. | ||
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| ```py | ||
| >>> l = list(enumerate("desync")) | ||
| >>> l[2] | ||
| (2, "s") | ||
| ``` | ||
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| This is because `enumerate()` actually returns an **iterator** which acts as a proxy to the original object. We’ll take a closer look at these a future issue! | ||
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| <br> | ||
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| ## Challenge | ||
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| Given a list of fruit and how many to purchase as tuples, can you print a shopping list? | ||
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| ```py | ||
| >>> shopping = [ | ||
| ("apricots", 3), | ||
| ("bloomerangs", 4), | ||
| ("carrots", 2), | ||
| ("dragonfruit", 1) | ||
| ] | ||
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| >>> (your_expression) | ||
| 1. apricots x3 | ||
| 2. bloomerangs x4 | ||
| 3. carrots x2 | ||
| 4. dragonfruit | ||
| ``` | ||
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| And bonus points for making it as fancy as you can :P | ||
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| ``` | ||
| ===================== | ||
| | 1 | durian | x7 | | ||
| | 2 | acai | x3 | | ||
| | 3 | lychee | x10 | | ||
| | 4 | pomelo | x2 | | ||
| --------------------- | ||
| | TOTAL | x22 | | ||
| ===================== | ||
| ``` | ||
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| <br> | ||
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| --- | ||
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| <div align="center"> | ||
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| [](http://thecodelesscode.com/case/158) | ||
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| [*The Codeless Code*, Case 158](http://thecodelesscode.com/case/158) | ||
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| </div> |
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