> For the complete documentation index, see [llms.txt](https://louisazhou.gitbook.io/notes/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://louisazhou.gitbook.io/notes/chapter1/section1.2.md). # Iterators ## Iterators vs. iterables #### Iterable * Examples: lists, strings, dictionaries, �file connections * An object with an associated `iter()` method * Applying `iter()` to an iterable creates an iterator #### Iterator * Produces next value with `next()` An *iterable* is an object that can return an *iterator*, while an *iterator* is an object that keeps state and produces the next value when you call `next()` on it. ### Iterating over iterables (list) ```python # Create a list of strings: flash flash = ['jay garrick', 'barry allen', 'wally west', 'bart allen'] # Print each list item in flash using a for loop for person in flash: print (person) # Create an iterator for flash: superhero superhero=iter(flash) # Print each item from the iterator print(next(superhero)) print(next(superhero)) print(next(superhero)) print(next(superhero)) ``` ### Iterating over dictionaries ```python pythonistas = {'hugo': 'bowne-anderson', 'francis': 'castro'} for key, value in pythonistas.items(): print(key, value) # francis castro # hugo bowne-anderson ``` ### Iterating over file connections ```python file = open('file.txt') it = iter(file) print(next(it)) #This is the first line. print(next(it)) #This is the second line. ``` ### Iterating over iterables (range) Not all iterables are *actual* lists. You can use `range()` in a `for` loop *as if* it's a list to be iterated over: ``` for i in range(5): print(i) ``` Recall that `range()` doesn't actually create the list; instead, it creates a range object with an iterator that produces the values until it reaches the limit (in the example, until the value 4). ```python # Create an iterator for range(10 ** 100): googol googol = iter(range(10**100)) # Print the first 5 values from googol print(next(googol)) print(next(googol)) ``` ### Iterators as function arguments There are also functions that take iterators and iterables as arguments. For example, the `list()` and `sum()` functions return a list and the sum of elements, respectively. ## enumerate() `enumerate()` returns an `enumerate` object that produces a sequence of tuples, and each of the tuples is an *index-value* pair. ```python avengers = ['hawkeye', 'iron man', 'thor', 'quicksilver'] e = enumerate(avengers) print(type(e)) # e_list = list(e) print(e_list) #[(0, 'hawkeye'), (1, 'iron man'), (2, 'thor'), (3, 'quicksilver')] ``` Then, we can use list to turn this enumerate object into a list of tuples and print to see what it contains ### enumerate() and unpack ```python avengers = ['hawkeye', 'iron man', 'thor', 'quicksilver'] for index, value in enumerate(avengers): #enumerate(avengers, start=10) print(index, value) #0 hawkeye #1 iron man #2 thor #3 quicksilver ``` ## zip() `zip()` takes any number of iterables and returns a `zip` object that is an iterator of tuples. ```python avengers = ['hawkeye', 'iron man', 'thor', 'quicksilver'] names = ['barton', 'stark', 'odinson', 'maximoff'] z = zip(avengers, names) print(type(z)) # z_list = list(z) print(z_list) #[('hawkeye', 'barton'), ('iron man', 'stark'), ('thor', 'odinson'), ('quicksilver', 'maximoff')] ``` ### zip() and unpack ```python avengers = ['hawkeye', 'iron man', 'thor', 'quicksilver'] names = ['barton', 'stark', 'odinson', 'maximoff'] for z1, z2 in zip(avengers, names): print(z1, z2) # hawkeye barton # iron man stark # thor odinson quicksilver maximoff ``` ### zip(\*object) use `*` in a call to `zip()` to unpack the tuples produced by `zip()`. ```python z1 = zip(mutants, powers) # 'Unzip' the tuples in z1 by unpacking with * and zip(): result1, result2 result1, result2 = zip(*z1) ``` ### Print zip with \* ```python avengers = ['hawkeye', 'iron man', 'thor', 'quicksilver'] names = ['barton', 'stark', 'odinson', 'maximoff'] z = zip(avengers, names) print(*z) #('hawkeye', 'barton') ('iron man', 'stark') ('thor', 'odinson') ('quicksilver', 'maximoff') ``` ## \* operator (splat operator) ```python word = 'Data' it = iter(word) print(*it) # D a t a ``` ## Use iterator to load data in chunks {% tabs %} {% tab title="enumerate for idx" %} ```python skills = ['ML', 'Stat', 'Coding', 'Business'] for idx, skill in enumerate(skills): print('{idx_} : {skill_}'.format(skill_ = skill, idx_ = idx)) ··· output： 0 : ML 1 : Stat 2 : Coding 3 : Business ··· ``` {% endtab %} {% tab title="占位符输出" %} ```python s1 = 'single quote' # use single quotes s2= "double quote" # use double quotes s4 = '{0} huozhe {1} {2}'.format(s1, s2, 100) //output：single quote huozhe double quote 100 ``` {% endtab %} {% tab title="enumerate in Set" %} ```python skills = {'ML', 'Stat', 'Coding', 'Business'} for idx, skill in enumerate(skills): print(f"{skill} : {idx}") ``` {% endtab %} {% endtabs %} {% hint style="info" %} 1. print中使用{}, 使用.format()格式化输出 括号内可以不包含前面的字段（此时需要一一对应） 2. index可以直接用enumerate，靠idx来call {% endhint %} 也可以用%d的占位符，就有点C的写法... ```python skills = ['ML', 'Stat', 'Coding', 'Business'] scores = [90, 80, 1, 59] print(scores[skills.index('Coding')]) score_dict = {'ML': 90, 'Stat': 80, 'Coding': 1, 'Business': 59} # create a dictionary print("current score: %d" % score_dict['Coding']) #current score: 1 ``` ```python skills = {'ML', 'Stat', 'Coding', 'Business'} skills_list = ['ML', 'Stat', 'Coding', 'Business'] # loop over a set for idx, skill in enumerate(skills): print(f"{skill} : {idx}") ··· Stat : 0 Coding : 1 ML : 2 Business : 3 ··· ```