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| ==Dictionaries==
| | * A <code>list</code> is useful to store an ordered set of items that are indexed by integers. |
| | | * A <code>dict</code> or dictionary is a way to store key/value pairs. |
| * Use dictionaries to store key/value pairs.
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| * Dictionaries do not guarantee ordering.
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| * A given key can only have one value, but multiple keys can have the same value. | |
| | |
| ====Initialization====
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| | |
| >>> my_dict = {}
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| >>> my_dict
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| {}
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| >>> your_dict = {"Alice" : "chocolate", "Bob" : "strawberry", "Cara" : "mint chip"}
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| >>> your_dict
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| {'Bob': 'strawberry', 'Cara': 'mint chip', 'Alice': 'chocolate'}
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| | |
| ====Adding elements to a dictionary====
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| | |
| >>> your_dict["Dora"] = "vanilla"
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| >>> your_dict
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| {'Bob': 'strawberry', 'Cara': 'mint chip', 'Dora': 'vanilla', 'Alice': 'chocolate'}
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| | |
| ====Accessing elements of a dictionary====
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| >>> your_dict["Alice"]
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| 'chocolate'
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| >>> your_dict.get("Alice")
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| 'chocolate'
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| >>> your_dict["Eve"]
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| Traceback (most recent call last):
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| File "<stdin>", line 1, in <module>
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| KeyError: 'Eve'
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| >>> "Eve" in your_dict
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| False
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| >>> "Alice" in your_dict
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| True
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| >>> your_dict.get("Eve")
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| >>> person = your_dict.get("Eve")
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| >>> print(person)
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| None
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| >>> print(type(person))
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| <type 'NoneType'>
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| >>> your_dict.get("Alice")
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| 'chocolate'
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| | |
| ====Changing elements of a dictionary====
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| | |
| >>> your_dict["Alice"] = "coconut"
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| >>> your_dict
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| {'Bob': 'strawberry', 'Cara': 'mint chip', 'Dora': 'vanilla', 'Alice': 'coconut'}
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| ====Histograms====
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| '''Challenge''': using wordplay example from last week, count the number of words that start with each letter.
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| This kind of problem is very common Data Science, and it is easy with a dictionary.
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| (note: I will post the solution after class)
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| ====For-loops and dictionaries====
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| There are two common ways to iterate through dictionaries:
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| | |
| >>> ages = {'Tommy': 32, Zula: '9', 'Joanna': 18}
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| >>> for key in ages:
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| >>> print(key + " is " + str(ages[key]) + " years old")
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| >>> for key, value in ages.items():
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| >>> print(key + " is " + str(value) + " years old")
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