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5: Dictionaries

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5: Dictionaries#

Dictionaries are unordered control structures, meaning that the values they contain are not ordered according to an index, but according to a single key.

A perfect use of dictionaries is to group together “variables” in a single directory. (these variables are not real variables, but keys).

*For example, you can create an inventory dictionary that groups several products (the keys) and their quantities (the values)

inventory = { 'apples': 100,
              'bananas': 80,
              'pears': 120}

inventory.values()

dict_values([100, 80, 120])

inventory.keys()

dict_keys(['apples', 'bananas', 'pears'])

len(inventory)

3

Here is how to add a key/value association in our dictionary (be careful if the key already exists it is replaced)

inventory['apricots'] = 30
print(inventory)

{'apples': 100, 'bananas': 80, 'pears': 120, 'apricots': 30}

Warning : if you look for a key that doesn’t exist in a dictionary, python returns an error. To avoid this, you can use the get() method

inventory.get('peaches') # does not exist

inventory.get('apples') # apple exists

100

the pop() method allows you to remove a key from a dictionary while returning the value associated to the key.

apricots = inventory.pop("apricots")
print(inventory) # no longer contains a key apricots
print(apricots) # apricots contains the value of the dictionary

{'apples': 100, 'bananas': 80, 'pears': 120}
30

To use a for loop with a dictionary, it is useful to use the items() method which returns both keys and values

for key, value in inventory.items():
    print(key, value)

apples 100
bananas 80
pears 120

Exercise and Solution#

Implement a function sort(folder, value) that places a value in a dictionary according to its sign

folder = { 'negative':[],
            'positive':[]
            }

def sort(folder, value):
    return folder

Solution#

Hide code cell content 
def sort(folder, value):
    if value >=0:
        folder['positive'].append(value)
    else:
        folder['negative'].append(value)
    return folder

sort(folder, 9)

{'negative': [], 'positive': [9]}
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