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Section 11.5 Multiple Assignment with Dictionaries

By combining items, tuple assignment, and for, you can make a nice code pattern for traversing the keys and values of a dictionary in a single loop: 
for key, val in list(d.items()):
    print(val, key)
This loop has two iteration variables because items returns a list of tuples and key, val is a tuple assignment that successively iterates through each of the key-value pairs in the dictionary.
For each iteration through the loop, both key and value are advanced to the next key-value pair in the dictionary (still in hash order).
The output of this loop is: 
10 a
22 c
1 b
Again, it is in hash key order (i.e., no particular order).

Activity 11.5.1.

11-9-1: How will the contents of list “lst” be ordered after the following code is run? 
d = {'a': 10, 'b': 15, 'c': 17, 'd': 4}
lst = []
for key, val in d.items():
    lst.append( (val, key) )
print(lst)
  • [(4, ’d’), (10, ’a’), (15, ’b’), (17, ’c’)]
  • Incorrect! Remember, key-value pairs aren’t in any particular order. Try again.
  • [(’a’, 10), (’b’, 15), (’c’, 17), (’d’, 4)]
  • Incorrect! Remember, key-value pairs aren’t in any particular order. Try again.
  • There will be no particular order
  • Correct! When running this type of iteration, we are left with a hash key order, meaning there is no particular order.
If we combine these two techniques, we can print out the contents of a dictionary sorted by the value stored in each key-value pair.
To do this, we first make a list of tuples where each tuple is (value, key). The items method would give us a list of (key, value) tuples, but this time we want to sort by value, not key. Once we have constructed the list with the value-key tuples, it is a simple matter to sort the list in reverse order and print out the new, sorted list.
By carefully constructing the list of tuples so that the value is the first element of each tuple and the key is the second element, we can sort our dictionary contents by value.

Activity 11.5.2.

Construct a block of code to iterate through the items in dictionary d and print out its key-value pairs.

Activity 11.5.3.

Write code to create a list called ‘lst’ and add the key-value pairs of dictionary d to list lst as tuples. Sort list lst by the values in descending order.
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