Iterating with Dictionaries

Many times in this book, we looped through the characters in a string or the elements in a list without using the index values.

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my_string = 'Rutabagas!'
numbers = [33, -25, 3.14, 86, 1168, 42, 6.022e23]

for character in my_string:
    print(character)

total = 0
for number in numbers:
    total += number*2

Each time the loop body repeats, the loop variable (character or number) gets assigned the next value in the collection.

We can do something similar with dictionaries.

Loop by Keys or Values

To loop through a dictionary, we need to specify whether to assign the loop variable the keys or the values from the collection. To do this, we use the keys() or values() method in the for statement:

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for key in dictionary_name.keys():
    # Loop body...

for value in dictionary_name.values():
    # Loop body...

In the first for loop, each time the code repeats, the loop variable key gets assigned one of the key names from the dictionary. In the second loop, the variable value gets assigned a new value each iteration.

Loop by Key/Value Pairs

The items() method returns each key/value pair as a unit, and it allows us to assign BOTH the key and value from the dictionary to separate variables. The general syntax for this is:

for (key, value) in dictionary_name.items():

In the for statement, we define a pair of variables (key, value) to hold a key name and its linked value from the dictionary. Each iteration, these two variables represent a new key/value pair from the collection.

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comics = {
    'Gary Larson' : 'The Far Side',
    'Terri Libenson' : 'Pajama Diaries',
    'Hilary B. Price' : 'Rhymes with Orange',
    'Jim Toomey' : "Sherman's Lagoon"
}

# Iterate by keys, and print out the dictionary key/value pairs:
for key in comics.keys():
    print(key, comics[key])

# Iterate by key/value pairs:
for (key, value) in comics.items():
    print(key, value)

Sorting by Keys

Dictionaries are unordered collections, so they do NOT include any type of sorting method. However, sometimes we might want to access or display the key/value pairs in a particular order—like alphabetically by key name.

If we want to sort a dictionary, the short answer is…we can’t. However, we can use a work-around.

By adding copies of the key names to a separate list, we can sort the list to get the order we want. Then we use the key names from the sorted list to access the values in the dictionary.

Check Your Understanding

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comics = {
    'Georgia Dunn' : 'Breaking Cat News',
    'Jan Eliot' : 'Stone Soup',
    'Wiley Miller' : 'Non Sequitur',         
    'Bill Watterson' : 'Calvin and Hobbs'
}

for key in comics.keys():
    print(key, comics[key])