11.7. Using Tuples as Keys in Dictionaries¶
Because tuples are hashable and lists are not, if we want to create a composite key to use in a dictionary we must use a tuple as the key.
We would encounter a composite key if we wanted to create a telephone
directory that maps from last-name, first-name pairs to telephone
numbers. Assuming that we have defined the variables last,
first, and number, we could write a dictionary
assignment statement as follows:
directory[last, first] = number
Write code to create a dictionary called ‘d1’, and in it give the tuple (1, ‘a’) a value of “tuple”.
- Dictionaries
- Incorrect! Dictionaries cannot be the keys of other dictionaries. Try again.
- Tuples
- Correct! It is fine to use tuples as keys in a dictionary.
- Strings
- Correct! Strings are used as keys of dictionaries all the time!
- Integers
- Correct! Integers are perfectly acceptable to be keys of dictionaries.
- Lists
- Incorrect! Lists cannot be used as the keys of dictionaries. Try again.
11-9-2: Which of these options can be keys of a dictionary? Select all that apply.
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The expression in brackets is a tuple. We could use tuple assignment in
a for loop to traverse this dictionary.
for last, first in directory:
print(first, last, directory[last, first])
This loop traverses the keys in directory, which are
tuples. It assigns the elements of each tuple to last and
first, then prints the name and corresponding telephone
number.
- my_dict['Go']
- Incorrect! You need both values in the tuple for the dictionary to recognize it as the correct key. Try again.
- my_dict['Blue']
- Incorrect! You need both values in the tuple for the dictionary to recognize it as the correct key. Try again.
- my_dict['Go', 'Blue']
- Correct! In this case, the parentheses of the tuple are not required in order to properly call its value.
- my_dict[('Go', 'Blue')]
- Correct! This is one way to grab the value associated with the tuple.
11-9-3: Which of the following lines of code correctly prints the value associated with (‘Go’, ‘Blue’)? Select all that apply.
my_dict = {}
my_dict[('Go', 'Blue')] = True