Dictionaries

Using a dictionary

Find out what each of the expressions does to the dictionary in the center.

Definitions

Dictionaries

Dictionaries are an unordered, associative array. They have a set of key/value pairs. They are very versatile data structures, but slower than lists. Dictionaries can be used easily as a hashtable.

Creating dictionaries

prices = {
    'banana':0.75,
    'apple':0.55,
    'orange':0.80
    }

Accessing elements in dictionaries

By applying square brackets with a key inside, the values of a dictionary can be requested. Valid types for keys are strings, integers, floats, and tuples.

print prices['banana']  # 0.75
print prices['kiwi']    # KeyError!

Looping over a dictionary:

You can access the keys of a dictionary in a for loop. However, their order is not guaranteed, then.

for fruit in prices:
    print fruit

Methods of dictionaries

There is a number of functions that can be used on every dictionary:

Checking whether a key exists:

prices.has_key('apple')

Retrieving values in a fail-safe way:

prices.get('banana')
prices.get('kiwi')

Setting values only if they dont exist yet:

prices.setdefault('kiwi', 0.99)
prices.setdefault('banana', 0.99)
# for 'banana', nothing happens

Getting all keys / values:

print prices.keys()
print prices.values()
print prices.items()

Exercises

Exercise 1.

What do the following commands produce?

d = {1:'A', 'B':1, 'A':True}
print(d['A'])

False
"B"
True
1