Week 3 part 1 - dictionaries

A list l has entries l[0], l[1], ..., l[len(l)-1] You could think of the list as being like a function l that sends a number i to l[i].

A Python dictionary d is like a more general version of this where the domain doesn't just have to be numbers. Elements of the domain are called keys, and if k is a key we call d[k] the value for that key. If you've programmed in Java or C++ before, a dictionary is similar to what is called a map in those languages.

As an example, suppose we are writing a program to store people's telephone numbers. We can do this with a dictionary where the keys are people's names, and the values are telephone numbers.

You can create an empty dictionary using {}

Once you've created a dictionary d you can add a key k with value v by assigning d[k] = v

Now phoneNumbers is a dictionary with two keys, whose values you can access as follows:

You can check if a dictionary has a value for a certain key using in...

...which is useful because asking for a key for which no value is set raises a KeyError:

We've already seen how to add a key, but we can also remove one using del:

Now that it's removed, asking for phoneNumbers["UCL Admissions"] raises a KeyError

Other ways to create dictionaries

Instead of starting with an empty dictionary and adding entries like we did above, you can create a new dictionary with some entries already specified like this:

You can even do a "dictionary comprehension", like the list comprehensions we met last week.

What do you think the keys and values are in counter? You can find out by calling counter.keys() and counter.values()

Looping over a dictionary

It's common to want to do something for every entry in a dictionary. You can use for loops: if d is a dictionary then

for k in d:
    # loop body

will run the loop body with k set to each of the keys of the dictionary d. For example:

Unassessed exercises

Exercise 1

In this exercise you are going to teach Python how to say certain numbers in English.

Create a dictionary unit_words whose keys are 1, 2, ..., 9 such that unit_words[1] = "one", unit_words[2] = "two", and so on.

Now create a dictionary tens_words whose keys are 2, 3, ..., 9 such that tens_words[2] = "twenty", tens_words[3] = "thirty", and so on.

Now use your dictionaries to write a function say(tens, units) which returns a string representing how the number 10 * tens + units is spoken in English. For example, say(4, 2) should be "forty-two", and say(2, 5) should be "twenty-five". You can assume tens >= 2

Further suggestions:

• modify say so that it can deal with the numbers 10, 11, ..., 19 as well

• modify say so that it can deal with all numbers between 0 and 99

• modify say so that instead of calling say(4, 2), for example, I can instead call say(42)