Overview

Teaching: 15 min
Exercises: 20 min

Questions

• How can I store and manipulate non-rectangular data?

Objectives

• Write programs that use sets to store unique values.

• Write programs that use dictionaries to store key/value pairs.

• Identify values that can and cannot be used as dictionary keys.

We already know about list [] which is immutable, how about Set, Dictionary and Tuple…?

Set

A set is an unordered collection of distinct elements.

Use a set to store unique values.

• Create with {...}
  • But must use set() to create an empty set.

primes = {2, 3, 5, 7}
print('is 3 prime?', 3 in primes)
print('is 9 prime?', 9 in primes)

is 3 prime? True
is 9 prime? False

• Intersection, union, etc.

odds = {3, 5, 7, 9}
print('intersection', odds & primes)
print('union', odds | primes)

intersection {3, 5, 7}
union {3, 5, 7, 9, 11}

Sets are mutable.

• But only store unique values.

primes.add(11)
print('primes becomes', primes)
primes.discard(7)
print('after removal', primes)
primes.add(11)
print('after adding 11 again', primes)

primes becomes {11, 2, 3, 5, 7}
after removal {11, 2, 3, 5}
after adding 11 again {11, 2, 3, 5}

Sets are unordered.

• Values are stored by hashing.
  • Which is intentionally as random as possible.

names = {'Darwin', 'Newton', 'Turing'}
for n in names:
    print(n)

Newton
Darwin
Turing

Dictionary

A dictionary stores (key, value) pairs.

Use a dictionary to store key/value pairs.

• Equivalently, to store extra information with elements of a set.

birthdays = {'Newton' : 1602, 'Darwin' : 1809}
print(birthdays['Newton'])
birthdays['Turing'] = 1612 # oops
birthdays['Turing'] = 1912 # that's better
print(birthdays)

1602
{'Darwin': 1809, 'Newton': 1602, 'Turing': 1912}

• Just an accident that keys are alphabetical in this case.
  • Like sets, dictionaries store keys by hashing, which is as random as possible.

Set values and dictionary keys must be immutable.

• Changing them after insertion would leave data in the wrong place.
• Use a tuple for multi-valued keys.

people = {('Isaac', 'Newton'): 1602, ('Charles', 'Darwin'): 1809}

Example: create a histogram.

numbers = [1, 0, 1, 2, 0, 0, 1, 2, 1, 3, 1, 0, 2]
count = {}
for n in numbers:
    if n not in count:
        count[n] = 1
    else:
        count[n] = count[n] + 1
print(count)

{0: 4, 1: 5, 2: 3, 3: 1}

Keys are often strings.

elements = {'H' : 1, 'He' : 2, 'Li' : 3, 'Be' : 4, 'B' : 5}
print('atomic number of lithium:', elements['Li'])

atomic number of lithium: 3

Tuples

A tuple is an (immutable) ordered list of values. A tuple is in many ways similar to a list; one of the most important differences is that tuples can be used as keys in dictionaries and as elements of sets, while lists cannot. Tuple is written as lis, (x, y). Here is a trivial example:

d = {(x, x + 1): x for x in range(10)}  # Create a dictionary with tuple keys
t = (5, 6)        # Create a tuple
print(type(t))    # Prints "<class 'tuple'>"
print(d[t])       # Prints "5"
print(d[(1, 2)])  # Prints "1"

How Heavy Is This Molecule?

Write a function that takes two parameters: a dictionary mapping atomic symbols to atomic weights, and a list of (atom, count) pairs for a molecule, and returns that molecule’s molecular weight.

weights = {'H' : 1.0079, 'C' : 12.0107, 'N' : 14.0067,
           'O' : 15.9994, 'P' : 30.9738, 'S' : 32.065}

methane = [('C', 1), ('H', 4)]
print('methane', mol_weight(weights, methane))

aminothiazole = [('C', 3), ('H', 4), ('N', 2), ('S', 1)]
print('aminothiazole', mol_weight(weights, aminothiazole))

methane 16.0423
aminothiazole 100.1421

How did you test your function?

Key Points

• Use sets to store unique unordered values.

• Use dictionaries to store extra information with those values.

• Use tuples to store list including key of dictionaries