Dictionaries

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.

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

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

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.