1.8 Iteration

Estimated time for this notebook: 10 minutes

Our other aspect of control is looping back on ourselves.

We use for … in to “iterate” over lists:

mylist = [3, 7, 15, 2]

for whatever in mylist:
    print(whatever**2)

9
49
225
4

Each time through the loop, the variable in the value slot is updated to the next element of the sequence.

1.8.1 Iterables

Any sequence type is iterable:

vowels = "aeiou"
sarcasm = []

for letter in "Okay":
    if letter.lower() in vowels:
        repetition = 3
    else:
        repetition = 1

    sarcasm.append(letter * repetition)

"".join(sarcasm)

'OOOkaaay'

The above is a little puzzle, work through it to understand why it does what it does.

Dictionaries are Iterables

All sequences are iterables. Some iterables (things you can for loop over) are not sequences (things with you can do x[5] to), for example sets and dictionaries.

current_year = 2022
founded = {"Barack Obama": 1961, "UCL": 1826, "The Alan Turing Institute": 2015}

for thing in founded:
    print(f"In {current_year} {thing} is {current_year - founded[thing]} years old.")

In 2022 Barack Obama is 61 years old.
In 2022 UCL is 196 years old.
In 2022 The Alan Turing Institute is 7 years old.

1.8.2 Unpacking and Iteration

Unpacking can be useful with iteration:

triples = [[4, 11, 15], [39, 4, 18]]

for whatever in triples:
    print(whatever)

[4, 11, 15]
[39, 4, 18]

for first, middle, last in triples:
    print(middle)

11
4

# A reminder that the words you use for variable names are arbitrary:
for hedgehog, badger, fox in triples:
    print(badger)

11
4

for example, to iterate over the items in a dictionary as pairs:

things = {
    "James": [1976, "Kendal"],
    "UCL": [1826, "Bloomsbury"],
    "Cambridge": [1209, "Cambridge"],
}

print(things.items())

dict_items([('James', [1976, 'Kendal']), ('UCL', [1826, 'Bloomsbury']), ('Cambridge', [1209, 'Cambridge'])])

for name, year in founded.items():
    print(name, "is", current_year - year, "years old.")

Barack Obama is 61 years old.
UCL is 196 years old.
The Alan Turing Institute is 7 years old.

1.8.3 Break, Continue

• Continue skips to the next turn of a loop

• Break stops the loop early

for n in range(50):
    if n == 20:
        break
    if n % 2 == 0:
        continue
    print(n)

1
3
5
7
9
11
13
15
17
19

These aren’t useful that often, but are worth knowing about. There’s also an optional else clause on loops, executed only if you don’t break, but I’ve never found that useful.