Lecture 8 – Lists and Strings

Data 6, Summer 2021

Lists

We use square brackets to create lists.

years = [1998, 2001, 2007]
years

[1998, 2001, 2007]

schools = ['cal', 1868, 'cal poly', 1901, 'columbia', 1754]
schools

['cal', 1868, 'cal poly', 1901, 'columbia', 1754]

nums = [2 + 2, 5, 5 - 1]
nums

[4, 5, 4]

Types and comparisons

type([3, 1, 2])

list

type([]) 

list

[3, 1, 2] == [3, 1, 2]

True

[3, 1, 2] == [3, 1, 2, -4]

False

Working with lists

len([9, 2.5, 7])

3

max([9, 2.5, 7])

9

# Earliest in dictionary
min(['hello', 'hi', 'abbey'])

'abbey'

# TypeError!
min(['hello', 2.5, 'abbey'])

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
/tmp/ipykernel_40/3620951238.py in <module>
      1 # TypeError!
----> 2 min(['hello', 2.5, 'abbey'])

TypeError: '<' not supported between instances of 'float' and 'str'

sum([9, 2.5, 7])

18.5

[1, 2] + [3, 4] * 2

[1, 2, 3, 4, 3, 4]

Append

groceries = ['eggs', 'milk']
groceries

['eggs', 'milk']

groceries.append('bread')

groceries

['eggs', 'milk', 'bread']

Containment

3 in [3, 1, 'dog']

True

10 not in [3, 1, 'dog']

True

not 10 in [3, 1, 'dog']

True

[3, 1] in [3, 1, 'dog']

False

Quick Check 1

A = [1] + [2] == [1 + 2]
A

False

B = sum([4, 5, 9]) / len([4, 5, 9])
B

6.0

C = [4, 0, 2]
C = C.append(min(C))
print(C)

None

Indexing

nums = [3, 1, 'dog', -9.5, 'berk']

nums[0]

3

nums[3]

-9.5

nums[5]

---------------------------------------------------------------------------
IndexError                                Traceback (most recent call last)
/tmp/ipykernel_40/3723135418.py in <module>
----> 1 nums[5]

IndexError: list index out of range

Slicing

nums = [3, 1, 'dog', -9.5, 'berk']

nums[1:3]

[1, 'dog']

nums[0:4]

[3, 1, 'dog', -9.5]

# If you don't include 'start',
# the slice starts at the
# beginning of the list
nums[:4]

[3, 1, 'dog', -9.5]

# If you don't include 'stop',
# the slice goes until the
# end of the list
nums[2:]

['dog', -9.5, 'berk']

Negative indexing

nums = [3, 1, 'dog', -9.5, 'berk']

nums[len(nums) - 1]

'berk'

nums[-1]

'berk'

nums[-3]

'dog'

nums[-3:]

['dog', -9.5, 'berk']

Quick Check 2

threes = [3, 6, 9, 12, 15]
fours = threes[1:4]
five = fours[-1] + fours[1]
five

21

Example: While loops with lists

def square_all(vals):
    output = []
    i = 0
    while i < len(vals):
        val_squared = vals[i] ** 2
        output.append(val_squared)
        i += 1
    return output

square_all([1, 10, 3, 4])

[1, 100, 9, 16]

.index

.index tells us the position of an element in a list – if it is in the list.

[9, 8, 14, -1].index(14)

2

[9, 8, 14, -1].index(15)

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
/tmp/ipykernel_40/3856422112.py in <module>
----> 1 [9, 8, 14, -1].index(15)

ValueError: 15 is not in list

# Two occurrences of 2
# Gives index of first one
[1, 2, 4, 2, 4].index(2)

1

[1, 2, 4, 2, 4].count(2)

2

Example: next day of the week

def next_day(day):
    week = ['Sunday', 'Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday']
    curr = week.index(day)
    return week[(curr + 1) % 7]

next_day('Wednesday')

'Thursday'

next_day('Saturday')

'Sunday'

Strings

university = 'uc berkeley'

list(university)

['u', 'c', ' ', 'b', 'e', 'r', 'k', 'e', 'l', 'e', 'y']

university[3:7]

'berk'

university[1]

'c'

university[-8:]

'berkeley'

# Weird slicing that can be used
# to reverse a string or list
university[::-1]

'yelekreb cu'

Finding characters

'alfalfa'.find('f')

2

'alfalfa'.rfind('a')

6

'alfalfa'.find('b')

-1

'alfalfa'.index('b')

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
/tmp/ipykernel_40/3743236407.py in <module>
----> 1 'alfalfa'.index('b')

ValueError: substring not found

Differences between strings and lists

test_list = [8, 0, 2, 4]
test_string = 'zebra'

test_list[1] = 99

test_list

[8, 99, 2, 4]

test_string[1] = 'f'

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
/tmp/ipykernel_40/3748101269.py in <module>
----> 1 test_string[1] = 'f'

TypeError: 'str' object does not support item assignment

test_string[:1] + 'f' + test_string[2:]

'zfbra'

Demo

Let's use data about survivors of the Titanic, downloaded from here.

from datascience import *
table = Table.read_table('data/titanic.csv').select(['Name', 'Age', 'Sex', 'Fare', 'Survived'])
table

Name	Age	Sex	Fare	Survived
Braund, Mr. Owen Harris	22	male	7.25	0
Cumings, Mrs. John Bradley (Florence Briggs Thayer)	38	female	71.2833	1
Heikkinen, Miss. Laina	26	female	7.925	1
Futrelle, Mrs. Jacques Heath (Lily May Peel)	35	female	53.1	1
Allen, Mr. William Henry	35	male	8.05	0
Moran, Mr. James	nan	male	8.4583	0
McCarthy, Mr. Timothy J	54	male	51.8625	0
Palsson, Master. Gosta Leonard	2	male	21.075	0
Johnson, Mrs. Oscar W (Elisabeth Vilhelmina Berg)	27	female	11.1333	1
Nasser, Mrs. Nicholas (Adele Achem)	14	female	30.0708	1

... (881 rows omitted)

Soon, we will learn how to load in data like the above and extract columns as lists. But for now, just run the following cell.

names = list(table.column('Name'))
ages = list(table.column('Age'))
survived = list(table.column('Survived'))

ages[:5]

[22.0, 38.0, 26.0, 35.0, 35.0]

names[:5]

['Braund, Mr. Owen Harris',
 'Cumings, Mrs. John Bradley (Florence Briggs Thayer)',
 'Heikkinen, Miss. Laina',
 'Futrelle, Mrs. Jacques Heath (Lily May Peel)',
 'Allen, Mr. William Henry']

100 * sum(survived) / len(survived)

38.38383838383838