LECTURE 3.2

Lists and tuples

Storing many values: a whole inbox in one variable

Introduction to Data Science and Machine Learning · Md Mahbub E Noor · University of Barishal

Today's key line of code

inbox = ["Hi", "Win cash", "Class at 10"]
print(inbox[1])

Where we are: Lecture 3, part 2 of 3

L2Python basics

Part 2 of 3L3Decisions and loops

L4Counting words

L5Real data

L6Build the model

Today you will

1Create lists and read items by position
2Slice out parts of a list
3Add, change and remove items
4Use tuples for fixed records, and unpack them
5Avoid the two-names-one-list trap

Md Mahbub E NoorLecture 3.2: Lists and tuplesUniversity of Barishal2 / 29

Why lists?

One variable per message does not scale to 5,000 messages
A list keeps many values, in order, under one name
len() counts the items
Lists are written with square brackets

Code

msg1 = "Win a free phone"
msg2 = "Class moved to 305"
msg3 = "Claim your prize"
inbox = [msg1, msg2, msg3]
print(inbox)
print(len(inbox))

Output

['Win a free phone', 'Class moved to 305', 'Claim your prize']
3

Md Mahbub E NoorLecture 3.2: Lists and tuplesUniversity of Barishal3 / 29

Anatomy of a list

Code

scores = [72, 85, 90]
print(scores)

Output

[72, 85, 90]

The name of the list

[ opens the list

Items: any values, here numbers

Commas separate the items

] closes the list

Md Mahbub E NoorLecture 3.2: Lists and tuplesUniversity of Barishal4 / 29

Positions start at 0

index from the startindex from the end

"Win cash"

-4

"Class 305"

-3

"Claim prize"

-2

"See you"

-1

Code

inbox = ["Win cash", "Class 305",
         "Claim prize", "See you"]
print(inbox[0])
print(inbox[2])
print(inbox[-1])
print(inbox[-2])

Output

Win cash
Claim prize
See you
Claim prize

Md Mahbub E NoorLecture 3.2: Lists and tuplesUniversity of Barishal5 / 29

Slicing: take a part of a list

inbox[1:3]

index from the startindex from the end

"Win cash"

-4

"Class 305"

-3

"Claim prize"

-2

"See you"

-1

Code

print(inbox[1:3])
print(inbox[:2])
print(inbox[2:])

Output

['Class 305', 'Claim prize']
['Win cash', 'Class 305']
['Claim prize', 'See you']

Md Mahbub E NoorLecture 3.2: Lists and tuplesUniversity of Barishal6 / 29

Predict the slices

Predict first

Code

nums = [10, 20, 30, 40, 50]
print(nums[1:4])
print(nums[-3:])
print(len(nums[2:]))

What will this print? Write your prediction as a comment before you run the cell.

Md Mahbub E NoorLecture 3.2: Lists and tuplesUniversity of Barishal7 / 29

Answer: Predict the slices

Code

nums = [10, 20, 30, 40, 50]
print(nums[1:4])
print(nums[-3:])
print(len(nums[2:]))

Output

[20, 30, 40]
[30, 40, 50]
3

[1:4] takes positions 1, 2, 3: stop before 4
[-3:] means the last three items
A slice is itself a list, so len() works on it

Md Mahbub E NoorLecture 3.2: Lists and tuplesUniversity of Barishal8 / 29

Changing a list

append(x) adds to the end
insert(i, x) adds at position i
list[i] = x replaces an item
remove(x) deletes the first x
pop() removes and returns the last item

Code

tasks = ["read", "code"]
tasks.append("sleep")
tasks.insert(0, "eat")
tasks[1] = "study"
tasks.remove("code")
last = tasks.pop()
print(tasks)
print(last)

Output

['eat', 'study']
sleep

Md Mahbub E NoorLecture 3.2: Lists and tuplesUniversity of Barishal9 / 29

Watching the list change, step by step

Each row runs one line. Follow how the list looks after it.

Code

tasks = ["read", "code"]
tasks.append("sleep")
tasks.insert(0, "eat")
tasks[1] = "study"
tasks.remove("code")
last = tasks.pop()

step	operation	tasks after this line
1	create	['read', 'code']
2	append	['read', 'code', 'sleep']
3	insert	['eat', 'read', 'code', 'sleep']
4	replace [1]	['eat', 'study', 'code', 'sleep']
5	remove	['eat', 'study', 'sleep']
6	pop -> 'sleep'	['eat', 'study']

Next row: arrow key or the ▶ button. Predict it first!

Md Mahbub E NoorLecture 3.2: Lists and tuplesUniversity of Barishal10 / 29

Asking questions about a list

len, max, min, sum work on whole lists
x in list checks membership
count(x) and index(x) find items
sorted(list) gives a new sorted list; the original stays

Code

nums = [5, 3, 8, 3, 1]
print(len(nums), max(nums), min(nums), sum(nums))
print(8 in nums)
print(nums.count(3))
print(nums.index(8))
print(sorted(nums))
print(nums)

Output

5 8 1 20
True
2
2
[1, 3, 3, 5, 8]
[5, 3, 8, 3, 1]

Md Mahbub E NoorLecture 3.2: Lists and tuplesUniversity of Barishal11 / 29

sort() versus sorted()

Predict first

Code

nums = [5, 3, 8]
result = nums.sort()
print(nums)
print(result)

What will this print? Write your prediction as a comment before you run the cell.

Md Mahbub E NoorLecture 3.2: Lists and tuplesUniversity of Barishal12 / 29

Answer: sort() versus sorted()

Code

nums = [5, 3, 8]
result = nums.sort()
print(nums)
print(result)

Output

[3, 5, 8]
None

.sort() sorts the list itself, in place
It returns nothing, which Python shows as None
sorted(nums) returns a new sorted list instead
Never write nums = nums.sort(): you lose the list

Md Mahbub E NoorLecture 3.2: Lists and tuplesUniversity of Barishal13 / 29

Strings behave like lists, almost

Strings have positions and slices too
len() counts characters
But strings cannot be changed in place
word[0] = "J" gives an error (see the mistakes slide)

Code

word = "PYTHON"
print(word[0], word[-1])
print(word[1:4])
print(len(word))

Output

P N
YTH
6

Md Mahbub E NoorLecture 3.2: Lists and tuplesUniversity of Barishal14 / 29

Anatomy of a tuple

Code

student = ("Nadia", 21, "CSE")
print(student[0])
print(len(student))

Output

Nadia
3

Round brackets make a tuple

Items, read with positions like a list

Commas separate items

Indexing works the same as lists

Md Mahbub E NoorLecture 3.2: Lists and tuplesUniversity of Barishal15 / 29

Tuples cannot be changed

Reading works; changing gives a TypeError
Use tuples for fixed records: a student, a date, a coordinate
Fixed means safe: no code can change it by accident
Functions use tuples to return several values (Lecture 4.1)

Code

student = ("Nadia", 21, "CSE")
print(student[1])
student[1] = 22

Output

21
TypeError: 'tuple' object does not support item assignment

Md Mahbub E NoorLecture 3.2: Lists and tuplesUniversity of Barishal16 / 29

Unpacking: one tuple, several names

name, age, dept = student gives each item a name
The number of names must match the number of items
a, b = b, a swaps two values in one line
You will use this constantly with functions

Code

name, age, dept = student
print(name)
print(age)
a, b = 1, 2
a, b = b, a
print(a, b)

Output

Nadia
21
2 1

Md Mahbub E NoorLecture 3.2: Lists and tuplesUniversity of Barishal17 / 29

List or tuple?

List [ ]

Square brackets
Can change: append, remove, replace
For collections that grow: messages, scores
Example: an inbox

Tuple ( )

Round brackets
Fixed once created
For records: a student, a coordinate
Example: several values returned by a function

Md Mahbub E NoorLecture 3.2: Lists and tuplesUniversity of Barishal18 / 29

Careful: two names, one list

b = a does not copy the list
Both names point to the same list
Change it through b, and a sees the change
Use a.copy() for a real copy

Code

a = [1, 2, 3]
b = a
b.append(4)
print(a)
c = a.copy()
c.append(5)
print(a)
print(c)

Output

[1, 2, 3, 4]
[1, 2, 3, 4]
[1, 2, 3, 4, 5]

Md Mahbub E NoorLecture 3.2: Lists and tuplesUniversity of Barishal19 / 29

Watch it run: changing a list, and two names

WATCH FOR

append, replace and pop change the list
pop hands the removed item back
b = tasks: the note says 'same object'
b.append changes what tasks shows too

Silent video: press play, pause any time.

Md Mahbub E NoorLecture 3.2: Lists and tuplesUniversity of Barishal20 / 29

Common mistakes with lists and tuples

Read the last line of an error first. It names the problem.

What you typed	What Python says	What it means
print(inbox[10])	IndexError: list index out of range	No position 10. Valid positions: 0 to len(inbox) - 1.
student[1] = 22	TypeError: 'tuple' object does not support item assignment	Tuples cannot be changed. Use a list, or build a new tuple.
nums.remove(99)	ValueError: list.remove(x): x not in list	99 is not in the list. Check with 99 in nums first.
word[0] = "J"	TypeError: 'str' object does not support item assignment	Strings cannot be changed. Build a new one: "J" + word[1:].

Md Mahbub E NoorLecture 3.2: Lists and tuplesUniversity of Barishal21 / 29

In-class exercises

Work in pairs. 15 minutes. No loops yet: use indexing, slicing and list functions.

Second largest

Use sorted() to print the second largest of [4, 9, 2, 7]. Expected: 7. Bonus: what goes wrong with [4, 9, 9, 2]?

Ends of the inbox

Print the first two and the last two messages of a 6-message list, using slices.

Swap ends

Swap the first and last items of a list. Test: [1, 2, 3, 4] becomes [4, 2, 3, 1].

Student record

Store (name, roll, cgpa) as a tuple, unpack it, and print one f-string sentence.

Md Mahbub E NoorLecture 3.2: Lists and tuplesUniversity of Barishal22 / 29

Solutions

Show these after students have tried the exercises.

Md Mahbub E NoorLecture 3.2: Lists and tuplesUniversity of Barishal23 / 29

Solutions 1 and 2

The bonus shows a real limitation: with a repeated maximum, [-2] returns the maximum again.

Code

nums = [4, 9, 2, 7]
print(sorted(nums)[-2])
print(sorted([4, 9, 9, 2])[-2])

inbox = ["m1", "m2", "m3", "m4", "m5", "m6"]
print(inbox[:2])
print(inbox[-2:])

Output

7
9
['m1', 'm2']
['m5', 'm6']

Md Mahbub E NoorLecture 3.2: Lists and tuplesUniversity of Barishal24 / 29

Solutions 3 and 4

Swapping uses tuple unpacking. Unpacking gives each field of a record its own name.

Code

items = [1, 2, 3, 4]
items[0], items[-1] = items[-1], items[0]
print(items)

record = ("Nadia", 1042, 3.71)
name, roll, cgpa = record
print(f"{name} (roll {roll}) has CGPA {cgpa}")

Output

[4, 2, 3, 1]
Nadia (roll 1042) has CGPA 3.71

Md Mahbub E NoorLecture 3.2: Lists and tuplesUniversity of Barishal25 / 29

Interactive lab: Lists Lab

In the lab you can

List playgroundAppend, insert, remove and pop: see each box change.
SlicingDrag start, stop and step and watch which items are taken.
Ask the listEdit marks: len, max, sum, sorted and a live chart.
Two names, one listArrows show when b = a shares one list.

Open the lab ►︎

Opens in a new tab and works offline. All labs: labs/START_HERE.html

Md Mahbub E NoorLecture 3.2: Lists and tuplesUniversity of Barishal26 / 29

Today's toolkit

`[a, b, c]`	Make a list
`x[0], x[-1]`	First item, last item
`x[1:3]`	Items 1 and 2 (stop before 3)
`append / insert`	Add items
`remove / pop`	Delete items

`sorted(x) / x.sort()`	New sorted list / sort in place
`(a, b, c)`	Make a tuple (fixed)
`n, a, d = t`	Unpack a tuple
`x.copy()`	A real, separate copy

Md Mahbub E NoorLecture 3.2: Lists and tuplesUniversity of Barishal27 / 29

Homework 3.2

Due before Lecture 3.3 · [date]

Predict, then run

Predict first: x = [3, 1, 2]; print(x[::-1]); print(x[5:]); print(sorted(x) == x).

Grade book

Make a list of 5 marks. Print the highest, lowest, average (sum / len) and the marks sorted.

Inbox editor

Start with 3 messages. Add 2, replace the second, remove one by value, and print the list after each step.

Trace it

Draw a trace table (step, list after) for the Inbox editor code by hand, then check it by printing.

Explain

In 2 sentences: why does b = a followed by b.append(4) change a?

Submit on [LMS / Google Classroom]: your notebook file HW3_2_<your ID>.ipynb, or a Colab link set to “Anyone with the link can view”.

Md Mahbub E NoorLecture 3.2: Lists and tuplesUniversity of Barishal28 / 29

Recap: Lecture 3.2 in 90 seconds

Watch it again at home to revise.

Md Mahbub E NoorLecture 3.2: Lists and tuplesUniversity of Barishal29 / 29