Contents

Collections

Collections

  • Lists

    • Indexing

    • Mutating

  • Tuples

Assignment Reminders

  • To make life easier on your wonderful TAs:

    • Please don’t change the assignment file name

    • Please don’t delete/modify cells in assignment (other than cells where you add code)

    • You can add cells and include print() statements to check your work.

  • There are hidden assert cells

    • If you follow instructions and pass all the asserts provided, you will receive credit

    • We cannot give points back for typos

  • You must submit your assignment for it to be graded.

    • best to “Kernel > Restart & Run All” before you submit

      • this executes code in your notebook from top to bottom

      • You can also use “Validate” button at top. Note that for A4 you may get an error b/c that notebook takes a while to run

    • submit as many times as you like

      • most recent submission is graded

      • but, if you submit before deadline and again after, it will be late

  • variables defined in %%writefile cells will overwrite variables in test (assert) cells

Collections: Lists

A list is a mutable collection of ordered items, that can be of mixed type. Lists are created using square brackets.

List examples

# Define a list
lst = [1, 'a', True]

# Print out the contents of a list
print(lst)

[1, 'a', True]

# Check the type of a list
type(lst)

list

Clicker Question #1

Which of the following specifies a list of 4 items?

item_A = [0, 'string', 18]
item_B = (0, 'string', 18, 'name')
item_C = [0, 'string', 18, 'name']
item_D = (0, 'string', 18)
item_E = [1234]

  • A) item_A

  • B) item_B

  • C) item_C

  • D) item_D

  • E) item_E

Indexing

Indexing refers to selecting an item from within a collection. Indexing is done with square brackets. 
# Define a list 
my_lst = ['Julian', 'Amal', 'Richard', 'Juan', 'Xuan']

# Indexing: Count forward, starting at 0, with positive numbers
print(my_lst[1])

Amal

# Indexing: Count backward, starting at -1, with negative numbers
print(my_lst[-1])

Xuan

# Indexing: Grab a group of adjacent items using `start:stop`, called a slice
print(my_lst[2:4])

['Richard', 'Juan']

# can determine type in list
type(my_lst[2])

str

my_lst[2:-1]

['Richard', 'Juan']

# indexing to end of list
print(my_lst[2:])

['Richard', 'Juan', 'Xuan']

# Indexing from beginning of list
print(my_lst[:4])

['Julian', 'Amal', 'Richard', 'Juan']

# slicing by skipping a value [start:stop:step]
print(my_lst[0:4:2])

['Julian', 'Richard']

Index Practices

# Define a list for the examples
example_lst = [1, 2, 3, 4, 5]

example_lst[2]

3

example_lst[-3]

3

example_lst[1:3]

[2, 3]

Clicker Question #2

What will be the output of the following piece of code:

q2_lst = ['a', 'b', 'c','d']
q2_lst[-3:-1]

['b', 'c']

  • A) [‘a’, ‘b’, ‘c’]

  • B) [‘c’, ‘b’, ‘a’]

  • C) [‘c’, ‘b’]

  • D) [‘b’, ‘c’, ‘d’]

  • E) [‘b’, ‘c’]

Note: The following has been added to the notes due to student questions in previous iterations. This and the following two cells are not someting you’ll be tested on. Including as an FYI for those curious.

You can return [‘c’,’b’] but it combines two different concepts.

  1. the start:stop now refers to indices in the reverse.

  2. -1 is used as the step to reverse the output.

More details about step:
step: the amount by which the index increases, defaults to 1. If it’s negative, you’re slicing over the iterable in reverse.

# slice in reverse
q2_lst[-2:-4:-1]

['c', 'b']

# you can use forward indexing
# makes this a little clearer
q2_lst[2:0:-1]

['c', 'b']

Clicker Question #3

What would be the appropriate line of code to return ['butter', '&', 'jelly']?

q3_lst = ['peanut', 'butter', '&','jelly']

  • A) q3_lst[2:4]

  • B) q3_lst[1:3]

  • C) q3_lst[:-2]

  • D) q3_lst[-3:]

  • E) q3_lst[1:4:2]

Reminders

  • Python is zero-based (The first index is ‘0’)

  • Negative indices index backwards through a collection

  • A sequence of indices (called a slice) can be accessed using start:stop

    • In this contstruction, start is included then every element until stop, not including stop itself

    • To skip values in a sequence use start:stop:step

SideNote: But why is it like this…

Starting at zero is a convention (some) languages use that comes from how variables are stored in memory , and 'pointers' to those locations.

Length of a collection

# Define a new list
another_lst = ['Peter', 'Hind', 'Jack', 'Pam', 'Barbara', 'Colin', 'Felix']

# Get the length of the list, and print it out
len(another_lst)

7

The in Operator

The in operator asks whether an element is present inside a collection, and returns a boolean answer. 
# Define a new list to work with
lst_again = [True, 13, None, 'apples']

# Check if a particular element is present in the list
True in lst_again

True

# The `in` operator can also be combined with the `not` operator
'19' not in lst_again

True

Practice with in

# Define a list to practice with
practice_lst = [1, True, 'alpha', 13, 'cogs18']

13 in practice_lst

True

False in practice_lst

False

'True' in practice_lst

False

#searching partial strings
'cogs' in practice_lst

False

'cogs18' not in practice_lst

False

Clicker #4

After executing the following code, what will be the value of output?

ex2_lst = [0, False, 'ten', None]

bool_1 = False in ex2_lst
bool_2 = 10 not in ex2_lst

output = bool_1 and bool_2

print(output)

True

  • a) True

  • b) False

  • c) This code will fail

  • d) I don’t know

Reminder

  • The in operator checks whether an element is present in a collection, and can be negated with not

Mutating a List

Lists are mutable, meaning after definition, you can update and change things about the list. 
# Define a list
updates = [1, 2, 3]

# Check the contents of the list
print(updates)

[1, 2, 3]

# Redefine a particular element of the list
updates[1] = 0

# Check the contents of the list
print(updates)

[1, 0, 3]

Clicker Question #5

What would the following code accommplish?

lst_update = [1, 2, 3, 0, 5]
lst_update[3] = 4

  • A) replace 0 with 4 in lst_update

  • B) replace 4 with 0 in lst_update

  • C) no change to lst_update

  • D) produce an error

  • E) I’m not sure

Collections: Tuples

A tuple is an immutable collection of ordered items, that can be of mixed type. Tuples are created using parentheses.

Tuple Examples

# Define a tuple
tup = (2, 'b', False)

# Print out the contents of a tuple
print(tup)

(2, 'b', False)

# Check the type of a tuple
type(tup)

tuple

# Index into a tuple
tup[0]

2

# Get the length of a tuple
len(tup)

3

Tuples are Immutable

# Tuples are immutable - meaning after they defined, you can't change them
# This code will produce an error.
tup[2] = 1

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-43-6b0fd3f24bc7> in <module>()
      1 # Tuples are immutable - meaning after they defined, you can't change them
      2 # This code will produce an error.
----> 3 tup[2] = 1

TypeError: 'tuple' object does not support item assignment

Clicker Question #6

Which of the following specifies a tuple of 4 items?

item_A = [0, 'string', 18]
item_B = (0, 'string', 18, 'name')
item_C = [0, 'string', 18, 'name']
item_D = (0, 'string', 18)
item_E = [1234]

  • A) item_A

  • B) item_B

  • C) item_C

  • D) item_D

  • E) item_E

A1: Clarification/Demo

  • variables created in %%writefile will be stored in file

  • these variables will overwrite variables created in test cells

%%writefile new_file.py
# You can ignore the line above - it is used to help check your code

a= 3
adfasf = 'asdf'
b = 'new'
print(status)

Overwriting new_file.py

status = False

%run -i new_file.py  
assert status == False

False

Student Question:

  • slice of a bigger slice

  • multiple slices

# slice of a bigger slice
my_list = ['a', 'b', 'c', 'd', 'e', 'f', 'g']

# create first slice
first_slice = my_list[4:]
print('first slice: ', first_slice)

# second slice with new indices
second_slice = first_slice[1:]
print('second slice: ', second_slice)

first slice:  ['e', 'f', 'g']
second slice:  ['f', 'g']

# multiple slices 
slice_one = my_list[4:]
slice_two = my_list[:2]

# put them together
new_list = slice_one + slice_two
new_list

['e', 'f', 'g', 'a', 'b']

Aside: Aliases

Note: This was introduced in the Variables lecture.

# Make a variable, and an alias
a = 1
b = a
print(b)

1

Here, the value 1 is assigned to the variable a.

We then make an alias of a and store that in the variable b.

Now, the same value (1) is stored in both a (the original) and b (the alias).

What if we change the value of the original variable (a) - what happens to b?

Clicker Question #7

After executing the following code, what will the values stored in a and b be?

# Make a variable & an alias
# change value of original variable
a = 1
b = a
a = 2

print(a)
print(b)

2
1

  • A) a and b both store 1

  • B) a and b both store 2

  • C) a stores 2 b stores 1

  • D) a stores 1 b stores 2

  • E) No clue

Reminder: integers are immutable.

Alias: mutable types

What happens if we make an alias of a mutable variable, like a list?

first_list = [1, 2, 3, 4]
alias_list = first_list
alias_list

[1, 2, 3, 4]

#change second value of first_list
first_list[1] = 29
first_list

[1, 29, 3, 4]

# check alias_list
alias_list

[1, 29, 3, 4]

For mutable type variables, when you change one, both change.

Clicker Question #8

After executing the following code, what will the second value stored in second_tuple?

# Make a variable & an alias
# change value of original variable
my_tuple = (1, 2, 3, 4)
second_tuple = my_tuple
my_tuple[1] = 29 

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
~/Desktop/Teaching/COGS18/Materials/new_file.py in <module>()
      3 my_tuple = (1, 2, 3, 4)
      4 second_tuple = my_tuple
----> 5 my_tuple[1] = 29

TypeError: 'tuple' object does not support item assignment

  • A) 1

  • B) 2

  • C) 29

  • D) This will Error

  • E) I’m lost.

Why allow aliasing?

Aliasing can get confusing and be difficult to track, so why does Python allow it?

Well, it’s more efficient to point to an alias than to make an entirely new copy of a a very large variable storing a lot of data.

Python allows for the confusion, in favor of being more efficient.

Strings as Collections

Strings act similarly to ordered collections of homogenous elements - specifically characters. But, they are immutable. 
# Define a string
my_str = 'TheFamousFive'

# Index into a string
my_str[2]

'e'

# Ask if an item is in a string
'Fam' in my_str

True

# Check the length of a string
len(my_str)

13

# Index into a string
# This code will produce an error
my_str[1:3] = 'HE'

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
~/Desktop/Teaching/COGS18/Materials/new_file.py in <module>()
      1 # Index into a string
      2 # This code will produce an error
----> 3 my_str[1:3] = 'HE'

TypeError: 'str' object does not support item assignment

SideNote: using counters

# Initialize a counter variable
counter = 0 
print(counter)

0

counter = counter + 1
print(counter)

2

counter = counter + 1
print(counter)

4

Pulling it Together: Collections, Membership & Conditionals

Clicker Question #9

What will be the value of counter after this code is run?

things_that_are_good = ['python', 'data', 'science', 'tacos']

counter = 0

if 'python' in things_that_are_good:
    counter = counter + 1

if len(things_that_are_good) == 4:
    counter = counter + 1
    
if things_that_are_good[2] == 'data':
    counter = counter + 1 
    
print(counter)

2

 A) 0   B) 1   C) 2   D) 3   E) 4

Clicker Question #10

What will be printed out from running this code?

lst = ['a', 'b', 'c']
tup = ('b', 'c', 'd')

if lst[-1] == tup[-1]:
    print('EndMatch')
elif tup[1] in lst:
    print('Overlap')
elif len(lst) == tup:
    print('Length')
else:
    print('None')

Overlap

 A) EndMatch   B) Overlap   C) Length   D) Overlap & Match   E) None

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Conditionals

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Control Flow - Loops