Section 08: Lists and Nested Lists

Question 1

Define Lists:

Answer 1

List: contains number of arbitrary ordered elements (values), each of which can be accosiated with an index

• Created w/ square brackets: [A1, ..., An]
• Index starts from $0$List w/ zero elements: my_list = []
• len(my_list) used to tell how many elements in a list
• Any combination of object type: numbers = [1, "two", 2.22]

Question 2

Accessing elements in a list

Answer 2

Access using name of list variable and index inside sqaure brackets

• Positive indexing: my_list[2]
• Negative indexing: my_list[-1]
• Range of elements: my_list[2:5]
• Whole list: my_list[:]

If not element in index[1] ⇒ IndexError

Question 3

Operators + and * in lists:

Answer 3

Concatenate Lists

(1) Concatenate two lists using + operator:
~~~
»>list1 = [1, 2, 3]
»>list2 = [4, 5, 6]

> > > list3 = list1 + list2
print(list3)

[1, 2, 3, 4, 5, 6]
~~~

(2) Create a new list from an existing list using * operator: (replicate)

>>>list4 = list1 * 3
>>>print(list4)

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

Question 4

in and not in in lists

Answer 4

• Test for membership
• Use to test if an object is an element of a list

>>> list5 = [10, 20, 30, 40]

#Check if 10 is in list
>>> if 10 in list5:
        print("Yes")
Yes

#Check if 50 is not in list
>>> if 50 not in list5:
        print("No")
No

Good Style:

• Not Good: if x == 5 or x == 7
• Better: if x in [5,7]

Question 5

Two options for loops for lists?

Answer 5

Option 02:

```python
# Option 01:
def print_list(my_list):
for index in range(len(my_list)): #range(3)
print(“[”, index,”] = “, my_list[index])

for element in my_list:
print(element)

> > > my_list = [4, 6, 19]
print_list(my_list)

[0] = 4
[1] = 6
[2] = 19
~~~

Question 6

How to modify lists:

Answer 6

• Strings - immutable; lists are not
• Add elements using append() and extend() methods
  
  • append() adds elements to end of list
  • extend() adds elements of one list to another list
  • insert() adds element at a given index
  • remove() removes element by value
  • pop() removes element by index
  • clear() removes all elements from list
  • sort() sorts list in place (ascending/descending)
  • reverse() reverses order of list in place

Question 7

Erros

TypeError

Answer 7

TypeError if elements are not comparable or right object type

Question 8

Errors

ValueError

Answer 8

ValueError is raised if no such element

Question 9

List objects:

Answer 9

1. len(a): returns num elements in a list a
2. sum(a): returns sum all elemetns in a (if they are numbers)
3. min(a), max(a): returns smallest/largest element in aTypeError if elements are not comparable or right object type
4. a.append(x) adds elements to end of list
5. a.extend(x) adds elements of one list to another list
6. a.insert(x) adds element at a given index
7. a.remove(x) removes element by valueValueError is raised if no such element
8. a.pop(i) : removes and returns the i-th element from the list a. If no input is specified, the last element is removed and returned.
9. a.count(x): returns num of occurrences of the element x in list a
10. a.index(x): returns index of first occurence of the element x in list a.ValueError is raised if no such element
11. a.sort(): sorts element of the list in a ‘in place’. This means that a is modified after execution of this method

Question 10

Common list pattersn

what are common list patterns?

Answer 10

1. reducing,
2. mapping,
3. filtering

Question 11

Common List Patterns

(1) reducing

Answer 11

Reducing a list to a value

Example: find the sum of all elements in a list

def sum_list(my_list):
	sum = 0
	for element in my_list:
		sum += element
	return sum

Question 12

Common List Patterns

(2) Mapping

Answer 12

Transforming a list to another list; traverse one list while building another ⇒ “map”

Output: new list

Example: double each element in a list

def double_list(my_list):
	double_list = []
	for element in my_list:
		double_list.append(element * 2)
	return double_list

Question 13

Common List Patterns

(3) Filtering

Answer 13

Selecting elements from a list

Output: a new list

Example: select even elements from a list
~~~
def even_list(my_list):
even_list = []
for element in my_list:
if element % 2 == 0:
even_list.append(element)
return even_list

~~~

Question 14

Altering lists

repeating lists

Answer 14

Repeating Lists:

• Use the * operator to duplicate elements

Question 15

Altering Lists

Comparing Lists

Answer 15

Comparing Lists:

• The == operator to each of two lists are equal
• Equal if they have same number of elements; elements
• Also use != to check if they are different

Question 16

Altering lists

Adding elements to a list:

Answer 16

Adding Elements to a list:

• Remember strings are immutable - cant change on index leve l
• Must concatinate:

```jsx
»> x = []
»> x = x + [‘first’]
OR
»> x = []
»> x.append(‘first’)
~~~

Question 17

Define: nested loop

Answer 17

Conditional statements: multiple levels of loops

Question 18

Print out a table with rows and columns using a nested loop:

Answer 18

```jsx
cols = 12
rows = 12

for j in range(1, rows+1)
for i in range(1, cols+1)
print(j*i, end = ‘\t’)
print(‘——’)
~~~

Question 19

Functions

Purpose of shift_up()

Answer 19

voidfunction shift_up() that takes a list as input and modifies it so that all the elem are moved up one position

```python
»> a = [1, 2, 3, 4, 5]
»> shift_up(a)
»> a
[5, 1, 2, 3, 4]
~~~

Question 20

Define nested lists:

Answer 20

What if the lists were lists themselves …. ? Nested Lists

Ex: some_numbers = [[1], [1,2,3], [1,2]]

• Length: 3 (containing lists of integers)
• First element: list of length 1 …

Lists are mutable

Question 21

Nested lists as matrices

Answer 21

• Use nested lists to represent matrix of numbers
• Inner lists = one “row” in the matrix

```python
movie_ave = [[81,75,90],
[80, 55, 10],
[40, 90, 99]]

print(movie_ave[1][2]) # Row index and Column index
10
~~~

Question 22

Row-Major Indexing:

Alternative: Column-Major Indexing:

Answer 22

```python
for r in range(num_rows):
# Sequence 0 … num of rows - 1
print(‘Printing row”, r)

for c in range(num_cols):
	# Sequence 0 ... num of cols - 1 
	print(r, c, matrix[r][c]) ~~~

Question 23

Column-Major Indexing:

Alternative: Row-Major Indexing

Answer 23

```python
for c in range(num_cols):
#Sequence 0 … num of cols - 1
print(‘Printing column,’ c)

for r in range(num_rows): 
	#Sequence 0 ... num rows - 1 
	print(r, c, matrix[r][c]) ~~~

Question 24

How to convert from a list to a string:

Answer 24

The join() function:

Join a list of strings into a single string (inverse of split())

• Can join any iterable (lists, tuples, sets, etc)

list_example = ['hello', 'world']
string_example = ' '.join(list_example)
print(string_example)

Output
hello world

Optional argument called delimiter specifies which character to use to join the elements
~~~
list_example = [‘hello’, ‘world’]
string_example = ‘,’.join(list_example)
print(string_example)

Output
hello,world
~~~

Question 25

How to convert from a string to a list:

Answer 25

The list() function:

**********List function breaks a string into individual letters**********

string_example = 'hello world'
list_example = list(string_example)
print(list_example)

Output
['h', 'e', 'l', 'l', 'o', ' ', 'w', 'o', 'r', 'l', 'd']

The split() function:

******Break a string into words******

string_example = 'hello world'
list_example = string_example.split()
print(list_example)

Output
['hello', 'world']

• Optional argument called delimiter specifies which character to use as word boundaries
  ~~~
  string_example = ‘hello,world’
  list_example = string_example.split(‘,’)
  print(list_example)

Output
[‘hello
world’]
~~~

Question 26

Differentiate between functions and methods:

Answer 26

int and round are built in function, get_pseudo is created

> Function: “stand-alone” program that you can call from your program

```python
a = int(‘1234’)
b = round(12.3)
c = get_pseudo()

~~~

> Method Function that “belongs” to an object, call using the dot notation
object.method_name()