Section 08: Lists and Nested Lists Flashcards
Define Lists:
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]
Accessing elements in a list
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
Operators + and * in lists:
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]
in and not in in lists
- 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]
Two options for loops for lists?
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
~~~
How to modify lists:
- Strings - immutable; lists are not
- Add elements using
append()andextend()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
-
Erros
TypeError
TypeError if elements are not comparable or right object type
Errors
ValueError
ValueError is raised if no such element
List objects:
-
len(a): returns num elements in a lista -
sum(a): returns sum all elemetns ina(if they are numbers) -
min(a), max(a): returns smallest/largest element inaTypeError if elements are not comparable or right object type -
a.append(x)adds elements to end of list -
a.extend(x)adds elements of one list to another list -
a.insert(x)adds element at a given index -
a.remove(x)removes element by valueValueError is raised if no such element -
a.pop(i): removes and returns the i-th element from the lista. If no input is specified, the last element is removed and returned. -
a.count(x): returns num of occurrences of the element x in lista -
a.index(x): returns index of first occurence of the element x in lista.ValueError is raised if no such element -
a.sort():sorts element of the list in a ‘in place’. This means thatais modified after execution of this method
Common list pattersn
what are common list patterns?
- reducing,
- mapping,
- filtering
Common List Patterns
(1) reducing
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
Common List Patterns
(2) Mapping
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
Common List Patterns
(3) Filtering
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
~~~
Altering lists
repeating lists
Repeating Lists:
- Use the
*operator to duplicate elements
Altering Lists
Comparing Lists
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
Altering lists
Adding elements to a list:
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’)
~~~
Define: nested loop
Conditional statements: multiple levels of loops
Print out a table with rows and columns using a nested loop:
```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(‘——’)
~~~
Functions
Purpose of shift_up()
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]
~~~
Define nested lists:
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
Nested lists as matrices
- 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
~~~
Row-Major Indexing:
Alternative: Column-Major Indexing:
```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]) ~~~
Column-Major Indexing:
Alternative: Row-Major Indexing
```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]) ~~~
How to convert from a list to a string:
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
~~~
How to convert from a string to a list:
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’]
~~~
Differentiate between functions and methods:
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()
