Lists & Tupels

Stack

a "stack" essentially has 2 functions
one with which you can put something on the stack
and one to remove something from the top level of a stack

method	meaning
push	adds something "on top" to the stack python does not have a function called push but append performs the same functionality
pop	return the top object of the stack, removing it in the process
peek	allows you to read the "top" value without removing it as with pop

Stacks In Python

• s.append(x)

  Add an item to the end of the list.

• s.pop(i)

  prints the i-th element of an lsite and removes it in the process

• s.pop()

  If i is not specified, the last object is used

colors = ["red", "green"]
colors.append("blue")
colors
# ['red', 'green', 'blue']

c = colors.pop()
c
# 'blue'

colors.append(colors.pop())
colors
# ['red', 'green']

Extend

extend is used to append multiple elements to a list

s.extend(t)

t must be an iterable object

fib = [0, 1, 1, 2, 3, 5]
fib.extend([8, 13, 21])
fib
# [0, 1, 1, 2, 3, 5, 8, 13, 21]

if you pass a string it will be split into its individual letters

list1 = []
list1.extend("Test123")
list1
# ['T', 'e', 's', 't', '1', '2', '3']

"+" Operator

bli = [4, 2]
bli = bli + [123]
bli
# [4, 2, 123]

this method is also possible but much slower than the append method

bli = [4, 2]
bli += [123]
bli
# [4, 2, 123]

this is better, but still not as fast as the append method

Remove a Value

s.remove(x)

with remove the first occurrence of the value x can be removed

if x is not present a ValueError is generated

Count

if you want to count how many times an element is included in a list you can use .count

bli = [4, 2, 123, 4, 2, 2, 2, 42]
bli.count(2)
# 4

bli.count(123)
#1

Position

with index you get the position of an element within a list

s.index(x[ , i [, j ] ] )

the index for the x is determined
if the parameter i is given the search starts at this position and ends at j

bli = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13]
bli.index(1)
# 0

bli.index(10 , 8)
# 9

bli.index(10 , 6, 8)
# Traceback (most recent call last):
#   File "<stdin>", line 1, in <module>
# ValueError: 10 is not in list

Insert

with append you can append an element to the end of a list but with insert you can insert an element at any place you want

s.insert(index, object)

bli = [1, 2, 3, 4, 5, 6, 7]
bli.insert(1, 42)
bli
#[1, 42, 2, 3, 4, 5, 6, 7]

Tupels

as we already know tuples are immutable means methods like append, insert , pop... do not exist/work

Empty tuples

empty tuples can be defined with an empty bracket

t = ()
type(t)
# <class 'tuple'>

Single Digit Tuples

let's try to define a tuple with only one element

t = (1)
type(t)
# <class 'int'>

does not look good but works

t = (1,)
type(t)
# <class 'tuple'>

Packing & Unpacking

another option to define a tuple goes like this:

f = "test", "test123"
type(f)
# <class 'tuple'>

it is called tuple unpacking when you assign the individual values of a tuple to variables

(first, second) = f
print(first, second)
# test test123

(first, *everything_else) = 1, 2, 3, 42, 123
print(first)
# 1

print(everything_else)
# [2, 3, 42, 123]

Unchangeable?

tuples cannot be modified
but we can edit objects like lists
because the tuple only contains a pointer to this list

test = ([],)
test[0].append("bli bla")
test
# (['bli bla'],)

Sort Lists

.sort

.sort will sort the list inplace

test123 = [42, 123, 2, 1, 4, 3,]
test123.sort()
test123
# [1, 2, 3, 4, 42, 123]

sorted

sorted returns a sorted list and the original list remains unchanged

test123 = [42, 123, 2, 1, 4, 3,]
test = sorted(test123)
test123
# [42, 123, 2, 1, 4, 3]

test
# [1, 2, 3, 4, 42, 123]

reverse

to reverse the sort order set the parameter reverse to True (reverse=True)

this works for .sort and sorted

test123 = [42, 123, 2, 1, 4, 3,]
test = sorted(test123, reverse=True)
test
# [123, 42, 4, 3, 2, 1]

test123.sort(reverse=True)
test123
# [123, 42, 4, 3, 2, 1]

Own Sorting Function

if you don't want to sort a list by the standard order relation you can use the parameter key

test = ["Green", "yellow", "blue", "Black", "red"]
sorted(test)
# ['Black', 'Green', 'blue', 'red', 'yellow']

test = ["Green", "yellow", "blue", "Black", "red"]
sorted(test, key=str.lower)
# ['Black', 'blue', 'Green', 'red', 'yellow']

itemgetter

if you want to sort lists containing tuples you can use itemgetter

sort by the 0-th index:

from operator import itemgetter

test123 = [("John", "Doe", "1337"),
            ("Maria", "Sigibert", "1230"),
            ("Folcmar", "Kester", "4200"),]

sorted(test123, key=itemgetter(0))
# [('Folcmar', 'Kester', '4200'),
#  ('John', 'Doe', '1337'),
#  ('Maria', 'Sigibert', '1230')]

sort after the first index:

sorted(test123, key=itemgetter(1))
# [('John', 'Doe', '1337'),
# ('Folcmar', 'Kester', '4200'),
# ('Maria', 'Sigibert', '1230')]

sort after the second index

sorted(test123, key=itemgetter(2))
# [('John', 'Doe', '1337'),
#  ('Maria', 'Sigibert', '1230'),
#  ('Folcmar', 'Kester', '4200')]