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List methods

Modifying Lists

  • lists are mutable (we can add, remove, or modify elements in a list.)
  • strings are not mutable. 
  • Changing the list means, keep the same box and add, remove, or change the elements inside that box.
method what it does
list[i] = x Replaces the element at index i with x
list.append(x) Inserts x at the end of the list
list.insert(i, x) Inserts x at index i
list.remove(x) Removes the first occurrence of x in the list
list.sort(<key=None>, <reverse=False>) Sorts the items in the list
list.reverse() Reverses the order of items of the list
list.clear() Removes all the items of the list
list.copy() Creates a copy of the list
list.extend(other_list) Appends all the elements of other_list at the end of list


returns values - pop() method 

  • list.pop(i)
  • remove elements
  • method receives an index.
  • returns the element that was removed at the index that was passed.
  • to change an item by assigning something else to that position 
  • Returns the element a index i, also removing it from the list. If i is omitted, the last element is returned and removed.
fruits = ["Orange", "Pineapple", "Banana", "Apple"]
fruits.pop(3)
print(fruits)
["Orange", "Pineapple", "Banana"]

returns values - count() method 

li.count('b') # number of occurrences
2

returns values - index() method 

  • list.index([,[, ]])

**returns values - count() method** 

  • list.count()


append() method

  • adding an element to a list (adds a new element at the end of the list.)
  • list.append(“New data”) would add the string “New data” to the end of the list.
  • mylist.append()
fruits = ["apple", "banana", "cherry"]
fruits.append("orange")
print(fruits)
['apple', 'banana', 'cherry', 'orange']


insert() method

  • to add an element to a list in a specific position, instead of at the end.
  • two parameters:
    • mylist.insert(,)an index as the first parameter
    • element as the second parameter. 
list.insert(0, "New data")
  • would add the string “New data” to the front of the list.
  • This wouldn’t overwrite the existing element at the start of the list.
  • It would just shift all the other elements by one.
  • If you specify an index that’s larger than the length of the list, the element will simply be added to the end of the list.
fruits = ["Pineapple", "Banana", "Apple", "Melon"]
fruits.insert(0,"Orange")
print(fruits)
["Orange", "Pineapple", "Banana", "Apple", "Melon"]

use a number larger than the length of the list

fruits.insert(25,"Peach")
print(fruits)
["Orange", "Pineapple", "Banana", "Apple", "Melon", "Peach"]

No errors. If an index higher is than the current length, the element just gets added to the end. Y

  • usually, you either
    • add at the beginning using insert() at the zero index
    • add at the end using append(). 
fruits = ["Orange", "Pineapple", "Banana", "Kiwi","Peach"]
fruits[2] = "Strawberry"
print(fruits)
["Orange", "Pineapple", "Strawberry", "Banana", "Kiwi","Peach"]

remove() method

li.remove('b') # remove first occurrence
li
['a', 'b', 'c']

sort() method

strs = ['aa', 'BB', 'zz', 'CC']
print(sorted(strs)) 
## ['BB', 'CC', 'aa', 'zz'] (case sensitive)

print(sorted(strs, reverse=True))   
## ['zz', 'aa', 'CC', 'BB']

li.sort() # sort the list *in place*
li
[2, 5, 6, 8]

reverse() method

li = [5, 2, 6, 8] #reverses the list in place (does not return it)
li.reverse() 
li
[8, 6, 2, 5]

extend() method

  • adds the specified list elements (or any iterable) to the end of the current list.
  • mylist.extend()
	fruits = ['apple', 'banana', 'cherry']
	cars = ['Ford', 'BMW', 'Volvo']
	fruits.extend(cars)
	print(fruits)
	['apple', 'banana', 'cherry', 'Ford', 'BMW', 'Volvo']

slice method

  • use indexes to create a slice of the list. 
# the range starts at index one and goes up to the second index minus one.

x[1:3]

# First value defaults to zero and the second value to the length of the list.

x[:2]

split() method

  • splits a string into a list.
# Example 1
txt = "welcome to the wild"
x = txt.split()
print(x)

#OUTPUT
#['welcome', 'to', 'the', 'wild']

replace via indexing you can change an element in a list

  • to overwrite the value stored at the specified index.
  • to overwrite the first element in a list with the new string “Old data”. 
      list[0] = "Old data"

remove() method

  • remove elements from the list using the value of the element we want to remove. 
  • removes the first occurrence of the element from the list.
  • If the element is not in the list - we get a value error (“element isn’t in the list”).  Takes an element as a parameter, and removes the first occurrence of the element. If the element isn’t found in the list, you’ll get a ValueError error explaining that the element was not found in the list.
list.remove(elmnt)
mylist.remove(<object>)

fruits = ["Orange", "Pineapple", "Banana", "Apple"]
fruits.remove("Apple")
print(fruits)
["Orange", "Pineapple", "Banana"]

same result accomplished with pop()

	fruits = ['apple', 'banana', 'cherry']
	fruits.pop(1)
	print(fruits)
	['apple', 'cherry']

This method differs from the remove method in that it takes an index as a parameter, and returns the element that was removed. This can be useful if you don’t know what the value is, but you know where it’s located. This can also be useful when you need to access the data and also want to remove it from the list.

Use cases

  • If the list contains hosts on a network, you could add or remove hosts as they come online or offline. 
  • If the list contains users authorized to run a certain process, you could add or remove users when permissions are granted or removed and so on. 
  • you’ll use a list whenever you need to write a program that’ll handle a variable amount of elements
  • You will use whenever you need to write a programm that’ll handle a sequence of a fixed amount of elements.

reversed() method

  • specify the sequence in a forward direction 
     for i in reversed(range(1, 10, 2)):
     print(i)
#OUTPUT
9
7
5
3
1

my_list = [1, 2, 3, 4, 5]
print(my_list)
# Output: [1, 2, 3, 4, 5]
  • depending on the Python version, there are different options to print
make = 'Mercedes'
range = 600
print(f"The GLC {name} has a range of {range} miles.")

Using a loop

my_list = [1, 2, 3, 4, 5]
for item in my_list:
    print(item)
# Output:
1
2
3
4
5

Using list comprehension

my_list = [1, 2, 3, 4, 5]
print(*my_list)
# Output
1, 2, 3, 4, 5

Using the join() method

my_list = [1, 2, 3, 4, 5]
print(' '.join(map(str, my_list)))
# Output
1 2 3 4 5

Zip and Enumerate

  • when dealing with loops

Zip

list(zip(['a', 'b', 'c'], [1, 2, 3])) 
# Output 
[('a', 1), ('b', 2), ('c', 3)].

x_coord = [23, 53, 2, -12, 95, 103, 14, -5]
y_coord = [677, 233, 405, 433, 905, 376, 432, 445]
z_coord = [4, 16, -6, -42, 3, -6, 23, -1]
labels = ["F", "J", "A", "Q", "Y", "B", "W", "X"]

points = []
for point in zip(labels, x_coord, y_coord, z_coord):
    points.append("{}: {}, {}, {}".format(*point))
    # tuple is unpacked using * in the format method

for point in points:
    print(point)

# OUTPUT
F: 23, 677, 4
J: 53, 233, 16
A: 2, 405, -6
Q: -12, 433, -42
Y: 95, 905, 3
B: 103, 376, -6
W: 14, 432, 23
X: -5, 445, -1

Use zip to write a for loop

Zip Lists to a Dictionary

Unzip Tuples

Enumerate

  • use this when you want the index along with each element of an iterable in a loop.
  • Iterating Over Lists
  • position index and corresponding value can be retrieved at the same time.
  • to access the elements in a list, along with the index of the element in question.
  • takes a list as a parameter and returns a tuple for each element in the list.
    • First value of the tuple is the index
    • Second value is the element itself.
for i, v in enumerate(['tic', 'tac', 'toe']):
    print(i, v)
0 tic
1 tac
2 toe

letters = ['a', 'b', 'c', 'd', 'e']
for i, letter in enumerate(letters):
    print(i, letter)
    0 a
1 b
2 c
3 d
4 e

Use enumerate to modify the list

index = 0
for value in values:
    print(index, value)
    index += 1
# OUTPUT
0 a
1 b 
2 c

for index in range(len(values)):
    value = values[index]
    print(index, value)
# OUTPUT
0 a
1 b
2 c

fruits = ['apple', 'banana', 'orange', 'grape']
for index, fruit in enumerate(fruits):
    print(f"Index: {index}, Fruit: {fruit}")

Index: 0, Fruit: apple
Index: 1, Fruit: banana
Index: 2, Fruit: orange
Index: 3, Fruit: grape

my_list = [1, 2, 3, 4, 5]
for index, value in enumerate(my_list):
    my_list[index] = value * 2

print(my_list)
[2, 4, 6, 8, 10]

List-specific methods

Data Structure
Lists
List methods
Tuples
Tuples methods
Dictionary
Dictionary methods
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