python-by-phitron

Day - 07: Introduction to list in Python

List in Python
7.1 Introduction to list in Python
7.2 More about list indexing
7.3 List slicing like slicing a cake
7.4 List methods Part-1
7.5 List methods Part-2

About list:

1. List is a Data structure
2. It can contains Different type data
3. It’s an Ordered Data structure
4. It is a Mutable Data structure

Introduction to List

Example of List:

a =  ["Mango", "Banana", "Apple"] # 1D List
b =  [[11, 12], ["Cake"], [-22]] # 2D list / Nested list

More about list indexing

Indexing is two types:

1. Positive indexing -> [0, 1, 2, 3, 4]
2. Negetive indexing <- [-4, -3, -2, -1]

Examples:

a = [12, 20, 34, "Phitron"]
print([0])
print(a[-3])
print(a[-20]) # Height index - 4 is possible
print(len(a))
a[-3] = 500 # Changed in Memory
print(a)
if 500 in a:
    print("Found")
else:
    print("Not found")

Traversing:

a = [12, 20, 34, "Phitron"]

for i in a:
    print(i)

# Positive indexing
for i in range(len(a)):
    print(i, a[i])

# Negetive indexing --> Reviersing
for i in range(-1, -len(a)-1, -1):
    print(i, a[i])

# Positive indexing --> Reversing
for i in range(len(a)-1, -1, -1):
    print(i, a[i])

List slicing

custom_list = [1, 2, 3, 4, 5]
reverse_list = custom_list[::-1] # create new reverse list with different referance
another_reverse_list = custom_list[-1::-1] # create new reverse list with different referance
empty_list = custom_list[-1:-1:-1] # return empty list
new_slice = custom_list[::2]
print(empty_list)
# slice tecnique works: [start:end:step]

List methods part-1

• list() convert list to string

message = 'Hello World!'
text_to_list = list(message)
print(text_to_list)

• append() adds an element at the ene of a list

num_list = [10, 11, 12, 13, 14, 15]
new_num_list = num_list.append(16) # The append() method returns nothing!
print(num_list)

• insert() adds an element at the specified position

num_list.insert(len(num_list), 17)
print(num_list)

• copy() returns copy of the list

copy_list = num_list.copy()
#print(copy_list)

• count() returns the number of element with the specified value

duplicate_num_list = [1, 2, 3, 4, 4, 4, 4, 4, 5, 4]
count_dupicates = duplicate_num_list.count(4)
print(count_dupicates)
print(duplicate_num_list.count('a')) # it will return 0 because there is no 'a' in this list

• extend() add the elements of a list to the end of the current list

random_num = [1, 3, 2]
random_num2 = [4, 7, 9]
final_list = random_num.extend(random_num2) # extend() retuns nothing!
print(random_num)

List methods part-2

• pop() the pop method removes the element at the specified positon and returns removed value

numbers = [1, 2, 3, 4, 5]
updated_list = numbers.pop() # returns removed value as it was 5
print(updated_list)

• remove() the remove method removes the first occurrence of the element with the specified value

num_list = [1, 2, 3, 4, 5]
removed_list = num_list.remove(5) # it returns nothing!
print(removed_list)

• clear() the clear method removes all the elements from a list

cleard_list = num_list.clear() # it returns nothing!
print(num_list)

• reverse() the reverse() method reverses the sorting order of the elements

new_list = [10, 11, 12, 13, 14, 15]
reversed_list = new_list.reverse() # it returns nothing!
print(new_list)

• sort() sort a list in ascending and descending ordered

random_numbers = [1, 3, 4, 5 , -2, -1, 0]
ascending_ordered= random_numbers.sort() # it returns nothing!
descending_ordered= random_numbers.sort(reverse=True) # it returns nothing!
print(random_numbers)

• max() function returns the item with the highest value, or the item with the highest value in an iterable. If the values are strings, an alphabetically comparison is done

all_numbers = [1, 2, 3, 4, 45, 60, 99]
max_value = max(all_numbers) # returns the max value
print(max_value)
min_value = min(all_numbers) # returns the min value
print(min_value)