Python
Python list
The list type is probably the most commonly used collection type in Python. Despite its name, a list is more like an array in other languages, mostly Java
int_list = [1, 2, 3] string_list = ['abc', 'defghi']
A list can be empty:
empty_list = []
The elements of a list are not restricted to a single data type, which makes sense given that Python is a dynamic
language:
mixed_list = [1, 'abc', True, 2.34, None]
A list can contain another list as its element:
nested_list = [['a', 'b', 'c'], [1, 2, 3]]
The elements of a list can be accessed via an index, or numeric representation of their position. Lists in Python are
zero-indexed meaning that the first element in the list is at index 0, the second element is at index 1 and so on:
names = ['Alice', 'Bob', 'Craig', 'Diana', 'Eric']
print(names[0]) # Alice
print(names[2]) # Craig
Indices can also be negative which means counting from the end of the list (-1 being the index of the last element). So, using the list from the above example:
print(names[-1]) # Eric
print(names[-4]) # Bob
Lists are mutable, so you can change the values in a list:
names[0] = 'Ann'
print(names)
# Outputs ['Ann', 'Bob', 'Craig', 'Diana', 'Eric']
Besides, it is possible to add and/or remove elements from a list:
Append object to end of list with L.append(object), returns None.
names = ['Alice', 'Bob', 'Craig', 'Diana', 'Eric']
names.append("Sia")
print(names)
# Outputs ['Alice', 'Bob', 'Craig', 'Diana', 'Eric', 'Sia']
Add a new element to list at a specific index. L.insert(index, object)
names.insert(1, "Nikki")
print(names)
# Outputs ['Alice', 'Nikki', 'Bob', 'Craig', 'Diana', 'Eric', 'Sia']
Remove the first occurrence of a value with L.remove(value), returns None
names.remove("Bob")
print(names) # Outputs ['Alice', 'Nikki', 'Craig', 'Diana', 'Eric', 'Sia']
Get the index in the list of the first item whose value is x. It will show an error if there is no such item.
name.index("Alice") 0
Count length of list
len(names)
6
count occurrence of any item in list
a = [1, 1, 1, 2, 3, 4] a.count(1) 3
Reverse the list
a.reverse()
[4, 3, 2, 1, 1, 1]
# or
a[::-1]
[4, 3, 2, 1, 1, 1]
Remove and return item at index (defaults to the last item) with L.pop([index]), returns the item
names.pop() # Outputs 'Sia'
You can iterate over the list elements like below:
for element in my_list:
print (element)