Working with Python
Lists
The primary data structure for collections in Python are lists. Lists are a built-in data structure and are ordered, mutable, and a single list can contain items of different types.
Create Lists
Python lists are defined using square brackets [ ]
a = ["a", "b", "c"]Append item to list
a = [1, 2, 3]
a.append(4)
a
>>> [1, 2, 3, 4]How to concatenate two lists
To concatenate two lists together in Python use the + operator.
a = [1, 2, 3]
b = [4, 5, 6]
a + b
>>> [1, 2, 3, 4, 5, 6]Count elements in a list
Use the len() function to get a count of all elements in alist.
a = ["a", "b", "c"]
len(a)
>>> 3Count specific elements in a list
Use .count() method to count how many occurrences of a specific item exists in a list:
a = ["a", "b", "b", "c"]
a.count("b")
>> 2Initialize a two dimensional array
To create a two dimensional array and initialize with all 0
width, height = 100, 50
matrix = [[o for x in range(width)] for y in range[height]]How to iterate over a list
To iterate over a list in Python:
a = ["a", "b", "c"]
for item in a:
print(item)
>>> a
>>> b
>>> cIterate over a list with index
Use the enumerate() function to iterate with an index:
a = ["a", "b", "c"]
for idx, item in enumerate(a):
print(f"{idx} -> {item}")
0 -> a
1 -> b
2 -> c
>>>Sorting Lists
Python has several built-in functions to sort lists.
Sort list by value
Use the .sort() method to alter the list sorting by value:
a = [3, 2, 1]
a.sort()
a
>>> [1, 2, 3]
b = ["c", "b", "a"]
b.sort()
b
>>> ['a', 'b', 'c']Reverse list order
Use the .reverse() method to alter the list reversing the order:
a = ["a", "b", "c"]
a.reverse()
a
>>> ['c', 'b', 'a']Iterate over list in reverse order
To iterate over a list in reverse order, not changing the list, use the reversed() iterator function:
a = ["a", "b", "c"]
for item in reversed(a):
print(item)
>>> c
>>> b
>>> aRemove item from list
You can use the .pop() method to remove an item at a specific index, if no index specified it will remove the last item. .pop() returns the item removed.
a =["a", "b", "c"]
item = a.pop(1)
print(f"List: {a}, Item: {item}")
>>> List: ['a', 'c'], Item: bRemove item by value from list
You can remove an item by value from list using .remove() but this is not as convenient to use, it will only remove the first item found and will raise a ValueError if not found.
a = ["a", "b", "b", "c"]
a.remove("b")
a
>>> ['a', 'b', 'c']
a.remove("z")
>>> ValueError: list.remove(x): x not in listUse the filter() function as a better means to remove all items matching a value. The filter function returns a filter object that needs to be converted back to a list:
a = ["a", "b", "b", "c"]
list(filter(lambda e: e != "b", a)
>>> ['a', 'c']