PyListstest.txt


Lists

Lists are another type of object in Python. They are used to store an indexed list of items. 
A list is created using square brackets with commas separating items.
The certain item in the list can be accessed by using its index in square brackets.
For example:
words = ["Hello", "world", "!"]
print(words[0])
print(words[1])
print(words[2])

Result:
>>>
Hello
world
!
>>>

The first list item's index is 0, rather than 1, as might be expected.



Lists

An empty list is created with an empty pair of square brackets.
empty_list = []
print(empty_list)

Result:
>>>
[]
>>>

Most of the time, a comma won't follow the last item in a list. However,
 it is perfectly valid to place one there, and it is encouraged in some cases.

Lists

Typically, a list will contain items of a single item type, but it is also 
possible to include several different types. 
Lists can also be nested within other lists.
number = 3
things = ["string", 0, [1, 2, number], 4.56]
print(things[1])
print(things[2])
print(things[2][2])

Result:
>>>
0
[1, 2, 3]
3
>>>

Lists of lists are often used to represent 2D grids, as Python lacks
 the multidimensional arrays that would be used for this in other languages.

Lists

Indexing out of the bounds of possible list values causes an IndexError. 
Some types, such as strings, can be indexed like lists. Indexing strings 
behaves as though you are indexing a list containing each character in the 
string. 
For other types, such as integers, indexing them isn't possible, and it causes 
a TypeError.
str = "Hello world!"
print(str[6])


Result:
>>>
w
>>>

List Operations

The item at a certain index in a list can be reassigned.
For example:
nums = [7, 7, 7, 7, 7]
nums[2] = 5
print(nums)


Result:
>>>
[7, 7, 5, 7, 7]
>>>

List Operations

Lists can be added and multiplied in the same way as strings.
For example:
nums = [1, 2, 3]
print(nums + [4, 5, 6])
print(nums * 3)

Result:
>>>
[1, 2, 3, 4, 5, 6]
[1, 2, 3, 1, 2, 3, 1, 2, 3]
>>>

Lists and strings are similar in many ways -
 strings can be thought of as
 lists of characters that can't be changed.
3/4
                               
List Operations

To check if an item is in a list, the in operator can be used. 
It returns True if the item occurs one or more times in the list, 
and False if it doesn't.

words = ["spam", "egg", "spam", "sausage"]
print("spam" in words)
print("egg" in words)
print("tomato" in words)
Try It Yourself

Result:
>>>
True
True
False
>>>

The in operator is also used to determine whether or not a string is a
 substring of another string.


print("wor" in "words")

print("or" in "words")

print("nor" in "words")

Result:
>>>
True
True
False
>>>


List Operations

To check if an item is not in a list, 
you can use the not operator in one of the following ways:
nums = [1, 2, 3]

print(not 4 in nums)

print(4 not in nums)

print(not 3 in nums)

print(3 not in nums)

print("55" not in "5566")

print("55" not in "6666")

print("on" not in "goon")

print("so" not in "goon")

Result:
>>>
True
True
False
False
False
True
False
True
>>>

List Functions

Another way of altering lists is using 
the append method. This adds an item to the end of an existing list.

nums = [1, 2, 3]

nums.append(4)

print(nums)

nums.append(5)

print(nums)

nums.append(6,)

print(nums)


words = ["hello"]

words.append("world")

print(words)

print(words[1])

Result:
>>>
[1, 2, 3, 4]
[1, 2, 3, 4, 5]
[1, 2, 3, 4, 5, 6]
['hello', 'world']
world
>>>

The dot before append is there because it is a method of the 
list class. 
Methods will be explained in a later lesson.

List Functions

To get the number of items in a list, you can use the len function.

nums = [1, 3, 5, 2, 4]
print(len(nums))

letters = ["a", "b", "c"]

letters.append("d")

print(len(letters))

Result:
>>>
5
4
>>>

Unlike append, len is a normal function, rather than a method.
 This means it is written before the list it is being called on, 
without a dot.

List Functions

The insert method is similar to append, except that it allows you to 
insert a new item at any position in the list, as opposed to just at the end.

words = ["Python", "fun"]


index = 1


words.insert(index, "is")


print(words)



words = ["CPython", "fun"]


words.insert(3, "is")

print(words)


words.insert(2, "C/C++")

print(words)

words.insert(index, "and")


print(words)


nums = [9, 8, 7, 6, 5]

nums.append(4)

nums.insert(2, 11)

print(len(nums))

Result:
>>>
['Python', 'is', 'fun']
['CPython', 'fun', 'is']
['CPython', 'fun', 'C/C++', 'is']
['CPython', 'and', 'fun', 'C/C++', 'is']
7
>>>

List Functions

The index method finds the first occurrence of a list item and returns its index.
If the item isn't in the list, it raises a ValueError.
letters = ['p', 'q', 'r', 's', 'p', 'u']
print(letters.index('r'))
print(letters.index('p'))
print(letters.index('z'))


Result:
>>>
2
0
ValueError: 'z' is not in list
>>> 

There are a few more useful functions and methods for lists. 
max(list): Returns the list item with the maximum value
min(list): Returns the list item with minimum value
list.count(obj): Returns a count of how many times an item occurs in a list
list.remove(obj): Removes an object from a list
list.reverse(): Reverses objects in a list


letters = ['p', 'q', 'r', 's', 'p', 'u', 9, "9", 0, .0, 7.0, 7, "7"]

print(letters.index('r'))

print(letters.index('p'))

print(letters.index('9'))

print(letters)

print(letters.count(7))

print(letters.remove(0))

print(letters)
print(letters.reverse())

print(letters)




nums=[1,2,0,.0,7,7.0,4/2]

print(nums)

print(max(nums))

print(nums)

print(min(nums))

Result:
>>>

2
0
7
['p', 'q', 'r', 's', 'p', 'u', 9, '9', 0, 0.0, 7.0, 7, '7']
2
None
['p', 'q', 'r', 's', 'p', 'u', 9, '9', 0.0, 7.0, 7, '7']
None
['7', 7, 7.0, 0.0, '9', 9, 'u', 'p', 's', 'r', 'q', 'p']
[1, 2, 0, 0.0, 7, 7.0, 2.0]
7
[1, 2, 0, 0.0, 7, 7.0, 2.0]
0

>>>


list = [1, 1, 2, 3, 5, 8, 13]
print(list[list[4]])

Result
>>>
8

>>>