List Indexes

Each item/element in a list has its own unique index. This index can be used to access any particular item from the list. The first item has index [0], second item has index [1], third item has index [2] and so on.

Example:

colors = ["Red", "Green", "Blue", "Yellow", "Green"]
#          [0]      [1]     [2]      [3]      [4]

 

Accessing list items:

 

Positive Indexing:

As we have seen that list items have index, as such we can access items using these indexes.

Example:

colors = ["Red", "Green", "Blue", "Yellow", "Green"]
#          [0]      [1]     [2]      [3]      [4]
print(colors[2])
print(colors[4])
print(colors[0])

Output:

Blue
Green
Red

 

Negative Indexing:

Similar to positive indexing, negative indexing is also used to access items, but from the end of the list. The last item has index [-1], second last item has index [-2], third last item has index [-3] and so on.

Example:

colors = ["Red", "Green", "Blue", "Yellow", "Green"]
#          [-5]    [-4]    [-3]     [-2]      [-1]
print(colors[-1])
print(colors[-3])
print(colors[-5])

Output:

Green
Blue
Red

 

Check for item:

We can check if a given item is present in the list. This is done using the in keyword.

colors = ["Red", "Green", "Blue", "Yellow", "Green"]
if "Yellow" in colors:
    print("Yellow is present.")
else:
    print("Yellow is absent.")

Output:

Yellow is present.

 

colors = ["Red", "Green", "Blue", "Yellow", "Green"]
if "Orange" in colors:
    print("Orange is present.")
else:
    print("Orange is absent.")

Output:

Orange is absent.

 

Range of Index:

You can print a range of list items by specifying where do you want to start, where do you want to end and if you want to skip elements in between the range. 

Syntax:

List[start : end : jumpIndex]

Note: jump Index is optional. We will see this in given examples.

 

Example: printing elements within a particular range:

animals = ["cat", "dog", "bat", "mouse", "pig", "horse", "donkey", "goat", "cow"]
print(animals[3:7])	#using positive indexes
print(animals[-7:-2])	#using negative indexes

Output:

['mouse', 'pig', 'horse', 'donkey']
['bat', 'mouse', 'pig', 'horse', 'donkey']

Here, we provide index of the element from where we want to start and the index of the element till which we want to print the values.

Note: The element of the end index provided will not be included. 

 

Example: printing all element from a given index till the end

animals = ["cat", "dog", "bat", "mouse", "pig", "horse", "donkey", "goat", "cow"]
print(animals[4:])	#using positive indexes
print(animals[-4:])	#using negative indexes

Output:

['pig', 'horse', 'donkey', 'goat', 'cow']
['horse', 'donkey', 'goat', 'cow']

When no end index is provided, the interpreter prints all the values till the end.

 

Example: printing all elements from start to a given index

animals = ["cat", "dog", "bat", "mouse", "pig", "horse", "donkey", "goat", "cow"]
print(animals[:6])	#using positive indexes
print(animals[:-3])	#using negative indexes

Output:

['cat', 'dog', 'bat', 'mouse', 'pig', 'horse']
['cat', 'dog', 'bat', 'mouse', 'pig', 'horse']

When no start index is provided, the interpreter prints all the values from start up to the end index provided. 

 

Example: print alternate values

animals = ["cat", "dog", "bat", "mouse", "pig", "horse", "donkey", "goat", "cow"]
print(animals[::2])		#using positive indexes
print(animals[-8:-1:2])	#using negative indexes

Output:

['cat', 'bat', 'pig', 'donkey', 'cow']
['dog', 'mouse', 'horse', 'goat']

Here, we have not provided start and index, which means all the values will be considered. But as we have provided a jump index of 2 only alternate values will be printed. 

 

Example: printing every 3rd consecutive withing given range

animals = ["cat", "dog", "bat", "mouse", "pig", "horse", "donkey", "goat", "cow"]
print(animals[1:8:3])

Output:

['dog', 'pig', 'goat']

Here, jump index is 3. Hence it prints every 3rd element within given index.