22 - Introduction to Lists
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2023-12-21 - 11:47
Python Lists
- Lists are an ordered collection of data items.
- They store multiple items in a single variable.
- List items are separated by commas and enclosed within square brackets [].
- Lists are changeable, meaning we can alter them after creation.
Example 1:
lst1 = [1,2,2,3,5,4,6]
lst2 = ["Red", "Green", "Blue"]
print(lst1)
print(lst2)
Output:
[1, 2, 2, 3, 5, 4, 6]
['Red', 'Green', 'Blue']
Example 2:
details = ["Abhijeet", 18, "FYBScIT", 9.8]
print(details)
Output:
['Abhijeet', 18, 'FYBScIT', 9.8]
As we can see, a single list can contain items of different data types.
List Index
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
We can access list items by using its index with the square bracket syntax []. For example, colors[0] will give "Red", colors[1] will give "Green" and so on...
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 whether an item in present in the list?
We can check if a given item is present in the list. This is done using theΒ inΒ 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 you want to start, where do you want to end and if you want to skip elements in between the range.
Syntax:
listName[start : end : jumpIndex]
Note: jump index is optional. We will see this in later 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 the 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: Printing alternate values
animals = ["cat", "dog", "bat", "mouse", "pig", "horse", "donkey", "goat", "cow"]
print2] #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 value within a 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 a given index.
List Comprehension
List comprehensions are used for creating new lists from other iterables like lists, tuples, dictionaries, sets, and even in arrays and strings.
Syntax:
List = [Expression(item) for item in iterable if Condition]
Expression: It is the item which is being iterated.
Iterable: It can be list, tuples, dictionaries, sets, and even in arrays and strings.
Condition: Condition checks if the item should be added to the new list or not.
Example 1: Accepts items with the small letter βoβ in the new list
names = ["Milo", "Sarah", "Bruno", "Anastasia", "Rosa"]
namesWith_O = [item for item in names if "o" in item]
print(namesWith_O)
Output:
['Milo', 'Bruno', 'Rosa']
Example 2: Accepts items which have more than 4 letters
names = ["Milo", "Sarah", "Bruno", "Anastasia", "Rosa"]
namesWith_O = [item for item in names if (len(item) > 4)]
print(namesWith_O)
Output:
['Sarah', 'Bruno', 'Anastasia']