Learn Python With Us - Lists

❗ Important Links

**Series Link : **https://makereading.com/series/python
**Challenges : **https://www.hackerrank.com/contests/makereading/challenges
**Google Classroom: ** https://classroom.google.com/c/NTA3NzI5NTUzMDQz?cjc=2qcqsu5

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What is a List?

A simple data structure in Python that is a mutable, or changeable, ordered sequence of elements.

Properties of the list

1. Lists are ordered, Which means they will maintain the order of insertion.

2. Lists can contain any arbitrary objects (eg: list, dictionary, set, tuple, string, numbers, None).

3. List elements can be accessed by index (position).

4. Lists are mutable.

5. Lists are dynamic, in memory allocation.

6. Lists can contain duplicate values; Since lists are indexed, lists can have items with the same value

How the list is built internally?

Python lists are internally built on top of Arrays using C. Specifically, they are dynamic arrays with exponential over-allocation. Python’s lists are really variable-length arrays. The implementation uses a contiguous array of references to other objects and keeps a pointer to this array and the array’s length in a list head structure.

When items are appended or inserted, the array of references is resized. Some cleverness is applied to repeatedly improve appending items' performance; when the array must be grown, some extra space is allocated so the next few times don’t require an actual resize.

💡 To know more about how the memory allocation happens in the dynamic array for python list, please check out this http://www.laurentluce.com/posts/python-list-implementation/

Declaration

A list data structure can be defined in two different methods,

1. [] using square brackets.

2. list() method.

1. Using square brackets

You can declare the list below,

data = ['d1', 'd2', 'd3']
print(data)

2. Using list() constructor

Using the list() constructor, we can convert a set, tuple, dictionary, or list to a list.

Set to list:

set_data = {1, 2, 3, 4}
print("set data type", type(set_data))
data = list(set_data)
print(data, type(data))

Dictionary to list

While converting Dictionary to a list, the list value comprises only the key values.

dict_data = {"name": "makereading", "type": "blog"}
print("dict_data type", type(dict_data))
data = list(dict_data)
print(data, type(data))

Tuple to list

tuple_data = (1, 2, 3)
print("Type of tuple_data", type(tuple_data))
data = list(tuple_data)
print(data, type(data))

Data Types

List is a mutable with both mutable & immutable data types.

data = [1, 'makereading', True, [1, 2, 3], {"name":"makereading"}, (1, 2, 3), {1, 2, 3}]
print(data)

As you see in the above list output, we can see that the list can contain all kind of datatypes. (i.e) number, string, boolean, list, dictionary, tuple, set.

Built-in Functions with a list

Length - len()

To find the length of the list,

data = [1, 2, 3, 4, 5, 6]
print("length of data list ,", len(data))

Type of data structure - type()

To find the type of data structure,

data = [1, 2, 3, 4, 5, 'string']
print(type(data))

For the list datastructure, type() method will output <class list>

max() & min()

To find the maximum value and the minimum value of a list,

data = [0, 10, 1, 89, 10]
print("Max - ", max(data), "Min - ", min(data))

any()

If any of the values in the set is true (i.e,) Non-Zero, Non-Empty String, False values then it will return True, else False.

data = [0, 10, 1, 89, 10]
print(any(data))

all()

The all() function returns True if all items in an iterable are true, otherwise, it returns False. If the iterable object is empty, the all() function also returns True.

data = [0, 10, 1, 89, 10]
print(all(data)) # Since it has zero, it will return False

data = [1, 10, 1, 89, 10]
print(all(data)) # Since it has all true values, it will return True

Accessing Elements of List

List items are indexed and you can access them by referring to the index number. Python supports positive zero-based indexing and negative indexing that starts from -1. Negative indexing in Python means the indexing starts from the end of the iterable. The last element is at index -1, the second last at -2, and so on.

consider the list, arr = ['a', 'b', 'c', 'd', 'e', 'f'], which is having 6 elements. From the below image, you can see the values of the positive indexing and negative indexing.

Positive Indexing

In this type of Indexing, we pass a positive index (which we want to access) in square brackets. The index number starts from index number 0 (which denotes the first element of a list).

fruits = ["apple", "banana", "cherry"]  
print(fruits[1])

Negative Indexing

Negative indexing means starting from the end. -1 refers to the last item, -2 refers to the second last item, etc.

fruits = ["apple", "banana", "cherry"]  
print(fruits[-1])

Slicing

**1. **You can specify a range of indexes by specifying where to start and where to end the range. When specifying a range, the return value will be a new list with the specified items.

list[start : stop : step]

The format for list slicing is [start: stop: step].

• start is the index of the list where slicing starts. (starting position is included)

• stop is the index of the list where slicing ends. (stop position is excluded)

• step allows you to select an nth item within the range start to stop (default value is 1)

🖊️ If the stop value is greater than the list of the array, it won't throw an error. It will just consider till the end of the array.

**Example: **

fruits = ["apple", "banana", "cherry", "orange", "kiwi", "melon", "mango"]  
print(fruits[2:5])

📝 Note: The search will start at index 2 (included) and end at index 5 (not included).

**2. ** If you aren't specifying the start value it will take index 0 as the start value. The same thing to the end, if you aren't specifying the end value it will take len(arr).

fruits = ["apple", "banana", "cherry", "orange", "kiwi", "melon", "mango"]
print(fruits[:4])

fruits = ["apple", "banana", "cherry", "orange", "kiwi", "melon", "mango"]
print(fruits[1:])

**3. ** If you aren't specifying both the start and end, then for start it's value will be 0 and for the end, its value will be len of the array.

**4. ** Let's try with negative indexing,

fruits = ["apple", "banana", "cherry", "orange", "kiwi", "melon", "mango"]  
print(fruits[-4:-1])

**5. ** Step Size specifies which element to picking while indexing. So a step size of 1 tells python to pick every element, a step size of 2 means picking alternate elements, and so on.

🗝️ Default step size is 1.

Examples:

Positive step size

Negative step size

Modifying List Elements

We can replace a value in the list based on the index, or the range of indexes.

1. Change value based on the index

fruits = ["apple", "banana", "cherry"]  
fruits[1] = "blackcurrant"  
print(fruits)

In the above list, we are changing the value of the list with index 1 from banana to blackcurrant.

2. Change the value based on ranges (slice)

fruits = ["apple", "banana", "cherry", "orange", "kiwi", "mango"]  
fruits[1:3] = ["blackcurrant", "watermelon"]  
print(fruits)

💥 If you insert more items than you replace, the new items will be inserted where you specified, and the remaining items will move accordingly.

Methods to add values to list

1. insert()

The insert() method takes two parameters:
index - the index where the element needs to be inserted
element - this is the element to be inserted in the list

Insert an item at a given position. The first argument is the index of the element before which to insert, so a.insert(0, x) inserts at the front of the list, and a.insert(len(a), x) is equivalent to a.append(x).

fruits = ['apple', 'papaya', 'banana', 'orange', 'pear', 'lemon']
fruits.insert(2, 'berry')
print(fruits)

All the elements after the element are shifted to the right.

After the insertion of the element,

This insert() method will work with the negative indexing as well.

fruits = ['apple', 'papaya', 'banana', 'orange', 'pear', 'lemon']
fruits.insert(-1, 'pear')
print(fruits)

2. append()

append() method will add an element at the end of the list. Since the list is a mutable data structure, this append() method is an in-place change.

fruits = ['apple', 'papaya', 'banana', 'orange', 'pear', 'lemon']
fruits.append('lemon orange')
print(fruits)

3. extend()

extend() method takes only a single argument. The single argument can be a set, list, tuples, or dictionary. It automatically converts into a list and adds to the list. extend() method will take only an iterable as a parameter.

With Sets

data = [1, 2, 3, 4, 5]
data.extend({'a', 'b', 'c', 'd'})
print(data)

With Dictionary

data = [1, 2, 3, 4, 5]
data.extend({"name": "makereading", "blog": "educational"})
print(data)

With String

data = [1, 2, 3, 4, 5]
data.extend("makereading")
print(data)

Method to remove values from a list

We have many methods to delete, remove the items from the list,

1. remove()

2. pop()

3. del()

4. clear()

1. remove()

remove() method will take one parameter (the element that is to be removed). It will delete the first occurrence of the element.

fruits = ["apple", "banana", "cherry"]  
fruits.remove("banana")  
print(fruits)

2. pop()

The pop() method removes the specified index. The default value of the index is -1.

**With default value: **

fruits = ["apple", "banana", "cherry"]  
fruits.pop()  
print(fruits)

**With specified index: **

fruits = ["apple", "banana", "cherry"]  
fruits.pop(1)  
print(fruits)

3. del

It removes the particular index; also the whole list.

**With the specified index: **

fruits = ["apple", "banana", "cherry"]  
del fruits[0]  
print(fruits)

Delete the object itself

fruits = ["apple", "banana", "cherry"]  
del fruits 
print(fruits)

4. clear

Clear the python lists. It removes all the elements in the list.

fruits = ["apple", "banana", "cherry"]  
fruits.clear()  
print(fruits)

Method to sort values from list

.sort()

Sorting is Case Sensitive in list. The sort() method sorts the list ascending by default. You can also make a function to decide the sorting criteria(s).

Ascending Order

states = ['Tamil Nadu', 'Kerala', 'Andra', 'Telengana']
states.sort()
print(states)

Descending Order

states = ['Tamil Nadu', 'Kerala', 'Andra', 'Telengana']
states.sort(reverse=True)
print(states)

With key

A function to specify the sorting criteria(s)

def sortFunc(e):
  return len(e)

fruits = ["big apple", "banana", "cherry"]  

fruits.sort(key=sortFunc)

Method to copy

The copy() method returns a shallow copy of the list.

# mixed list
fruits = ["apple", "orange", "lemon"]

# copying a list
fruits_fav = fruits.copy()

print('Copied List:', fruits_fav)

# Output: Copied List: ["apple", "orange", "lemon"]

Exercise

1: Reverse a list in Python. clue : use slicing.
2: Remove empty strings from the list of strings. clue : use remove, del, pop
3: Add a new item to the list after a specified item. **clue: ** use insert
4: Extend the nested list by adding the sublist. **clue: ** use extend
5: Replace the list’s item with a new value if found. **clue: ** use remove, replace
6: Remove all occurrences of a specific item from a list. clue use removeAll
7: Reverse a list. clue use reverse() or slicing method

Challenges

• Hackerrank https://www.hackerrank.com/contests/makereading