# Sorting in Python: How to Use sorted() and .sort()

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As programmers, we often deal with large datasets that need to be sorted efficiently. Python provides several built-in functions and methods to sort data, making it easier for us to organize and analyze our data. In this article, we will explore the fundamentals of Python sorting and learn how to use the sorted() function and .sort() method to sort our data in ascending and descending order.

Sorting is an essential skill for any Python programmer as it helps in organizing data, improving search efficiency, and optimizing algorithms’ performance. Sorting involves rearranging elements in a specific order, depending on our use case. In Python, we can sort various data types, including lists, tuples, and strings containing numerical values.

Before diving into Python’s sorting methods, let’s first understand what sorting algorithms are and their importance. A sorting algorithm is a set of instructions that determine the order in which data elements are arranged. These algorithms are crucial in organizing data, determining the most significant values, and sorting them in a specific order.

## Introduction to Python Sorting

Sorting is an essential skill for Python programmers, allowing them to efficiently organize data in their programs. Sorting algorithms are used to rearrange data in ascending or descending order, making it easier to search and analyze. In Python, there are various sorting methods available, including the sorted() function and the .sort() method.

Python sorting involves sorting lists, tuples, and other data types based on specific criteria, such as alphabetic order, numerical order, or even custom sorting rules. Understanding how to implement sorting algorithms in Python is crucial for any programmer looking to work with large datasets effectively.

## The sorted() Function in Python

Python provides an in-built function called sorted() to sort various data types, including lists, tuples, and dictionaries. The function takes an iterable as the argument and returns a new iterable with elements sorted in ascending order.

The syntax for the sorted() function is as follows:

Let’s consider an example where we sort a list of integers using the sorted() function.

### Sorting a List using sorted()

Suppose we have the following list of integers:

We can apply the sorted() function to sort the list in ascending order as follows:

After sorting the list, the sorted_lst variable will contain the following result:

We can also sort the list in descending order by setting the reverse parameter to True, as shown below:

After sorting the list in descending order, the sorted_lst_desc variable will contain the following result:

The sorted() function can also be used to sort other data types such as tuples, dictionaries, and strings.

## The .sort() Method in Python

Another way to sort lists in Python is to use the .sort() method. The .sort() method sorts the list in place, directly modifying the original list.

To sort a list in ascending order using the .sort() method:

To sort a list in descending order using the .sort() method:

Similar to the sorted() function, the .sort() method can also sort lists of strings and tuples.

### The .sort() Method with Key Function

The .sort() method can also take a key function as an argument to customize the sorting behavior. The key function should take in one argument and return a value that will be used to determine the order of the elements.

For example, if we have a list of strings representing names and we want to sort them in alphabetical order by the second character:

Here, we passed a lambda function as the key argument to the .sort() method. The lambda function takes in a string and returns its second character. The .sort() method uses this value to sort the list in alphabetical order based on the second character.

## Sorting Strings with Numerical Values in Python

Sometimes, we need to sort strings with numerical values in Python. For example, consider a list of strings containing ages, such as [’19 years old’, ’35 years old’, ’26 years old’]. The default sorting order will not give us the desired output as the numerical values are not being sorted correctly.

To sort such strings in numerical order, we can use a custom sort key with the sorted() function. The key parameter takes a function that returns a value based on which the sorting is done.

### Sorting strings with numbers separated by whitespace

If the numerical values are separated by whitespace, we can use the split() method to split the string into a list of its parts. We can then convert the numerical part to an integer and return it as the sort key.

“`
ages = [’19 years old’, ’35 years old’, ’26 years old’]
sorted_ages = sorted(ages, key=lambda x: int(x.split()))
“`

This will return a list of strings sorted by the numerical value in ascending order:

“`
[’19 years old’, ’26 years old’, ’35 years old’]
“`

To sort in descending order, we can add the reverse parameter with a value of True:

“`
sorted_ages = sorted(ages, key=lambda x: int(x.split()), reverse=True)
“`

Which will return:

“`
[’35 years old’, ’26 years old’, ’19 years old’]
“`

### Sorting strings with numbers embedded in them

If the numerical values are embedded in the strings, we can use regular expressions to extract them and return them as the sort key.

Both examples use regular expressions to extract the numerical values from the strings and convert them to integers. The sorting is then done based on the list of integers returned for each string.

## Sorting in Python: How to Use sorted() and .sort()

### Sorting with Index in Python

When sorting data in Python, you may come across scenarios where you need to maintain the original index of the elements. Sorting with index can be useful when you want to preserve the position of elements relative to the original data.

One way to perform sorting with index in Python is to use the `enumerate()` function. The `enumerate()` function adds a counter to an iterable, returning a tuple containing the index and the original value. By sorting the iterable based on the values, the original index can be preserved.

Let’s take a look at an example:

In the example above, we used the `enumerate()` function to add the index to the original list. We then sorted the list based on the values and stored the sorted index using a list comprehension.

Here’s the Python code:

``````
fruits = ['apple', 'banana', 'cherry']
sorted_fruits = sorted(fruits)
sorted_index = [i for _, i in sorted(enumerate(fruits), key=lambda x:x)]

print(sorted_fruits)
print(sorted_index)
``````

The output should be:

``````
['apple', 'cherry', 'banana']
[0, 2, 1]
``````

As you can see, the sorted index list matches the order of the sorted fruits list, allowing you to maintain the original index while still sorting the data.

## Performance and Efficiency of Sorting Algorithms in Python

Sorting is a fundamental operation in computer science and is crucial for many applications. In Python, there are various sorting algorithms available for use, each with different performance characteristics.

When selecting a sorting algorithm, it is important to consider the size and nature of the data set, as well as the performance requirements. Sorting algorithms can have different time complexities, with some being more efficient for smaller data sets and others for larger ones.

### Time Complexity of Sorting Algorithms

The time complexity of a sorting algorithm determines how long it takes to sort a given set of data. It is usually measured in terms of the number of operations it performs on the data set. The most common measures of time complexity are Big O notation and Theta notation.

Here are the time complexities of some popular sorting algorithms:

It is important to note that the best, average, and worst-case scenarios can be different for each algorithm. Choosing the right algorithm depends on the characteristics of the data set and the performance requirements.

### Comparing Sorting Algorithms

When comparing sorting algorithms, there are several factors to consider, including time complexity, memory usage, stability, and ease of implementation.

For example, bubble sort and selection sort are easy to implement but are not very efficient for large data sets. Insertion sort is more efficient than bubble sort and selection sort but is still not ideal for very large data sets. Merge sort and quick sort are more efficient for larger data sets but can be more difficult to implement.

### Choosing the Right Sorting Algorithm

Choosing the right sorting algorithm depends on the size and nature of the data set, as well as the performance requirements. For small data sets, less efficient algorithms like bubble sort or insertion sort may suffice. For larger data sets, more efficient algorithms like merge sort or quick sort may be necessary.

It is also worth considering the stability of the sorting algorithm. A stable sorting algorithm maintains the relative order of equal elements in the input data. Unstable sorting algorithms do not guarantee this property.

Ultimately, the best way to choose the right sorting algorithm for a specific use case is to experiment with different algorithms and measure their performance.

## Merge Sort in Python

### Introduction

In computer science, merge sort is an efficient, general-purpose, comparison-based sorting algorithm. The algorithm divides the input array into two halves, calls itself for the two halves, and then merges the sorted halves.

### Implementation

To implement merge sort in Python, we first define a function called `merge_sort` that takes an input list as an argument. The function will then recursively divide the input list into two halves until each half contains only one element.

After dividing the list into halves, we merge the two halves in a sorted manner by comparing the elements in both partitions. We then repeat the merging process until the entire list is sorted.

### Pseudocode

Here is the pseudocode for the merge sort algorithm in Python:

Note that the `merge` function is used to merge the two halves of the list.

### Example

Here is an example of the merge sort algorithm implemented in Python:

“`
def merge_sort(lst):
if len(lst) <= 1:
return lst
middle = len(lst) // 2
left_half = lst[:middle]
right_half = lst[middle:]
left_half = merge_sort(left_half)
right_half = merge_sort(right_half)
return merge(left_half, right_half)

def merge(left_half, right_half):
result = []
left_index = 0
right_index = 0
while left_index < len(left_half) and right_index < len(right_half):
if left_half[left_index] <= right_half[right_index]:
result.append(left_half[left_index])
left_index += 1
else:
result.append(right_half[right_index])
right_index += 1
if left_index == len(left_half):
result += right_half[right_index:]
else:
result += left_half[left_index:]
return result

“`

### Efficiency

The merge sort algorithm has a time complexity of `O(n log n)`, which makes it one of the most efficient sorting algorithms. It has a relatively stable performance and is suitable for sorting large datasets.

The merge sort algorithm has the following advantages:

• Efficient for large datasets
• Stable performance
• Not sensitive to initial order of elements

The merge sort algorithm has the following disadvantages:

• Requires extra memory for the partitioned arrays
• May not be the best choice for small datasets

## Quick Sort in Python

Quick sort is another popular sorting algorithm used in Python that follows a divide-and-conquer approach. It works by selecting a pivot element and partitioning the other elements into two sub-arrays, according to whether they are less than or greater than the pivot. The sub-arrays are then recursively sorted.

Quick sort has an average time complexity of O(n log n), making it faster than bubble sort and selection sort for larger datasets. However, quick sort has a worst-case time complexity of O(n^2), which occurs when the pivot element is the smallest or largest in the array. This can happen if the input data is already sorted or nearly sorted.

### Steps of Quick Sort

The following steps outline the process of quick sort:

### Implementation of Quick Sort

Here is an implementation of quick sort in Python:

``````
def quick_sort(arr):
if len(arr) <= 1:
return arr
pivot = arr
less = []
greater = []
for element in arr[1:]:
if element < pivot:
less.append(element)
else:
greater.append(element)
return quick_sort(less) + [pivot] + quick_sort(greater)
``````

This implementation uses recursion to sort the sub-arrays. The base case is when the length of the array is less than or equal to 1. The pivot element is selected as the first element of the array. The less and greater sub-arrays are then created by comparing each element to the pivot element.

Quick sort has several advantages over other sorting algorithms:

• It has an average time complexity of O(n log n), making it faster than bubble sort and selection sort for larger datasets.
• It is an in-place sorting algorithm, meaning it does not require additional memory to sort the elements.
• It is a stable sorting algorithm, meaning it can handle duplicate elements and maintain their relative order.

Quick sort also has some disadvantages:

• It has a worst-case time complexity of O(n^2), which can occur if the pivot element is the smallest or largest in the array.
• It is not a stable sorting algorithm, meaning it may change the relative order of duplicate elements.

Overall, quick sort is a popular sorting algorithm in Python due to its efficiency and simplicity. However, it is important to consider the input data and possible worst-case scenarios when choosing a sorting algorithm.

## Bubble Sort in Python

The bubble sort algorithm is a simple and intuitive sorting algorithm that works by repeatedly swapping adjacent elements if they are in the wrong order. The algorithm gets its name from the way elements “bubble” up to their correct position. Although it is not the most efficient sorting algorithm, it is an essential algorithm to understand for anyone learning sorting in Python.

### How Bubble Sort Works

Bubble sort works by comparing adjacent elements and swapping them if they are in the wrong order. The algorithm goes through the entire list repeatedly until there are no more swaps to be made. Here’s a step-by-step breakdown of the bubble sort algorithm:

As you can see, the algorithm repeatedly swaps adjacent elements until the list is sorted in ascending order.

### Implementation of Bubble Sort in Python

Here’s an example implementation of the bubble sort algorithm in Python:

“`python
def bubble_sort(arr):
n = len(arr)
for i in range(n):
for j in range(0, n-i-1):
if arr[j] > arr[j+1] :
arr[j], arr[j+1] = arr[j+1], arr[j]
return arr
“`

This implementation uses two nested loops to go through the list and swap adjacent elements if necessary. The outer loop iterates over the length of the list, and the inner loop iterates over the remaining unsorted elements.

### Performance of Bubble Sort

Bubble sort has a time complexity of O(n^2), which means that it is not the most efficient sorting algorithm for large datasets. However, for small datasets, bubble sort can be a simple and effective sorting algorithm.

In comparison to other sorting algorithms like merge sort and quick sort, bubble sort is less efficient. However, it is still an important algorithm to learn as it provides a foundation for understanding sorting algorithms.

## Selection Sort in Python

Selection sort is a simple and intuitive sorting algorithm that is often used in small datasets or as a building block for more complex sorting algorithms. Its basic idea is to find the smallest element in the unsorted part of an array and move it to the front of the sorted part. The sorted portion of the array grows progressively larger as the algorithm iterates through the unsorted portion.

### How Selection Sort Works

Selection sort works by dividing the input array into two parts – the sorted part and the unsorted part. Initially, the sorted part is empty, and the unsorted part contains the entire array.

The algorithm then repeatedly finds the minimum element in the unsorted part and swaps it with the first element in the unsorted part. This effectively moves the minimum element to its correct position in the sorted part and grows the size of the sorted part by one element.

The algorithm continues this process until the entire array is sorted.

### Implementation of Selection Sort in Python

Here is an implementation of the selection sort algorithm in Python:

Here, arr is the input array to be sorted.

The outer loop iterates through the entire array, while the inner loop searches for the minimum element in the unsorted portion of the array. The minimum element is then swapped with the first element in the unsorted portion of the array. This process is repeated until the entire array is sorted.

### Time Complexity of Selection Sort

The time complexity of selection sort is O(n^2), where n is the number of elements in the array. This means that selection sort is not suitable for large datasets, as its performance degrades rapidly as the size of the array increases.

However, selection sort has a space complexity of O(1), meaning that it does not require any additional memory aside from the input array itself.

### Comparison to Other Sorting Algorithms

Although selection sort is simple to implement and understand, it is not as efficient as other sorting algorithms, such as merge sort or quick sort. This is because selection sort requires multiple passes through the unsorted portion of the array to find the minimum element, leading to a time complexity of O(n^2).

In comparison, merge sort and quick sort have time complexities of O(n log n), making them more appropriate for large datasets.

Despite its inefficiency, selection sort is still useful as a building block for more complex sorting algorithms and for small datasets where performance is not a critical concern.

## Conclusion

In conclusion, mastering the art of sorting is an essential skill for any Python programmer. Whether you are working with small or large datasets, being able to efficiently organize and sort your data is crucial for optimal performance and accuracy.

We hope this article has shed some light on the different sorting techniques available in Python, including the sorted() function and .sort() method. We have also discussed how to sort strings with numerical values, perform sorting while maintaining the original index, and explored various sorting algorithms such as merge sort, quick sort, bubble sort, and selection sort.

### Practice Makes Perfect

Now that you have a better understanding of Python sorting, we encourage you to practice sorting using different methods and algorithms. By doing so, you will not only enhance your programming skills but also gain confidence in your ability to tackle complex datasets.

Remember to consider the size and complexity of your data when choosing the most appropriate sorting algorithm. A little bit of research and planning can go a long way in optimizing your code and achieving better results in less time.

Thank you for taking the time to read this article. We hope you found it informative and useful in your Python programming journey. Happy sorting!