Count & Sort Methods in Python Lists: Tutorial with Examples

The count() & Sort() methods are built-in methods in Python lists, useful for counting occurrences of elements and sorting of elements in the list. Lists in Python can store duplicate elements and elements of various data types, making these methods essential for data handling.

In previous article, we learned about Loops and List Comprehension in Python. In this article we will learn about Python Count Method & Python Sort Methods.

Python Count Method in List

The count() method is used to determine how many times a specific object appears in a list. Syntax of the count() method => count_value = list.count(object)

• The count() method takes a single object as an argument and returns the number of occurrences of that object in the list.
• Argument object, can be an element of any datatype or of any collection (list, tuple, set, dictionary etc.)
• It compares the elements with case sensitivity, so make sure to pass the exact value you are counting.

There are multiple scenarios in which we can use count(object) method:

1. Count of Numeric or String Element
2. Count of Case-Sensitive String Element
3. Count of Collection Element
4. Error: Count Method Passed with Two Parameters

Count of Numeric or String Element

The count() method can be used to count the occurrences of numeric or string elements in a list.

# Count of Numeric or String Element
print ("count of numeric element")
a = [4, 2, 3, 4, 9, 4, 2]                   # list of numeric elements, 4 occurs three times
print(a)
print("count of 4 in list - ", a.count(4))  # count method, return count of 4 in the list

print("\n---------------------------------------------------\n")

# count of string element
print("count of string element")             # list of string elements
b = ["AWS", "Azure", "Python", "Python", "Data Science", "Java Advanced"]   
print(b)
print("count of 'Python' in list - ", b.count("Python"))    # count method, return count of 'Python' in the list

In this example program we have used the count() method with list of numeric elements and a list of string elements. For numeric list a, we are using a.count(4) method to get the count of 4, which is present 3 times in the list. For string list b, we are using b.count("Python") method to get the count of Python, which is present 2 times in the list.

Output – Example Program – Count of Numeric or String Element

count of numeric element
[4, 2, 3, 4, 9, 4, 2]
count of 4 in list -  3
---------------------------------------------------
count of string element
['AWS', 'Azure', 'Python', 'Python', 'Data Science', 'Java Advanced']
count of 'Python' in list -  2

From the program output, we got count 3 for element 4 in list and count 2 for element ‘Python’ in list.

Count of Case-Sensitive String Element

The count() method is case-sensitive. For example, counting Python will not count python.

# Count of Case-Sensitive String Element
print("count of case-sensitive string element")
string_list = ["AWS", "Azure", "Python", "Python", "Data Science", "Java Advanced"]
print(string_list)
print("count of case-sensitive 'python' in list - ", string_list.count("python"))        # count of case-sensitive 'python'

In this example program, we have defined string_list of string elements. We are using string_list.count("python") method to get the count of python. Note – we are counting for small case letter python but list contains element Python with capital P. count("python") method will compare elements with case-sensitivity and 0 occurrences of python will be found.

Output – Example Program – Count of Case-Sensitive String Element

count of case-sensitive string element
['AWS', 'Azure', 'Python', 'Python', 'Data Science', 'Java Advanced']
0  # No occurrences of small letter python in the list

From the program output, we got count 0 for python in the list. But element Python with capital ‘P’ exists 2 times.

Count of Collection Element

The count() method can also count collection elements within a list.

# Count of Collection Element
print("count of collection element")

nested_list = [1, 'Python', 'Azure', ['AWS',2]]
print(nested_list)
print(nested_list.count(['AWS',2]))   # count of collection element
print(nested_list.count([]))	  # count of empty list

In this example program, we have defined a nested_list. We are using nested_list.count(['AWS',2]) method to get the count of a collection element ['AWS',2] and nested_list.count([]) to get the count of empty list.

Output – Example Program – Count of Collection Element

count of collection element
[1, 'Python', 'Azure', ['AWS', 2]]
1  # occurrence of ['AWS', 2] in the list
0  # occurrence of [] in the list

As return value from this program, count of ['AWS', 2] in the list is 1 and empty list count is 0.

Error: Count Method Passed with Two Parameters

The count() method accepts only one argument. If multiple parameters are passed, it will raise an error.

# Error: Count Method Passed with Two Parameters
print("Error - count method passed with two parameters")
e = ["AWS","Azure","Python","Python"]
print(e)
print(e.count("Python", 2))    # count method passed with 2 parameters

Output – Example Program – Error: Count Method Passed with Two Parameters

Error - count method passed with two parameters
['AWS', 'Azure', 'Python', 'Python']
Traceback (most recent call last):
  File "D:-count-element-list.py", line 41, in <module>
    print(e.count("Python", 2))
          ^^^^^^^^^^^^^^^^^^^^
TypeError: list.count() takes exactly one argument (2 given)

Python Sort Method in List

The sort() method is used to sort the list in Python, either in ascending or descending order. Syntax of the sort method => unsorted_list.sort(reverse)

• By default, sort() method sorts in ascending order.
• reverse (optional) – This is an optional parameter. By default its value if False. If passed as True, then sorting happens in descending order.

Lets see sorting of numeric and string list with two scenarios.

• Python Sort Method – List Sorting in Ascending Order
• Python Sort Method – List Sorting in Descending Order

Python Sort Method – List Sorting in Ascending Order

Sorting a list in ascending order can be done using the sort() method without the reverse parameter.

# sorting of alphanumeric list in ascending order
alphanumeric_list = ['Python', 'AWS', 'Java', 'Azure', 'DataScience']
print(alphanumeric_list)
alphanumeric_list.sort()       # sorting of alphanumeric list in ascending order
print(alphanumeric_list)

# sorting of numeric list in ascending order
numeric_list = [23,45,78,13, 34, 89, 67]
print(numeric_list)
numeric_list.sort()            # sorting of numeric list in ascending order
print(numeric_list)

In this example program, we have defined two lists – alphanumeric_list with string elements and numeric_list with numeric elements. Elements of both the lists are printed before sorting. sort() method (without reverse parameter) is called on both lists to sort them in ascending order.

• alphanumeric_list.sort() => This will sort the alphanumeric_list in ascending order based on the alphanumeric characters.
• numeric_list.sort() => This will sort the numeric_list in ascending order based on the numeric values.

Output – Example Program – List Sorting in Ascending Order

['Python', 'AWS', 'Java', 'Azure', 'DataScience']
['AWS', 'Azure', 'DataScience', 'Java', 'Python']      # alphanumeric sorting - sorted list in ascending order

[23, 45, 78, 13, 34, 89, 67]
[13, 23, 34, 45, 67, 78, 89]        # numeric sorting - sorted list, ascending order

From the program output, elements of alphanumeric_list are sorted in ascending order based on elements alphanumeric value. Elements of numeric_list are sorted in ascending order based on elements numeric value.

Python Sort Method – List Sorting in Descending Order

# sorting of alphanumeric list in descending order
alphanumeric_list = ['Python', 'AWS', 'Java', 'Azure', 'DataScience']
print(alphanumeric_list)
alphanumeric_list.sort(reverse = True)    # sorting of alphanumeric list in descending order
print(alphanumeric_list)

# sorting of numeric list in descending order
numeric_list = [23,45,78,13, 34, 89, 67]
print(numeric_list)
numeric_list.sort(reverse = True)        # sorting of numeric list in descending order
print(numeric_list)

In this example program, we have defined two lists – alphanumeric_list with string elements and numeric_list with numeric elements. Elements of both the lists are printed before sorting. sort(reverse = True) method (with reverse = True parameter) is called on both lists to sort them in descending order.

• alphanumeric_list.sort(reverse = True) => This will sort the alphanumeric_list in descending order based on the alphanumeric characters.
• numeric_list.sort(reverse = True) => This will sort the numeric_list in descending order based on the numeric values.

Output – Example Program – List Sorting in Descending Order

['Python', 'AWS', 'Java', 'Azure', 'DataScience']
['Python', 'Java', 'DataScience', 'Azure', 'AWS']     # sorted list, descending order, alphanumeric sorting

[23, 45, 78, 13, 34, 89, 67]
[89, 78, 67, 45, 34, 23, 13]     # sorted list, descending order, numeric sorting

From the program output, elements of alphanumeric_list are sorted in descending order based on elements alphanumeric value. Elements of numeric_list are sorted in descending order based on elements numeric value.

Summary

In this article, we explored the use of the count() and sort() methods in Python lists. Here’s a quick recap:

• Python Count Method in List
  • Count of Numeric or String Element
  • Count of Case-Sensitive String Element
  • Count of Collection Element
  • Error: Count Method Passed with Two Parameters
• Python Sort Method in List
  • Python Sort Method – List Sorting in Ascending Order
  • Python Sort Method – List Sorting in Descending Order

By understanding these methods, you can efficiently manipulate and manage lists in Python.

Code snippets and programs related to Count & Sort Methods in Python Lists, can be accessed from GitHub Repository. This GitHub repository all contains programs related to other topics in Python tutorial.

Interview Questions & Answers

Q: What is the time complexity of count() method?

The time complexity of the count() method is O(n), where n is the number of elements in the list. This is because the method needs to traverse the entire list (iterate over all the elements of the list) to count the occurrences of the specified element.

Q: How can we count the occurrences of multiple elements in a list?

We can use a loop or a dictionary comprehension to count multiple elements in a list.

elements_to_count = [22, 32, 22, 22, 52, 32]
counts = {element: elements_to_count.count(element) for element in elements_to_count}
print(counts)  # count of elements: {22: 3, 32: 2, 52: 1}

Q: What is the time complexity of the sort() method?

The time complexity of the sort() method is O(n log n) where n is the number of elements in the list. Python’s sort implementation uses Timsort (data sorting algorithm), which is highly efficient for real data.

Q: What is the difference between sort() and sorted() in Python?

• sort() method modifies the original list and returns None
• sorted() method returns a new sorted list and does not modify the original list. sorted() can be used on any iterable, not just lists. Example of sorted() method:

numbers_list = [42, 22, 27, 11, 34]
sorted_numbers = sorted(numbers_list)
print(numbers_list)         # original order of elements: [42, 22, 27, 11, 34]
print(sorted_numbers)  # sorted elements: [11, 22, 27, 34, 42]

Q: What happens if we try to sort a list with mixed data types?

If we try to sort a list that contains incompatible data types (like integers and strings), Python will raise a TypeError. To handle such situations, you should ensure that all elements in the list are of a comparable type.

mixed_list = [32, 'shbytes', 22]
try:
    mixed_list.sort()
except TypeError as e:
    print(f"Error: {e}")  # Error: '<' not supported between instances of 'str' and 'int'

Q: How to sort a list in a custom order using the sort() method?

To sort a list in a custom order using sort() method, we can provide a custom function to the key parameter. This custom function can be used to define the sorting order.

courses_list = ['Python', 'Power BI', 'Java']
courses_list.sort(key=lambda x: len(x))   # sorting based on length of string
print(courses_list)  # sorted list - ['Java', 'Python', 'Power BI'] (sorted by length)

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