str.count() Function in Python

The str.count() function in Python is used to count the number of occurrences of a specified substring within a string. This function is useful for determining how many times a particular substring appears in a string.

Syntax:

string.count(substring, start, end)

• substring: The substring you want to count.
• start (optional): The starting index from where the count should begin. The default is 0.
• end (optional): The ending index where the count should stop. The default is the end of the string.

Example 1: Basic usage

# Example 1: Counting occurrences of a substring
my_string = "Hello, world! Hello, everyone!"
count = my_string.count("Hello")
print(count)  # Output: 2

In this example:

• The substring "Hello" appears 2 times in the string.

Example 2: Specifying start and end indices

# Example 2: Specifying start and end indices
my_string = "banana"
count = my_string.count("a", 1, 5)  # Count 'a' from index 1 to index 5
print(count)  # Output: 2

Here:

• The count of "a" is done only within the substring "anan" (from index 1 to index 5), resulting in 2 occurrences.

Example 3: Case sensitivity

# Example 3: Case sensitivity
my_string = "Hello, world! hello, everyone!"
count = my_string.count("hello")
print(count)  # Output: 1

In this case:

• The count of "hello" (lowercase) returns 1, while the uppercase "Hello" is not counted, demonstrating that str.count() is case-sensitive.

Example 4: Counting a substring not present

# Example 4: Substring not present
my_string = "Hello, world!"
count = my_string.count("Python")
print(count)  # Output: 0

Here:

• The substring "Python" is not found in the original string, so the count returns 0.

Key Points:

• str.count() returns the number of non-overlapping occurrences of the specified substring.
• It is case-sensitive, meaning that different cases (uppercase vs. lowercase) are treated as different characters.
• If the substring is not found, it returns 0.
• You can specify optional start and end parameters to limit the search range within the string.
• The function does not modify the original string.

Example 5: Counting overlapping substrings

If you want to count overlapping occurrences, you'll need a different approach, as str.count() does not count overlapping substrings.

# Example 5: Counting overlapping substrings (manual approach)
my_string = "aaa"
substring = "aa"

# Using a loop to count overlapping occurrences
count = 0
start = 0

while start < len(my_string):
    start = my_string.find(substring, start)
    if start == -1:  # No more occurrences
        break
    count += 1
    start += 1  # Move to the next character to count overlapping

print(count)  # Output: 2

In this example:

• The loop counts overlapping occurrences of "aa" in the string "aaa" and outputs 2.