Python index() method: check whether a string contains a substring

In this tutorial, we'll learn about the index() method in Python, which is a string method that searches for a specified substring within a string and returns the index of the first occurrence. If the substring is not found, the method raises a ValueError.

The index() method is similar to the find() method, but raises an error when the substring is not found instead of returning -1.

The index() method takes the following arguments:

sub (required): The substring to search for within the string.
start (optional): The starting index from which to search for the substring. By default, it is 0.
end (optional): The ending index up to which to search for the substring. By default, it is the length of the string.

Example 1: Basic usage of the index() method

text = "Hello, world! Welcome to the world of Python."
substring = "world"

index = text.index(substring)
print(index)  # Output: 7

Example 2: Using start and end arguments

text = "Hello, world! Welcome to the world of Python."
substring = "world"

index = text.index(substring, 10)  # Start searching from index 10
print(index)  # Output: 28

text = "Hello, world! Welcome to the world of Python."
substring = "world"

index = text.index(substring, 0, 10)  # Search between index 0 and 10
print(index)  # Output: 7

Example 3: Substring not found

text = "Hello, world! Welcome to the world of Python."
substring = "Java"

try:
    index = text.index(substring)
except ValueError:
    print(f"'{substring}' not found in the text.")

Note that the index() method is case-sensitive. If you want to perform a case-insensitive search, you can convert both the text and the substring to lowercase (or uppercase) before using the index() method.

text = "Hello, world! Welcome to the World of Python."
substring = "WORLD"

index = text.lower().index(substring.lower())
print(index)  # Output: 28

In summary, the index() method is used to search for a specified substring within a string and returns the index of the first occurrence. If the substring is not found, it raises a ValueError. You can also specify a start and end index to search within a specific range.

How to Use index() to Check Substring Presence in Python:
- The index() method is used to find the index of the first occurrence of a substring in a string. If the substring is not found, it raises a ValueError.
```
# Example
sentence = "Python is powerful"
try:
    index = sentence.index("is")
except ValueError:
    index = -1
```

Checking for Substring Existence with index() in Python:

Use index() to check if a substring exists in a string.

# Example
sentence = "Python programming"
try:
    index = sentence.index("java")
except ValueError:
    index = -1

if index != -1:
    print("Substring found")
else:
    print("Substring not found")

Find Substring Index in a String with Python index():

Retrieve the index of the first occurrence of a substring using index().

# Example
sentence = "Python is versatile"
try:
    index_python = sentence.index("Python")
except ValueError:
    index_python = -1

Using index() to Search for a Substring Occurrence in Python:

Search for a specific substring occurrence using index().

# Example
sentence = "Python programming is fun. Python is powerful."
try:
    index_second_python = sentence.index("Python", sentence.index("Python") + 1)
except ValueError:
    index_second_python = -1

Handling Missing Substrings with index() in Python:

Check for the presence of a substring and handle missing substrings.

# Example
sentence = "Python is amazing"
substring = "Java"
try:
    index = sentence.index(substring)
except ValueError:
    index = -1

if index != -1:
    print(f"Substring found at index {index}")
else:
    print(f"Substring '{substring}' not found")

Case-Insensitive Substring Search with index() in Python:

Perform a case-insensitive substring search using lower().

# Example
sentence = "Python is CASE-INSENSITIVE"
substring = "case-insensitive"
try:
    index = sentence.lower().index(substring.lower())
except ValueError:
    index = -1

Python index() vs find() for Substring Detection:

While find() returns -1 for missing substrings, index() raises a ValueError.

# Example with find()
sentence = "Python is great"
index_find = sentence.find("Java")  # Returns -1 if not found

# Example with index()
try:
    index_index = sentence.index("Java")  # Raises ValueError if not found
except ValueError:
    index_index = -1

Detecting Multiple Substring Occurrences with index() in Python:

Iterate to find multiple occurrences of a substring using index().

# Example
sentence = "Python is easy. Python is versatile. Python is powerful."
substring = "Python"
start_index = 0
while True:
    try:
        index = sentence.index(substring, start_index)
        print(f"Found at index: {index}")
        start_index = index + len(substring)
    except ValueError:
        break