Python String find Method

To begin with, let us review the concept of a String in Python:

In Python, a string refers to a sequence of characters that can be enclosed in either single, double, or triple quotation marks. It represents a series of Unicode characters, each with a defined length. Consequently, once a string is instantiated, its value cannot be modified.

In Python, strings are utilized to represent text, numerical values, symbols, and various other types of data. They are commonly employed for storing and manipulating textual information, reading from and writing to files, handling user input, and generating output in a specified format.

Python strings support a variety of operations, including concatenation, slicing, and indexing. In addition to these functionalities, strings can be modified and altered using a range of built-in methods. These methods allow for operations such as replacing specific characters, converting text to either uppercase or lowercase, and extracting substrings.

my_string = "Hello, world!"

A variety of functions are employed to alter strings or to perform operations involving Strings.

One of them is the find method.

find Function in String

The find method in Python is used to locate a substring within an entire string and returns the index of its first occurrence. If the specified substring is not present, it returns -1.

Python String find Function Syntax:

The syntax of the find method is as follows:

string.find(substring, start, end)

Parameters:

Where:

• ✅ String is the String in which you want to search for the substring
• ✅ Substring is the String you want to search for
• ✅ Start is the index from which the search should begin (optional, defaults to 0)
• ✅ End is the index at which the search should end (optional, defaults to the end of the String)

Return Type:

The find function provides the position of the initial instance of the specified substring within the designated range. If the substring is absent, it will yield -1.

Different Examples for Python String find Function

Below is an illustration of employing the find method to locate a substring within a given string:

Example 1:

Let us explore a program that utilizes the find function in Python to locate a substring within a string.

string = "Hello, world!"

substring = "world"

index = string.find(substring)

print(index)

Output:

Example 2:

Locating a substring within a string using specified start and end indices:

string = "Hello, world!"

substring = "l"

start = 3

end = 8

index = string.find(substring, start, end)

print(index)

Output:

Example 3:

Identifying a substring that does not exist within a given string can be achieved through a systematic approach. Below are several methods to accomplish this task effectively:

1. Brute Force Method:

• ✅ Generate all possible substrings of the target string.
• ✅ Compare each generated substring against the string in question. If a substring is not found within the string, it qualifies as a substring that is absent.
• ✅ This method, while straightforward, can become computationally expensive for larger strings.

1. Using Regular Expressions:

• ✅ Employ regular expressions to search for the absence of a substring within a string. You can utilize the negative lookahead assertion.
• ✅ For example, using Python, you might implement this with:
Example
Copy
     import re

     string = "example string"
     substring = "notpresent"
     if not re.search(substring, string):
         print(f"'{substring}' is not present in the string.")
Try it Yourself

1. Set Operation Method:

• ✅ Create a set of all substrings from the original string, then check if the desired substring is included in that set.
• ✅ This approach can be particularly efficient since checking membership in a set is typically O(1).

1. Iterative Comparison:

• ✅ Iteratively check for the presence of the substring by looping through the original string and examining segments of the string that match the length of the substring.
• ✅ If, after checking all possible segments, the substring is never found, then it is confirmed to be absent.

1. String Methods in Programming Languages:

• ✅ Many programming languages provide built-in methods to check for substring presence. For instance, in Python, you can use the in keyword:
Example
Copy
     string = "example string"
     substring = "notpresent"
     if substring not in string:
         print(f"'{substring}' is not found in the string.")
Try it Yourself

By employing any of the aforementioned techniques, one can effectively determine whether a given substring is absent from the specified string.

string = "Hello, world!"

substring = "python"

index = string.find(substring)

print(index)

Output:

Example 4:

Locating a substring within a string that contains several instances:

string = "Hello, world!"

substring = "l"

index = string.find(substring)

while index != -1:

    print(index)

    index = string.find(substring, index + 1)

Output:

2

3

10