How to Create a Substring of a String in Python?

Last Updated on April 27, 2023 by Prepbytes

One of the fundamental data types in Python is a string. Python Strings are defined as a sequence of characters that includes alphabets, digits, punctuation marks, and other symbols. Sometimes, we need to extract some of the portions of the string for further processing or analysis. In this article, we will learn about Python substring and will also look at some methods of creating a Python substring.

What is a Python Substring?

A Python substring is a contiguous sequence of characters within a string. It can be a single character, a group of characters, or the entire string itself. For example, suppose we are given the string "PrepBytes". For this specified string, some of the substrings are listed as follows: “P”, “rep”, “PrepByt”, “Bytes”, “PrepBytes”, etc.

It is important to note that for a group of characters to be considered a substring, it must be present in a contiguous sequence in the original string. There are different ways of creating Python substrings, which will be discussed in detail in the next section.

String Slicing for Creating Python Substrings

Python does not come with any built-in function that performs the task of finding substrings like the subString function in Java or the substr() function in C++. However, Python provides an alternative method called string slicing, which involves extracting a specific portion of the string by specifying its start and end indices.

Syntax of String Slicing

The syntax of String Slicing for generating Python substrings of a string is given as:

my_str [start: stop: step]

Here, “my_str” denotes the original string of which we want to find the substring.

Parameters of String Slicing

The parameters of string slicing are as follows:

1. Start index: The position of the first character of the substring. It is included in the substring. The default value of this parameter is 0.
2. End index: The position of the last character of the substring. It is excluded from the substring. If not specified, it will take a value equal to the length of the string.
3. Step size (optional): The step size determines the number of characters to jump over to get the next character in the substring. By default, the step size is 1.

Return Type of String Slicing Notation

The return type of string slicing notation in Python is always a string. When a slice of a string is extracted using slicing notation, the result is always a new string containing the specified portion of the original string. The length of the resulting string depends on the difference between the start and end indices of the slice, and it will always be less than or equal to the length of the original string.

If the start and end indices of the slice are the same, an empty string is returned. If the start index is greater than the end index, an empty string is also returned. If the specified indices are out of range, an IndexError is raised.

Examples of Python Substring

Here are some examples of how to create a Python Substring using the String Slicing Operator.

• Python Substring Example 1
  Getting the First 4 Characters of a String

  Code Implementation:

  • Python

  # Declaring string
  my_str = "PrepBytes"
  
  
  # Using string slicing
  new_substr = my_str[:4]
  
  
  print("The first 4 Characters of string are ", new_substr)
  

  Output:

  The first 4 Characters of string are  Prep

  Explanation:
  Here in the above code, we have generated a Python substring by using the slicing. The slice notation [:4] starts at the beginning of the string (index 0) and it stops at position 4, not including the last position.

• Python Substring Example 2
  Getting the Last 5 Characters of the String

  Code Implementation:

  • Python

  # Declaring string
  my_str = "PrepBytes"
  
  
  # Using string slicing
  new_substr = my_str[-5:]
  
  
  print("The last 5 Characters of string are ", new_substr)

  Output:

  The last 5 Characters of string are  Bytes

  Explanation:
  In this example of Python Substring using String Slicing, we have first declared a string “PrepBytes”, then we used my_str[-5:0]. This slice notation starts from the -5 index i.e., from the last fifth character of the string to the end of the string.

• Python Substring Example 3
  Generating substring from some specific index in the String

  Code Implementation:

  • Python

  # Declaring string
  my_str = "PrepBytes"
  
  
  # Using string slicing
  new_substr = my_str[3:]
  
  
  print("The substring starting from index 3 is ", new_substr)
  

  Output:

  The substring starting from index 3 is  pBytes

  Explanation:
  In this example, we have generated a Python Substring which is starting from index 3. We have achieved this by specifying the slicing notation as my_str[3:], which means starting index is 3 and the end index is the end of the string,

• Python Substring Example 4
  Generating substring between two specific indices

  Code Implementation:

  • Python

  # Declaring string
  my_str = "PrepBytes"
  
  
  # Using string slicing
  new_substr = my_str[2:8]
  
  
  print("The substring between the specified indices are ", new_substr)

  Output:

  The substring between the specified indices are  epByte

  Explanation:
  In the above-coded example, we have written my_str[2:8]. This will retrieve a substring starting from index 2(inclusive) to index 8(exclusive) and then we have shown the result as an output on the console screen.

• Python Substring Example 5
  Getting every second character in the string

  Code Implementation:

  • Python

  # Declaring string
  my_str = "PrepBytes"
  
  
  # Using string slicing
  new_substr = my_str[::2]
  
  
  print("The output is ", new_substr)

  Output:

  The output is  PeBts

  Explanation:
  In the above code, we have declared a string and then we have used a slicing operator [::2], which means that the starting position is 0, and ending position is 0 and the step size is specified as 2. Hence, we get the output as “PeBts”, which is every other character in the string.

Conclusion
In conclusion, we learned that a Python substring is a sequence of characters within a string. We explored how to create Python substrings using string-slicing notation, which involves specifying the start and stop indices of the substring in the original string. We also discussed the parameters and return type of string slicing notation and provided examples to illustrate its usage. Overall, string slicing is a powerful and efficient way to create Python substrings.

Python Substring: FAQs

Here are some Frequently Asked Questions on Python Substring.

Q1: What is the difference between a Python substring and a Python string?
Ans: A Python string is a sequence of characters, while a Python substring is a contiguous sequence of characters within a string.

Q2: How can I check if a substring is present in a string in Python?
Ans: You can use the in keyword to check if a substring is present in a string. For example, the expression "abc" in "abcdef" will return True.

Q3: How can I get the length of a Python substring?
Ans: You can use the len() function to get the length of a Python substring.

Q4: Can I modify a Python substring in place?
Ans: No, you cannot modify a Python substring in place. However, you can create a new string that contains the modified substring using slicing and concatenation.

Q5: Can I use regular expressions to extract substrings from a string in Python?
Ans: Yes, you can use regular expressions to extract substrings from a string in Python. The re-module provides functions for working with regular expressions.

Q6: Can I use negative indices to specify the start and stop indices of a Python substring?
Ans: Yes, you can use negative indices to specify the start and stop indices of a Python substring. A negative index counts from the end of the string, with -1 being the last character.

Q7: How can I extract every other character from a Python string as a substring?
Ans: You can use slicing with a step parameter of 2 to extract every other character from a Python string.