How to Substring a String in Python

String

Unicode characters collection is referred to as a String. It consists of a succession of characters, including alphanumeric and special characters.

Substring: Core Concept

A substring is a piece of a string. Python offers a variety of techniques for producing substrings, as well as for determining the index of a substring and more. Let's examine a few substring-related operations.

What is Slicing?

Slicing is a Python technique for gaining access to a subset of data from given sequences such as strings, lists, tuples, etc. A slice object is created based on a set of indices. You can choose a start index, stop index, or step-index.

Index in Python

In Python, the index starts at 0 in forwarding and -1 in reverse order.

Think about the string "String Substring" stored in a variable word.

Note: Single (' ') or double (") quotes are always used to contain strings.

0   1   2   3   4   5   6   7   8   9   10   11   12   13   14   15  

   S        t      r      i       n      g      S      u   b     s      t     r     i     n    g

-16   -15   -14   -13   -12   -11   -10   -9   -8   -7    -6   -5   -4   -3   -2   -1

It is evident from the above that space is likewise regarded as an index. In addition to being considered when indexing and determining a variable's length, space is also treated as a character.

Code:

String3 = "String and Substring in Python"
print (len(String3))

Output:

Python has an internal function called len() that can be used to determine a variable's length. We determined the length of the variable phrase in the aforementioned example. It is obvious that it contains spaces; therefore, when determining the variable's length, space is also considered.

• Spaces: 4
• Characters: 26
• Total: 30

Methods for Making a Substring

There are numerous ways to produce a substring. String slicing is one of the common techniques.

Depending on a certain delimiter present in a text, the split() function can produce a list of substrings.

In Python, slicing can be used to create a substring, as seen below:

string[begin: end: step]

Here,

Begin: The substring's starting index is begun. The substring contains this component. If begin is not supplied while slicing, it is considered 0.

Step: The character that must be added following the current character is called Step. If the step's default value is not specified, it is considered one.

slicing a string– Syntax:

• string [begin: end]: The substring contains characters from the start through end-1.

Example:

s= 'String and Substring.'


print(s[2:10])

Output:

• string[: end]: The substring contains characters up to end-1. An illustration of slicing without a beginning index is as follows:

Example:

s= 'String and Substring.'
print(s[:6])

Output:

• string[begin:]:characters from the starting index to the string's end are included in the substring. Example for slicing without end index:

Example:

s= "String and Substring."
print(s[1:])

Output:

• String [begin:end: step]: substring consists of characters from start to end-1, except for each step character.

Example:

s= "String and substring"
print(s[2:8:3])

Output:

• string[:]: The complete string is included. Using slicing without begin or end index.

Example:

s= " String and Substring"
print(s[:])

Output:

• string[index]: A single character is included in the substring. Giving a character at a specific index, for instance:

Example:

s= "String and Substring"
print(s[4])

Output

• Negative slicing: Get a substring by using a negative index.

Example:

s= "string and substring "
print(s[0:-4])

Output:

• Reversing a string: By employing a negative step, slicing can be utilized to return the string's reverse.

Example:

s= 'String and Substring'
print(s[::-1])

Output:

How to Create a Substring in Python

Code1:

string1= " String and Substring " # substring using slicing
st = string1[7:]
print(st)

Output:

Code2:

string2= “ string and substring “ # Creating list of substrings using split function
st = string2.split()
print(st)

Output:

How to Determine Whether a Substring is Present

To determine whether a substring is present in the provided string, use the find() method or the in operator.

Example:

string1 = 'String and Substring '
if 'Substring' in string1: # using the in Operator.
 print('Substring found')
if string1.find('String') != -1: # using the find() method.
 print('Substring found')
else:
 print('Substring not found')

Output:

The count() Method

Use the count() method as follows to determine how many times a specific substring appears in the input string:

string3 = "using the count function "
print('Substring u count =', string3.count('u'))

Output:

Substring Indexes

There is no built-in function in Python that will find all substring indexes. However, we can define one using the find() function to get a list of all the indexes for a substring. As follows:

Code:

def indexes(string, substring):


    list1 = []


    length = len(string)


    index = 0


    while index < length:


        x = string.find(substring, index)


        if x == -1:


            return list1


        list1.append(x)


        index = x + 1


    return list1


st1 = "String and Substring"


print(indexes(st1, 'n')

Output:

How to Use List Slicing to Extract the Substring from a Given String

In Python, list slicing allows us to obtain the substring from a given string. Here are a few illustrations:

• Python Code to Extract a Substring From a String:

# Initialise


string1 = 'string and substring'


print ("Initial string: ", string1) 
start = string1[:4] # from the start.
end = string1[2:]  # from the end.
print ("Resultant substring from start:", start) 


print ("Resultant substring from end:", end)

Output:

• Construct a Substring from Characters in a Specific Gap (step).

# Initialise 
string2 = 'String and Substring'
print ("Initial String: ", string2) 
# create substring by taking element after certain position gap and define length upto which substring is required 
alt1 = string2[::3] 
gap = string2[::5] 
print ("Resultant substring from start:", alt1) 
print ("Resultant substring from end:", gap)

Output:

• Consider a string from the middle with a small space between the characters as you create a substring.

# Initialise  
string3 = 'substring in python'
print ("Initial string: ", string3) 
# create substring by taking element after certain step gap in a defined length 
st = string3[2:11:2] 
print ("Resultant substring:", st)  # Output

Output:

Conclusion

Alphanumeric and special characters are both included in Python String collections. They are also known as character arrays; the sole distinction is that they behave somewhat differently from arrays. But most of the time, strings may be thought of as arrays.

In the Python language, a substring is a group of characters with another string attached. Another name for it is "Slicing of String."

Our strings can be processed using a feature called "slicing." Slicing is a versatile technique with a wide range of applications.Different functions are available in many programming languages to extract the substring from the source or main string.