Add Substring at Specific Index Python

Adding a substring at a specific index in Python means creating a new string by combining slices of the original string with the inserted text, since strings are immutable. For example, inserting "for" at index 5 in "GeeksGeeks" results in "GeeksforGeeks".

Let’s explore several efficient ways to achieve this.

Using list slicing

This is the most straightforward method. We split the string into two parts and insert the new string in between.

s = "GeeksGeeks"
add = "for"
i = 5

res = s[:i] + add + s[i:]
print(res)

GeeksforGeeks

Explanation:

• s[:i] gives "Geeks" before index i , s[i:] gives "Geeks" after index i and add = "for" is inserted in between.
• s[:i] + add + s[i:] → "Geeks" + "for" + "Geeks" = "GeeksforGeeks"

Using join()

We convert the string to a list, insert the new string at the desired position and then join it back into a string.

s = "GeeksGeeks"
add = "for"
i = 5

a = list(s)
a.insert(i, add)
res = ''.join(a)
print(res)

GeeksforGeeks

Explanation:

• list(s) creates a list of characters from the string s and a.insert(i, add) inserts the string "for" at index i as a single element.
• ''.join(a) combines the list back into a single string, inserting "for" at the specified position.

Using str.format()

format() function insert a substring at a specific index by slicing the original string and combining the parts with placeholders.

s = "GeeksGeeks"
add = "for"
i = 5

res = "{}{}{}".format(s[:i], add, s[i:])
print(res)

GeeksforGeeks

Explanation:

• {} placeholders are used for formatting
• s[:i], add and s[i:] are passed into format() in order

Using f-string

An f-string is a cleaner and modern way of formatting strings. It makes the code more readable and concise, especially when inserting values directly into strings.

s = "GeeksGeeks"
add = "for"
i = 5

res = f"{s[:i]}{add}{s[i:]}"
print(res)

GeeksforGeeks

Explanation:

• f-strings evaluate expressions inside {}
• This directly inserts "for" at the desired index

Using re.sub()

're' module insert a substring at a specific index. We match the first i characters using a regex pattern and then add the new string right after the match.

import re
s = "GeeksGeeks"
add = "for"
i = 5

res = re.sub(f"(.{{{i}}})", f"\\1{add}", s)
print(res)

GeeksforGeeksfor

Explanation:

• The pattern (.{{{i}}}) captures the first i characters
• \\1{add} inserts the substring after the matched portion

Related Articles:

• How to Slice Strings in Python
• join() Function
• str.format()
• f-Strings in Python
• Regex (re) Module
• Remove a Substring from a String in Python