Strings in Python

In Python, strings are immutable.

This means once a string is created, it cannot be changed.

Example:

s = "python"

s[0] = "P"   # ❌ This will give an error

You cannot modify a character directly inside a string.

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What Does “Immutable” Mean?

If you try to change a string, Python does not modify the original string.
Instead, it creates a new string in memory.

Example:

s = "python"
s = "Python"

Here, a new string is created.

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Some of the most common and useful string methods are:

Case Conversion: upper(), lower(), capitalize(), title(), and swapcase() allow for changing the casing of strings.

Searching: Methods like find(), index(), and count() locate or count substrings, while startswith() and endswith() check string boundaries.

Modification: strip() (and lstrip()/rstrip()) removes whitespace, replace() substitutes substrings, split() divides strings into lists, and join() concatenates iterables.

Formatting/Validation: format() or f-strings are used for string formatting, while isalpha(), isdigit(), and isalnum() validate string content.

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Why Is This Important in DSA?

Many interview problems are based on strings:

• Palindrome problems

• Anagram checking

• Substring problems

• Pattern matching

• Sliding window on strings

Because strings are immutable:

• You cannot modify characters directly.

• You often convert strings into lists when modification is required.

Example:

s = "python"
s_list = list(s)
s_list[0] = "P"
s = "".join(s_list)

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Searching in a String → O(N)

You can check whether a character exists in a string using:

if "a" in s:
    print(True)

This operation takes O(N) time because Python may need to traverse the entire string.

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Time Complexity :

• Index access → O(1)

• Searching → O(N)

• Concatenation inside a loop → Can become O(N²) (be careful!)

For large string manipulation problems, consider:

• Using a list for modification

• Using join() instead of repeated concatenation