# Strings in Python

In Python, **strings are immutable**.

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

### Example:

```python
s = "python"

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

You cannot modify a character directly inside a string.

* * *

## 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:

```python
s = "python"
s = "Python"
```

Here, a new string is created.

* * *

### 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.

* * *

## 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:

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

* * *

## Searching in a String → O(N)

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

```python
if "a" in s:
    print(True)
```

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

* * *

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