Strings

Task: Reverse this string in place

Why we need Strings

Imagine a necklace made of beads.
Each bead is a character, and together they form a string.
You can count the beads, replace one, or cut the necklace into smaller parts.

What is a string?

A string is a sequence of characters stored together.

"Hello" is made up of 'H', 'e', 'l', 'l', 'o'.

Strings are how programs represent text.

String operations

Operation	What it does	Example
Concatenation	Joins two strings	"Hello" + "World"
Substring	Takes part of a string	"Hello"[0:2]
Length	Counts the characters	len("Hello")
Search	Finds a character or word	"World" in "HelloWorld"

Try it yourself

A run of characters you can read, not rewrite

Hello

"Hello"Choose an operation

length 5

Press concat and watch every character flash, not just the new one. Then press edit, which is the one that will not work. Both of those have a reason, further down the page.

Example in code

Code

s = "Hello" print(len(s))        # 5print(s.upper())     # HELLOprint(s[1])          # eprint(s + " World")  # Hello World

Immutable nature

In many languages, including Python and Java, strings are immutable. Once a string is created it cannot be changed directly.

"Hello"[0] = 'J' is an error, not an edit. To get "Jello" you have to build a new string.

So an operation that looks like a change is really a replacement: the variable is pointed at a fresh string, and the old one is left behind untouched.

That rule has an upside. Because the characters can never change, a language is free to let two variables share one stored copy instead of giving each its own.

Advantages

Easy to represent text and data.
A rich set of built-in operations comes for free.
The foundation for parsing, searching and data handling.

Limitations

Immutable strings are inefficient when you modify them repeatedly, because each change builds a whole new string.
Large string operations can use a lot of memory.

Strings vs arrays

	Array	String
Elements	Numbers or objects	Characters
Mutability	Often mutable	Often immutable
Use case	Store a collection	Represent text

A string behaves like an array of characters that you are not allowed to write into. Reading by index works the same way, and it starts at 0 the same way.

Quick recap

Think of a sentence in a book. Each letter is a character, and together they form words and sentences.

A string is a sequence of characters
Operations: concatenate, substring, length, search
Immutable in many languages, so a change builds a new string
Used everywhere text is needed

Beads on a necklace, and in most languages the necklace cannot be restrung.