String

Definition

A string is an ordered collection of characters enclosed in a pair of quotation marks, such as "Hello" or 'Programming', depending on the programming language.

More formally, a string is a sequence of zero or more characters treated as a single data item. Each character has a position in the sequence, and the positions are usually counted starting from zero.

Example:

"A" is a string of length 1
"Unit 1" is a string containing letters, a space, and a digit
"" is an empty string, which contains no characters

Main Content

1. First Concept

String as a Sequence of Characters

A string is not just a random group of letters; it is an ordered sequence of characters.
The order matters greatly. For example, "cat" and "tac" contain the same letters but are different strings because their character order is different.

A string may contain:

Alphabets: A, b, x
Numbers: 1, 7, 9
Spaces: "Hello World"
Symbols: @, #, %
Mixed content: "User123!"

Indexing and Position

Each character in a string has a specific position called an index.
In most languages, indexing begins at 0.

Example:

String:  H   e   l   l   o
Index:   0   1   2   3   4

This means:

H is at index 0
e is at index 1
o is at index 4

Length of a String

The length of a string is the total number of characters it contains.
Spaces and symbols are counted as characters too.

Examples:

"Hello" has length 5
"Hello World" has length 11
"" has length 0

2. Second Concept

String Representation in Memory

Computers store strings in memory as a sequence of character codes.
Each character is represented using a numerical encoding such as ASCII or Unicode.
This allows the computer to store, compare, and process text efficiently.

Example:

A may be stored as a numeric code
B as another numeric code
Different systems may use different encodings, but Unicode is widely used today because it supports many languages and symbols

Mutability and Immutability

In some languages, strings are immutable, meaning once created they cannot be changed directly.
If a modification is needed, a new string is created.
In other languages, strings may be mutable, meaning their contents can be changed.

Example of immutability:

Original string: "Hello"
If you change H to J, the result is not the same object; a new string like "Jello" may be created

This is important because it affects:

performance
memory usage
how string operations are implemented

String Literals

A string written directly in code is called a string literal.
It is usually enclosed in single quotes, double quotes, or backticks depending on the language.

Examples:

"Computer"
'Science'
"12345"

Some languages allow escape characters inside strings, such as:

\n for newline
\t for tab
\" for a double quote inside a string

Example:

"She said, \"Hello\""

3. Third Concept

Common String Operations

Strings are powerful because many operations can be performed on them.

Concatenation

: Joining two or more strings Example: "Hello" + " " + "World" → "Hello World"

Comparison

: Checking whether two strings are equal or which one comes first alphabetically Example: "apple" comes before "banana"

Substring extraction

: Getting part of a string Example: From "Programming", the substring "gram" can be extracted

Searching

: Finding whether a character or pattern exists in a string Example: Checking if "World" is inside "Hello World"

Replacement

: Substituting one part of a string with another Example: "I like cats" → "I like dogs"

Splitting

: Dividing a string into smaller parts Example: "red,green,blue" → ["red", "green", "blue"]

Trimming

: Removing spaces from the beginning or end Example: " hello " → "hello"

String Traversal

Traversal means visiting each character one by one.
This is useful when counting vowels, checking for palindromes, or analyzing text.

Example:

String: "DATA"
Characters: D → A → T → A

String as Input and Output

Strings are frequently used for user input and output.
When a user types a name or message, the program usually receives it as a string.
Programs also display messages as strings.

Example:

Input: "Alice"
Output: "Welcome, Alice!"

Working / Process

1. Creation of the String

A string is created when text is written inside quotation marks or received from user input.
Example: "School" or a name typed into a text field.

2. Storage and Encoding

The system stores each character using a numeric code such as ASCII or Unicode.
The string is kept in memory as a sequence of encoded values.

3. Processing and Manipulation

The program performs operations like searching, comparing, joining, or splitting.
The string may be read character by character, modified, or used in calculations such as counting words or checking patterns.

Example process for reversing a string:

Original:  "abcde"
Step 1:    read last character first
Step 2:    continue moving backward
Step 3:    result becomes "edcba"

Example diagram for character positions:

String:   P   r   o   g   r   a   m
Index:    0   1   2   3   4   5   6

This helps explain how each character is individually accessible.

Advantages / Applications

Text handling in software

: Strings are used to store names, addresses, messages, documents, and labels in nearly all applications.

Easy communication between user and program

: Input and output data are often exchanged as strings, making programs interactive and user-friendly.

Wide range of text processing tasks

: Strings support important operations such as searching, formatting, sorting, validation, and data extraction.

Summary

A string is a sequence of characters used to represent text.
Characters in a string have order, position, and length.
Strings are essential for storing and processing human-readable information.
Important terms to remember: string, character, index, length, concatenation, substring, immutable