String-handling operations are essential in programming for manipulating textual data, solving real-world problems like input validation, data parsing, and information formatting effectively.

What is a string?

A string is a data type used to represent a sequence of characters. These characters can include letters, digits, spaces, punctuation marks, and special symbols. Strings are enclosed in quotation marks. Depending on the programming language, you may use single quotes ('Hello') or double quotes ("Hello"). Strings are used frequently in applications where text needs to be displayed, stored, or processed, such as user inputs, file content, messages, and system logs.

String operations

High-level programming languages like Python, Java, and C# offer built-in support for various string-handling operations. These operations help perform specific tasks on strings such as measuring their length, accessing specific characters, extracting substrings, and combining multiple strings.

Length

The length of a string is the total number of characters it contains, including spaces and punctuation. This is useful when validating input, such as passwords or form fields.

• In Python, the built-in len() function is used:
  len("Computer Science") returns 17

• In Java, use the .length() method:
  "Computer Science".length() returns 17

• In C#, use .Length property:
  "Computer Science".Length returns 17

Use cases:

• Checking that user inputs meet minimum or maximum length requirements

• Looping through all characters in a string using a for-loop

• Validating that a string is not empty before processing

Position (Indexing)

Indexing allows access to individual characters within a string by their position. Most languages use zero-based indexing, meaning the first character is at position 0.

Example in Python:

s = "Computer"
print(s[0])  # Output: 'C'
print(s[7])  # Output: 'r'

In Java:

String s = "Computer";
System.out.println(s.charAt(0));  // Output: 'C'

Negative indexing is supported in Python (not in Java or C#), which allows access from the end of the string:

print(s[-1])  # Output: 'r'

Use cases:

• Accessing initials in a name (e.g., first letter of each word)

• Getting the file extension from a filename (e.g., .txt from "file.txt")

• Verifying that a character at a specific position matches a required value

Substring and slicing

A substring is a portion of a string. Slicing refers to extracting this portion using start and end indices.

In Python, slicing is done using the format string[start:end], where start is inclusive and end is exclusive:

text = "ComputerScience"
print(text[0:8])   # Output: 'Computer'
print(text[8:])    # Output: 'Science'

In Java, use substring():

String s = "ComputerScience";
System.out.println(s.substring(0, 8));  // Output: 'Computer'

In C#:

string s = "ComputerScience";
string sub = s.Substring(0, 8);  // Output: 'Computer'

Use cases:

• Extracting first name and surname from a full name

• Breaking up a date in YYYY-MM-DD format

• Parsing values from structured inputs such as postcodes or product codes

Concatenation

Concatenation is the process of joining two or more strings together to form a single string. This is done using the + operator or specific string methods depending on the language.

Python:

python

first = "Hello"
second = "World"
message = first + " " + second  # Output: 'Hello World'

Java:

String message = "Hello" + " " + "World";  // Output: 'Hello World'

C#:

string message = "Hello" + " " + "World";  // Output: 'Hello World'

Use cases:

• Generating user-friendly messages by joining variables and strings

• Dynamically creating file names or paths

• Formatting data for output to the screen or logs

Character and character code conversions

Sometimes you need to convert a character to its numeric representation or vice versa. These numeric values are typically based on the ASCII or Unicode character encoding systems.

In Python:

ord('A')    # Output: 65
chr(65)     # Output: 'A'

In Java:

int code = (int) 'A';  // 65
char ch = (char) 65;   // 'A'

In C#:

int code = Convert.ToInt32('A');   // 65
char ch = Convert.ToChar(65);      // 'A'

Use cases:

• Encrypting and decrypting text using algorithms like Caesar cipher

• Sorting strings based on ASCII values

• Validating whether a character is a digit or a letter by checking its code

Type conversion operations

String to integer/float

Strings that contain numeric values can be converted into integers or floats for arithmetic operations.

Python:

python

int("42")       # Output: 42
float("3.14")   # Output: 3.14

Java:

java

Integer.parseInt("42");      // 42
Double.parseDouble("3.14");  // 3.14

C#:

csharp

int.Parse("42");      // 42
float.Parse("3.14");  // 3.14

If the string is not a valid number (e.g., "abc"), an exception is raised. Always validate input before conversion.

Integer/float to string

You can convert numbers to strings using built-in functions or methods.

Python:

str(123)      # Output: '123'

Java:

Integer.toString(123);  // '123'

C#:

123.ToString();  // '123'

Use cases:

• Displaying calculated values in a user interface

• Saving numeric data to text files

• Joining numbers with other strings in logs or messages

Date/time to string and vice versa

Converting date/time to string

Python:

from datetime import datetime
now = datetime.now()
now.strftime("%Y-%m-%d")  # Output: '2025-06-21'

Java:

LocalDate date = LocalDate.now();
String formatted = date.format(DateTimeFormatter.ofPattern("yyyy-MM-dd"));

C#:

DateTime now = DateTime.Now;
string formatted = now.ToString("yyyy-MM-dd");

Converting string to date/time

Python:

datetime.strptime("2025-06-21", "%Y-%m-%d")

Java:

LocalDate.parse("2025-06-21", DateTimeFormatter.ofPattern("yyyy-MM-dd"));

C#:

DateTime.ParseExact("2025-06-21", "yyyy-MM-dd", CultureInfo.InvariantCulture);

Use cases:

• Validating date input in forms

• Storing and retrieving timestamps from databases

• Calculating intervals such as time until deadline

Solving common problems

Input sanitisation

This is the process of cleaning user input to prevent unexpected behaviour or security vulnerabilities. It includes:

Trimming whitespace:

name = "  Alice  ".strip()  # Output: 'Alice'

Standardising case:

email = "John.DOE@example.com".lower()  # 'john.doe@example.com'

Removing unwanted characters:

''.join(c for c in text if c.isalnum())  # Keeps only letters and numbers

Importance:

• Prevents injection attacks

• Reduces inconsistencies in data storage

• Improves user experience by avoiding rejections on valid inputs

Parsing strings

Parsing involves breaking a string down into parts or converting it into a different data structure.

Examples:

Splitting a string:

"John Smith".split()  # ['John', 'Smith']

Parsing CSV values:

"23,45,67".split(",")  # ['23', '45', '67']

Checking prefixes/suffixes:

filename.endswith(".txt")  # Returns True for text files

Use cases:

• Extracting values from form fields

• Processing lines in configuration files

• Analysing structured logs

Best practices

Use meaningful variable names

Descriptive variable names like email_address or first_name improve code clarity and make programs easier to understand and maintain.

Consider case sensitivity

String comparisons are case-sensitive in most languages. To compare accurately, convert both strings to a common case:

python

if a.lower() == b.lower():
    ...

Use built-in methods

Languages provide efficient and safe built-in string methods. Avoid reinventing the wheel.

Examples:

• .replace()

• .find()

• .split()

• .join()

• .strip()

Handle large string operations efficiently

In loops, repeated use of the + operator to concatenate strings can lead to performance issues. In loops, repeated use of the + operator to concatenate strings can lead to performance issues. Instead, use lists and join them at the end:

python

lines = []
for i in range(10):
    lines.append("Line " + str(i))
result = "\n".join(lines)

This is faster and uses less memory than building the string directly with +.

Common mistakes

Index errors

Accessing beyond the end of a string will cause an error. Always check that the index is within bounds.

python

if i < len(s):
    char = s[i]

Invalid conversions

Converting a non-numeric string to an integer will raise a runtime error. Always validate before converting:

python

if s.isdigit():
    val = int(s)

Modifying immutable strings

Strings are immutable in languages like Python and Java, so changes create a new string:

python

s = "hello"
s = "H" + s[1:]  # Correct way to change first letter