Strings are sequences of characters used to represent text or data. Operations on strings include manipulation, pattern matching, and encoding. In coding interviews, questions about strings evaluate a candidate's skill in handling textual data, understanding common algorithms and methods related to strings, and optimizing solutions for various string-based challenges.

Check out our carefully selected list of basic and advanced Strings questions and answers to be well-prepared for your tech interviews in 2024.

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A string represents a sequence of characters and is fundamental for storing textual information in programming languages like Python, Java, and C++.

• Character Encoding: Strings use encodings like ASCII, UTF-8, or UTF-16, setting rules for character representation.

• Mutability: A string's content can be either mutable (modifiable) or immutable (unchangeable). The mutability often varies across languages.

• Concatenation: Joining two or more strings.
• Access: Retrieving characters using an index.
• Length: Measuring the number of characters.
• Substrings: Extracting portions of the string.

Strings in Python are sequences of Unicode characters and are immutable. Python offers a rich set of methods for string manipulation.

s = "Hello, World!"
print(s[0])  # Output: "H"

Strings in Java are immutable sequences of characters. For efficiency, Java often stores its strings in a "string constant pool."

String s = "Hello, World!";
System.out.println(s.charAt(0));  // Output: "H"

In C, strings are represented using null-terminated character arrays. C++ provides both C-style strings and a more advanced std::string class.

#include <string>
std::string s = "Hello, World!";
std::cout << s[0];  // Output: "H"

In the context of strings, mutability refers to the ability to change or modify an existing string object, whereas immutability implies that a string, once created, cannot be altered.

Many programming languages have predefined ways in which strings are treated. For instance, in Python and Java, strings are primarily immutable, but these languages provide mutable alternatives for specific use-cases.

• Behavior: Any operation that seems to modify the string actually creates a new string with the changes and returns it, leaving the original string unaltered.

• Example: In Python, strings are immutable. For instance:

  text = "Hello"
  lowercase_text = text.lower()
  # Returns a new string in lower-case without altering 'text'.

• Advantages:

  • Thread-Safety: Immutable strings are inherently thread-safe, making them easier to use in concurrent environments.

• Disadvantages:

  • Memory Overhead: Generating a new string for every modification can lead to a higher memory footprint, especially with large strings.
  • Efficiency Concerns: Creating a new string object can be more resource-intensive than modifying an existing one in-place.

• Behavior: Mutable strings allow direct changes to their content without creating a new string.

• Example: In Java, while the String class is immutable, the StringBuilder and StringBuffer classes provide mutable string implementations. Similarly, in C++, std::string objects are mutable and can be modified directly using functions like string.replace() or by accessing individual characters.

• Advantages:

  • Memory Efficiency: In-place modifications can save on memory by avoiding the creation of new string objects.
  • Performance: Direct access and modification of string characters can be faster than creating new strings.

• Disadvantages:

  • Thread-Safety: Mutable strings can pose synchronization challenges in parallel or multi-threaded applications, necessitating extra precautions for proper management.

A null-terminated string is characterized by its ending '\0' ASCII character (or 0-byte). This method was essential for memory management before dynamic memory allocation became common. Today, it's mainly used for compatibility.

In C, character arrays act as null-terminated strings. Functions like printf and strlen depend on the null character to determine string termination. While this approach is memory-efficient, it can be prone to overflow and length errors if the null terminator isn't verified.

• Performance: Null-terminated strings might slow down some operations.
• Safety: Always ensure strings are NULL-terminated to prevent memory issues.
• Compatibility: Use them when needed for specific systems or libraries.

Modern languages like Python and C++ offer advanced string types, such as Python's str and C++'s std::string, which are more convenient and manage memory better, reducing the need for null-terminated strings.

Strings and Character Arrays have many similarities but also key differences in their usage and behavior.

• Strings: Dynamically allocated in many languages.
• Character Arrays: Explicitly allocated, typically on the stack.

• Strings: Immutable in many languages; once created, their content cannot be modified.
• Character Arrays: Mutable; allows for modification of individual elements.

• Strings: Provide built-in string manipulation methods, like concatenate.
• Character Arrays: Lack such convenience; require manual character handling.

• Strings: Might be less efficient due to potential dynamic resizing and metadata overhead.
• Character Arrays: More compact and efficient because of the absence of dynamic resizing overhead.

• Strings: Termination is managed internally in many languages and doesn't always rely on explicit terminators.
• Character Arrays: In languages like C, they require a null character ('\0') to indicate termination.

• Strings: Preferred for text processing and higher-level abstractions.
• Character Arrays: Suited for low-level manipulations, like specific I/O operations.

While Strings are versatile, they are not the ideal storage for passwords or critical data due to their immutability and the associated risks of data remanence.

For sensitive data, a character array is often more secure, as it allows for explicit clearing and reduces chances of unintended exposure.

• Data Security: char arrays can be overwritten in memory, effectively erasing sensitive information. Conversely, Strings persist in memory until garbage collection, posing a risk.

• Memory Protection: Since Strings are immutable, once created, their contents remain accessible in memory.

• Log Files and Dumps: Unintended logging or memory dumps could expose Strings. Overwriting char arrays after use minimizes this risk.

• Thread Safety: In concurrent environments, char arrays provide a level of control and safety that Strings may not.

Here is the Java code:

char[] password = {'p', 'a', 's', 's', 'w', 'o', 'r', 'd'};
// Clearing the password from memory
for(int i = 0; i < password.length; i++) {
    password[i] = 0;
}

Pascal Strings have been historically employed in the Pascal programming language.

Their unique feature is that they explicitly store the string's length, which provides both safety and efficiency advantages over null-terminated strings, commonly used in languages like C.

• Length Prefix: Pascal Strings begin with a distinct length indicator. This length is typically stored in one byte, allowing for strings of lengths from 0 to 255.
• Absence of Null-Terminator: Given the length prefix, there's no need for a trailing null-terminator as seen in C strings.
• $O(1)$ Length Access: Thanks to the length stored at the beginning, retrieving the length of a Pascal String is a constant-time operation.

• Safety: Pascal Strings reduce certain risks associated with string handling, like buffer overflows, which can arise with null-terminated strings. However, if the length prefix is tampered with or not correctly validated, it can lead to vulnerabilities.

• Memory Efficiency: They can be slightly less memory efficient because of the additional length byte. However, in cases with many short strings, the absence of a null-terminator can make Pascal Strings more memory-efficient.

• Binary Data Limitations: Pascal Strings can be used for binary data, but care must be taken. The length byte itself could be misinterpreted as content. Moreover, using only one byte for length limits the string to 255 characters.

Here is the C++ code:

#include <iostream>

int getPascalStringLength(const char * pascalString) {
    unsigned char length = static_cast<unsigned char>(*pascalString);
    return static_cast<int>(length);
}

int main() {
    const char * pascalString = "\x05" "Hello";
    int length = getPascalStringLength(pascalString);
    std::cout << "Length: " << length << std::endl;
    
    return 0;
}

👉🏼 Check out all 20 answers here: Devinterview.io - Strings

👉🏼 Check out all 20 answers here: Devinterview.io - Strings

👉🏼 Check out all 20 answers here: Devinterview.io - Strings

👉🏼 Check out all 20 answers here: Devinterview.io - Strings

👉🏼 Check out all 20 answers here: Devinterview.io - Strings

👉🏼 Check out all 20 answers here: Devinterview.io - Strings

👉🏼 Check out all 20 answers here: Devinterview.io - Strings

👉🏼 Check out all 20 answers here: Devinterview.io - Strings

👉🏼 Check out all 20 answers here: Devinterview.io - Strings

👉🏼 Check out all 20 answers here: Devinterview.io - Strings

👉🏼 Check out all 20 answers here: Devinterview.io - Strings

👉🏼 Check out all 20 answers here: Devinterview.io - Strings

👉🏼 Check out all 20 answers here: Devinterview.io - Strings

👉🏼 Check out all 20 answers here: Devinterview.io - Strings