String Basics and Memory Representation in DSA Typescript - Time & Space Complexity

Time Complexity: String Basics and Memory Representation

O(n)

Understanding Time Complexity

We want to understand how the time to work with strings grows as the string gets longer.

How does the length of a string affect the time needed to read or copy it?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

function copyString(input: string): string {
  let result = [];
  for (let i = 0; i < input.length; i++) {
    result.push(input[i]);
  }
  return result.join('');
}
    

This code copies a string character by character into a new string.

Identify Repeating Operations

• Primary operation: Looping through each character of the input string.
• How many times: Exactly once for each character, so as many times as the string length.

How Execution Grows With Input

As the string gets longer, the number of steps grows roughly the same as the string length.

Input Size (n)	Approx. Operations
10	About 10 steps
100	About 100 steps
1000	About 1000 steps

Pattern observation: The time grows directly with the string length.

Final Time Complexity

Time Complexity: O(n)

This means the time to copy the string grows in direct proportion to its length.

Common Mistake

[X] Wrong: "Copying a string is instant and does not depend on length."

[OK] Correct: Each character must be handled, so longer strings take more time.

Interview Connect

Understanding how string operations scale helps you write efficient code and answer questions about performance clearly.

Self-Check

"What if we used built-in string methods that copy the whole string at once? How would the time complexity change?"

Practice

1. What does the term immutable mean when talking about strings in TypeScript?

easy

A. Once created, the string cannot be changed.

B. The string can be changed anytime.

C. The string is stored in a mutable array.

D. The string is stored as a number.

Solution

1. Step 1: Understand string immutability

   Strings in TypeScript cannot be changed after creation. Any modification creates a new string.

2. Step 2: Compare options with definition

   Only Once created, the string cannot be changed. correctly states that strings cannot be changed once created.

3. Final Answer:

   Once created, the string cannot be changed. -> Option A

4. Quick Check:

   Immutable means unchangeable = A [OK]

Hint: Remember: strings can't be changed after creation [OK]

Common Mistakes:

• Thinking strings can be changed like arrays
• Confusing mutable and immutable
• Assuming strings are stored as numbers

2. Which of the following is the correct way to declare a string variable in TypeScript?

easy

A. let name: number = 'Alice';

B. let name = 123;

C. let name: string = 'Alice';

D. let name: string = 123;

Solution

1. Step 1: Check variable declaration syntax

   TypeScript uses let variableName: type = value; syntax.

2. Step 2: Match type and value

   let name: string = 'Alice'; declares a string variable with a string value correctly.

3. Final Answer:

   let name: string = 'Alice'; -> Option C

4. Quick Check:

   Correct string declaration = A [OK]

Hint: Match variable type with value type exactly [OK]

Common Mistakes:

• Assigning number to string type
• Missing type annotation
• Using wrong syntax for declaration

3. What will be the output of the following TypeScript code?

let greeting = 'Hello';
greeting[0] = 'Y';
console.log(greeting);

medium

A. Yello

B. undefined

C. Error: Cannot assign to string index

D. Hello

Solution

1. Step 1: Understand string immutability in TypeScript

   Strings cannot be changed by index assignment; such operations do not modify the string.

2. Step 2: Analyze the code behavior

   Assigning greeting[0] = 'Y' does nothing; greeting remains 'Hello'.

3. Final Answer:

   Hello -> Option D

4. Quick Check:

   Strings immutable, no change = Hello [OK]

Hint: Strings can't be changed by index assignment [OK]

Common Mistakes:

• Expecting first letter to change
• Thinking string is mutable like array
• Assuming error will be thrown

4. What best describes the result of the following TypeScript code snippet:

let str: string = 'Test';
str[1] = 'a';
console.log(str);

medium

A. No error, output is 'Test'

B. TypeError: Cannot assign to string index

C. SyntaxError: Invalid assignment

D. ReferenceError: str is not defined

Solution

1. Step 1: Check string mutability rules

   Strings are immutable; assigning to an index does nothing.

2. Step 2: Analyze the code behavior

   str[1] = 'a' does nothing; str remains 'Test', console.log outputs 'Test'. No error.

3. Final Answer:

   No error, output is 'Test' -> Option A

4. Quick Check:

   Strings immutable, index assignment ignored = 'Test' [OK]

Hint: Strings cannot be changed by index assignment [OK]

Common Mistakes:

• Assuming no error and string changes
• Confusing syntax error with runtime error
• Expecting ReferenceError

5. Given the following code, what will be the output?

let s1 = 'abc';
let s2 = s1 + 'd';
console.log(s1);
console.log(s2);

Choose the correct output considering string immutability and memory representation.

hard

A. abcabc

B. abcabcd

C. abcdabcd

D. Error at runtime

Solution

1. Step 1: Understand string concatenation and immutability

   Concatenating strings creates a new string; original strings remain unchanged.

2. Step 2: Trace variable values after operations

   s1 remains 'abc'; s2 becomes 'abcd'.

3. Final Answer:

   abc
   abcd -> Option B

4. Quick Check:

   Original unchanged, new string created = abc and abcd [OK]

Hint: Concatenation creates new string; originals stay same [OK]

Common Mistakes:

• Expecting s1 to change after concatenation
• Thinking concatenation modifies original string
• Assuming runtime error on concatenation