Character arrays and strings in MATLAB - Time & Space Complexity

Time Complexity: Character arrays and strings

O(n)

Understanding Time Complexity

When working with character arrays and strings in MATLAB, it is important to understand how the time to process them grows as the string gets longer.

We want to know how the number of steps changes when we do operations like copying or searching in strings.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

str = 'hello world';
newStr = '';
for i = 1:length(str)
    newStr(i) = str(i);
end
    

This code copies each character from one string to another using a loop.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Copying one character from the original string to the new string inside the loop.
• How many times: The loop runs once for each character in the string, so it repeats n times where n is the string length.

How Execution Grows With Input

As the string gets longer, the number of copy steps grows directly with the number of characters.

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

Pattern observation: The work grows in a straight line with the string length.

Final Time Complexity

Time Complexity: O(n)

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

Common Mistake

[X] Wrong: "Copying a string takes the same time no matter how long it is."

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

Interview Connect

Understanding how string operations scale helps you write efficient code and explain your reasoning clearly in interviews.

Self-Check

"What if we used MATLAB's built-in string copy function instead of a loop? How would the time complexity change?"

Practice

1. Which of the following correctly creates a character array in MATLAB?

easy

A. name = [Hello];

B. name = "Hello";

C. name = 'Hello';

D. name = (Hello);

Solution

1. Step 1: Understand character array syntax

   Character arrays in MATLAB use single quotes to enclose text.

2. Step 2: Check each option

   name = 'Hello'; uses single quotes correctly. name = "Hello"; uses double quotes which create string objects, not character arrays. Options A and D are invalid syntax.

3. Final Answer:

   name = 'Hello'; -> Option C

4. Quick Check:

   Single quotes create character arrays [OK]

Hint: Use single quotes for character arrays, double for strings [OK]

Common Mistakes:

• Using double quotes for character arrays
• Missing quotes around text
• Using brackets or parentheses incorrectly

2. Which of the following is the correct way to create a string in MATLAB?

easy

A. str = "MATLAB";

B. str = 'MATLAB';

C. str = {"MATLAB"};

D. str = ("MATLAB");

Solution

1. Step 1: Recall string creation syntax

   Strings in MATLAB are created using double quotes.

2. Step 2: Evaluate options

   str = "MATLAB"; uses double quotes correctly. str = 'MATLAB'; creates a character array, not a string. Options C and D use invalid or different data types.

3. Final Answer:

   str = "MATLAB"; -> Option A

4. Quick Check:

   Double quotes create strings [OK]

Hint: Double quotes create strings, single quotes create char arrays [OK]

Common Mistakes:

• Confusing single and double quotes
• Using curly braces which create cell arrays
• Using parentheses incorrectly

3. What is the output of the following MATLAB code?

charArray = 'abc';
str = "def";
disp([charArray str])

medium

A. abcdef

B. [abc def]

C. Error: Cannot concatenate char array and string

D. abc def

Solution

1. Step 1: Understand concatenation of char arrays and strings

   In MATLAB, concatenating a character array and a string with [] converts the string to a char array and joins them.

2. Step 2: Evaluate the concatenation result

   Concatenating 'abc' and "def" results in 'abcdef' as a character array, displayed without spaces.

3. Final Answer:

   abcdef -> Option A

4. Quick Check:

   Concatenation joins text without spaces [OK]

Hint: Concatenate char arrays and strings with [] to join text [OK]

Common Mistakes:

• Expecting spaces between concatenated strings
• Thinking concatenation causes error
• Confusing display output with variable type

4. Identify the error in the following MATLAB code:

str1 = 'Hello';
str2 = "World";
result = strcat(str1, str2);

medium

A. strcat cannot concatenate character arrays and strings

B. No error, code runs correctly

C. Missing semicolon after str2 assignment

D. str1 and str2 must both be strings or both char arrays

Solution

1. Step 1: Understand strcat behavior

   strcat can concatenate character arrays and strings by automatically converting strings to character arrays.

2. Step 2: Verify the code

   str1 is a character array, str2 is a string, but strcat(str1, str2) returns 'HelloWorld' as a character array. No error occurs.

3. Final Answer:

   No error, code runs correctly -> Option B

4. Quick Check:

   strcat handles mixed types [OK]

Hint: strcat automatically handles char and string inputs [OK]

Common Mistakes:

• Assuming strcat requires matching types
• Confusing with + operator which doesn't work for mixed types
• Looking for syntax errors where none exist

5. You have a character array chars = 'MATLAB' and want to convert it to a string and append " is fun". Which code correctly does this?

hard

A. newStr = chars + " is fun";

B. newStr = chars + ' is fun';

C. newStr = strcat(chars, " is fun");

D. newStr = string(chars) + " is fun";

Solution

1. Step 1: Convert character array to string

   Use string(chars) to convert the character array to a string type.

2. Step 2: Append text using + operator

   Strings can be concatenated with + and another string, so adding " is fun" works.

3. Step 3: Check other options

   newStr = chars + ' is fun'; tries to add char array and char array with + which is invalid. newStr = strcat(chars, " is fun"); mixes types but produces a character array, not a string. newStr = chars + " is fun"; adds char array and string with + which errors.

4. Final Answer:

   newStr = string(chars) + " is fun"; -> Option D

5. Quick Check:

   Convert char to string, then use + to join [OK]

Hint: Convert char array to string first, then use + to append [OK]

Common Mistakes:

• Using + to add char arrays
• Mixing char arrays and strings in strcat
• Not converting char array before concatenation