String vs character array in MATLAB - Performance Comparison

Time Complexity: String vs character array

O(n^2)

Understanding Time Complexity

We want to understand how working with strings and character arrays affects the time it takes to run code in MATLAB.

How does the choice between string and character array change the speed of common operations?

Scenario Under Consideration

Analyze the time complexity of concatenating characters to build a text.

str = "";
for k = 1:n
    str = str + "a";
end

charArr = '';
for k = 1:n
    charArr = [charArr 'a'];
end
    

This code builds a string and a character array by adding one character at a time in a loop.

Identify Repeating Operations

Both loops run from 1 to n, repeating the concatenation operation.

• Primary operation: Adding one character to the existing text each time.
• How many times: Exactly n times for both string and character array.

How Execution Grows With Input

Each time we add a character, MATLAB creates a new text by copying the old one and adding the new character.

Input Size (n)	Approx. Operations
10	About 55 copies (1+2+...+10)
100	About 5050 copies
1000	About 500,500 copies

Pattern observation: The total work grows roughly like the square of n because each addition copies all previous characters.

Final Time Complexity

Time Complexity: O(n^2)

This means the time to build the text grows much faster than the number of characters added, because each step copies all existing characters.

Common Mistake

[X] Wrong: "Adding one character in a loop always takes the same small time, so total time is O(n)."

[OK] Correct: Each addition copies the entire existing text, so the time grows with the length of the text, making the total time much larger.

Interview Connect

Understanding how string building works helps you write faster code and shows you know how data structures affect performance.

Self-Check

"What if we used a pre-allocated character array and filled it instead of concatenating? How would the time complexity change?"

Practice

1. Which of the following correctly represents a string in MATLAB?

easy

A. 'Hello World'

B. ('Hello World')

C. {'Hello World'}

D. "Hello World"

Solution

1. Step 1: Understand string syntax in MATLAB

   Strings in MATLAB are enclosed in single quotes, like 'Hello World'.

2. Step 2: Identify string and character array syntax

   In MATLAB, single quotes denote character arrays (which are often used as strings). Double quotes denote string arrays introduced in newer MATLAB versions, but the classic string representation is single quotes.

3. Final Answer:

   'Hello World' -> Option A

4. Quick Check:

   Single quotes denote character arrays (strings) = B [OK]

Hint: Strings use single quotes in MATLAB [OK]

Common Mistakes:

• Confusing single quotes for strings
• Using curly braces which create cell arrays
• Using parentheses which are invalid for strings

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

easy

A. charArray = "abc";

B. charArray = ["a", "b", "c"];

C. charArray = { 'a', 'b', 'c' };

D. charArray = 'abc';

Solution

1. Step 1: Recognize character array syntax

   Character arrays are created using single quotes around text, like 'abc'.

2. Step 2: Differentiate from string and cell array

   Double quotes create string arrays, and curly braces create cell arrays, so only single quotes create character arrays.

3. Final Answer:

   charArray = 'abc'; -> Option D

4. Quick Check:

   Character arrays use single quotes = C [OK]

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

Common Mistakes:

• Using double quotes for character arrays
• Using curly braces which create cell arrays
• Using square brackets with commas incorrectly

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

str = "MATLAB";
charArr = 'MATLAB';
disp(class(str));
disp(class(charArr));

medium

A. string
char

B. char
string

C. string
string

D. char
char

Solution

1. Step 1: Identify variable types

   Variable str is created with double quotes, so it is a string type. Variable charArr uses single quotes, so it is a character array (char).

2. Step 2: Understand class() output

   The class() function returns the type name as a string. So class(str) returns 'string' and class(charArr) returns 'char'.

3. Final Answer:

   string char -> Option A

4. Quick Check:

   Double quotes = string, single quotes = char [OK]

Hint: class() shows 'string' for double quotes, 'char' for single quotes [OK]

Common Mistakes:

• Assuming both are 'char' type
• Confusing output order
• Thinking class() returns variable content

4. Consider this MATLAB code snippet:

str = 'Hello';
str(1) = "h";

What is the error in this code?

medium

A. Indexing starts at 0, so str(1) is invalid

B. Cannot assign double quotes to a character array element

C. Strings cannot be indexed like arrays

D. No error, code runs fine

Solution

1. Step 1: Identify variable type

   Variable str is created with single quotes, so it is a character array.

2. Step 2: Understand assignment rules for character arrays

   Character array elements must be assigned single characters using single quotes. Assigning a double-quoted string "h" causes a type mismatch error.

3. Final Answer:

   Cannot assign double quotes to a character array element -> Option B

4. Quick Check:

   Character arrays need single quotes for elements [OK]

Hint: Assign single characters with single quotes to char arrays [OK]

Common Mistakes:

• Using double quotes to assign char array elements
• Thinking MATLAB indexing starts at 0
• Assuming strings can't be indexed

5. You want to concatenate the words "Data" and 'Science' in MATLAB to form a string. Which code correctly does this and results in a string type?

hard

A. result = "Data" + 'Science';

B. result = "Data" + "Science";

C. result = "Data" + string('Science');

D. result = "Data" + char('Science');

Solution

1. Step 1: Understand types of inputs

   "Data" is a string. 'Science' is a character array. To concatenate strings, both must be string type.

2. Step 2: Convert character array to string before concatenation

   Using string('Science') converts the char array to string, allowing + operator to concatenate strings properly.

3. Final Answer:

   result = "Data" + string('Science'); -> Option C

4. Quick Check:

   Convert char to string before + concatenation [OK]

Hint: Convert char arrays to strings before using + for concatenation [OK]

Common Mistakes:

• Trying to add string and char directly
• Using + with char arrays (causes error)
• Not converting char to string before concatenation