Structures and field access in MATLAB - Time & Space Complexity

Time Complexity: Structures and field access

O(n)

Understanding Time Complexity

We want to understand how the time it takes to access data in a structure grows as the structure gets bigger.

Specifically, how does accessing fields in a MATLAB structure change with more data?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

data = struct();
for i = 1:n
    data(i).value = i * 2;
end

for i = 1:n
    x = data(i).value;
end

This code creates a structure array with n elements, each having a field 'value'. Then it accesses each field once.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Looping through the structure array to access the 'value' field.
• How many times: Exactly n times, once for each element in the structure array.

How Execution Grows With Input

As n grows, the number of field accesses grows directly with n.

Input Size (n)	Approx. Operations
10	10 field accesses
100	100 field accesses
1000	1000 field accesses

Pattern observation: The time grows in a straight line as the number of elements increases.

Final Time Complexity

Time Complexity: O(n)

This means the time to access all fields grows directly in proportion to the number of elements.

Common Mistake

[X] Wrong: "Accessing a field in a structure is instant and does not depend on the number of elements."

[OK] Correct: While accessing one field is quick, doing it repeatedly for many elements adds up, so total time grows with the number of elements.

Interview Connect

Knowing how data access time grows helps you write efficient code and explain your choices clearly in interviews.

Self-Check

"What if we accessed multiple fields inside the loop instead of just one? How would the time complexity change?"

Practice

1. What is the correct way to access the field age from a structure person in MATLAB?

easy

A. person['age']

B. person.age

C. person->age

D. person.age()

Solution

1. Step 1: Understand MATLAB structure field access

   In MATLAB, fields of a structure are accessed using dot notation without parentheses or brackets.

2. Step 2: Identify correct syntax for field access

   The correct syntax is structure.field, so person.age is correct.

3. Final Answer:

   person.age -> Option B

4. Quick Check:

   Use dot notation for fields [OK]

Hint: Use dot notation to access structure fields in MATLAB [OK]

Common Mistakes:

• Using parentheses like a function call
• Using brackets like a dictionary
• Using arrow notation from other languages

2. Which of the following is the correct way to create a structure car with fields make and year in MATLAB?

easy

A. car = struct(make = 'Toyota', year = 2020);

B. car = {make: 'Toyota', year: 2020};

C. car = struct['make' => 'Toyota', 'year' => 2020];

D. car = struct('make', 'Toyota', 'year', 2020);

Solution

1. Step 1: Recall MATLAB struct creation syntax

   MATLAB uses the struct function with pairs of field names and values as strings and values.

2. Step 2: Match correct syntax

   car = struct('make', 'Toyota', 'year', 2020); uses struct('make', 'Toyota', 'year', 2020), which is correct MATLAB syntax.

3. Final Answer:

   car = struct('make', 'Toyota', 'year', 2020); -> Option D

4. Quick Check:

   Use struct('field', value, ...) to create structures [OK]

Hint: Use struct('field', value) pairs to create structures [OK]

Common Mistakes:

• Using JSON or dictionary syntax
• Using equal signs inside struct()
• Using brackets instead of parentheses

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

person.name = 'Alice';
person.age = 30;
disp(person.age);

medium

A. 'Alice'

B. name

C. 30

D. Error: Undefined variable

Solution

1. Step 1: Understand structure field assignment

   The code assigns 'Alice' to person.name and 30 to person.age.

2. Step 2: Identify what disp(person.age) shows

   The disp function prints the value of person.age, which is 30.

3. Final Answer:

   30 -> Option C

4. Quick Check:

   disp shows the value of the accessed field [OK]

Hint: disp(struct.field) prints the field's value [OK]

Common Mistakes:

• Confusing field names with values
• Expecting the field name instead of value
• Syntax errors from missing semicolons

4. Consider this MATLAB code snippet:

data = struct('x', 10, 'y', 20);
disp(data.z);

What will happen when this code runs?

medium

A. Error: Reference to non-existent field 'z'.

B. Displays 0

C. Displays empty array []

D. Displays 20

Solution

1. Step 1: Check structure fields

   The structure data has fields x and y, but no field z.

2. Step 2: Accessing a non-existent field

   Trying to access data.z causes MATLAB to throw an error about the missing field.

3. Final Answer:

   Error: Reference to non-existent field 'z'. -> Option A

4. Quick Check:

   Accessing missing fields causes an error [OK]

Hint: Access only existing fields; missing fields cause errors [OK]

Common Mistakes:

• Assuming missing fields return 0 or empty
• Ignoring error messages
• Confusing field names with variable names

5. You have a structure array students with fields name and score. How do you create a new structure passed containing only students with score >= 50?

hard

A. passed = students([students.score] >= 50);

B. passed = struct('name', students.name, 'score', students.score >= 50);

C. passed = students.score >= 50;

D. passed = students{students.score >= 50};

Solution

1. Step 1: Understand structure arrays and logical indexing

   In MATLAB, [students.score] creates an array of scores. Logical indexing selects elements where condition is true.

2. Step 2: Apply logical indexing to filter students

   The expression students([students.score] >= 50) selects only students with scores 50 or above.

3. Final Answer:

   passed = students([students.score] >= 50); -> Option A

4. Quick Check:

   Use logical indexing on structure arrays [OK]

Hint: Use logical indexing with [struct.field] to filter arrays [OK]

Common Mistakes:

• Trying to assign logical array directly
• Using curly braces instead of parentheses
• Creating new struct without filtering