Cell array indexing (curly vs parentheses) in MATLAB - Practice Questions

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of cell array indexing with parentheses

What is the output of the following MATLAB code?

MATLAB

C = {10, 20, 30};
result = C(2);
disp(result);

A The numeric value 20

B A 1x1 cell array containing the value 20

C An error because of wrong indexing

D A 1x3 cell array containing all elements

💡 Hint

Parentheses return a cell array subset, not the content inside.

Using parentheses () on a cell array returns a cell array containing the selected cells. So C(2) returns a 1x1 cell array with the second element inside, not the element itself.

❓ Predict Output

intermediate

Output of cell array indexing with curly braces

What is the output of this MATLAB code?

MATLAB

C = {10, 20, 30};
result = C{2};
disp(result);

A The numeric value 20

B A 1x1 cell array containing the value 20

C An error because of wrong indexing

D A 1x3 cell array containing all elements

💡 Hint

Curly braces extract the content inside the cell.

Using curly braces {} extracts the content inside the cell. So C{2} returns the numeric value 20 directly.

❓ Predict Output

advanced

Result of combining parentheses and curly braces

What is the output of this MATLAB code snippet?

MATLAB

C = {1, {2, 3}, 4};
result = C{2}(1);
disp(result);

A The numeric value 3

B A 1x1 cell array containing 2

C An error because of invalid indexing

D The numeric value 2

💡 Hint

C{2} extracts the second cell content, which is itself a cell array.

C{2} extracts {2,3}, a cell array. Then (1) indexes the first element of that cell array with parentheses, returning a 1x1 cell array containing 2.

❓ Predict Output

advanced

Output when mixing parentheses and curly braces incorrectly

What error or output does this MATLAB code produce?

MATLAB

C = {1, 2, 3};
result = C{1}(2);
disp(result);

A Index exceeds matrix dimensions error

B The numeric value 2

C A 1x1 cell array containing 2

D No error, outputs 1

💡 Hint

C{1} extracts the numeric 1, which is scalar and cannot be indexed at (2).

C{1} is the numeric value 1. Trying to index (2) on a scalar number causes an 'Index exceeds matrix dimensions' error.

🧠 Conceptual

expert

Difference between parentheses and curly braces in cell arrays

Which statement correctly describes the difference between using parentheses () and curly braces {} when indexing cell arrays in MATLAB?

A Parentheses return the content inside the cell; curly braces return a cell array containing the selected cells.

B Both parentheses and curly braces return a cell array but parentheses allow multiple indexing.

C Parentheses return a cell array containing the selected cells; curly braces return the content inside the cell.

D Both parentheses and curly braces return the content inside the cell but in different data types.

💡 Hint

Think about what type of output you get from each indexing style.

Parentheses () return a subset of the cell array as a cell array. Curly braces {} extract the actual content inside the cell.

Practice

1. What does using curly braces {} do when indexing a cell array in MATLAB?

easy

A. It returns a smaller cell array containing the selected cells.

B. It creates a new cell array with empty cells.

C. It accesses the actual content inside the cell.

D. It deletes the selected cells from the array.

Solution

1. Step 1: Understand curly brace indexing

   Curly braces {} in MATLAB cell arrays access the content inside the cell, not the cell itself.

2. Step 2: Compare with parentheses

   Parentheses () return a cell array containing the selected cells, but curly braces extract the actual data.

3. Final Answer:

   It accesses the actual content inside the cell. -> Option C

4. Quick Check:

   Curly braces = content access [OK]

Hint: Curly braces get content, parentheses get cells [OK]

Common Mistakes:

• Confusing curly braces with parentheses
• Thinking curly braces return a cell array
• Assuming curly braces delete cells

2. Which of the following is the correct syntax to extract the content of the first cell in a cell array C?

easy

A. content = C{1};

B. content = C[1];

C. content = C(1);

D. content = C{1,1};

Solution

1. Step 1: Identify correct indexing for content extraction

   Curly braces {} are used to extract content from a cell. So C{1} extracts the content of the first cell.

2. Step 2: Check other options

   C(1) returns a cell array, not content. C[1] is invalid syntax in MATLAB. C{1,1} is valid only if C is 2D, but C{1} is standard for 1D cell arrays.

3. Final Answer:

   content = C{1}; -> Option A

4. Quick Check:

   Curly braces extract content [OK]

Hint: Use curly braces with single index for content [OK]

Common Mistakes:

• Using parentheses instead of curly braces
• Using square brackets which are invalid
• Confusing 1D and 2D cell indexing

3. Given C = {10, 20; 30, 40}; what is the output of result = C(1,2);?

medium

A. Syntax error

B. result is the number 20

C. result is the number 10

D. result is a 1x1 cell array containing 20

Solution

1. Step 1: Understand parentheses indexing on cell arrays

   Using parentheses () returns a cell array containing the selected cells, not the content inside.

2. Step 2: Apply to given code

   C(1,2) returns a 1x1 cell array containing the element at row 1, column 2, which is 20 inside a cell.

3. Final Answer:

   result is a 1x1 cell array containing 20 -> Option D

4. Quick Check:

   Parentheses return cell array [OK]

Hint: Parentheses keep cell structure, curly braces extract content [OK]

Common Mistakes:

• Thinking parentheses extract content
• Confusing row and column indices
• Expecting a numeric output instead of cell

4. Consider the code:

C = {1, 2, 3};
val = C(2);
disp(val + 1);

What is the error in this code?

medium

A. Cannot add 1 to a cell array; use curly braces to get content.

B. Indexing with parentheses is invalid syntax.

C. The cell array C is not defined properly.

D. disp cannot display numeric values.

Solution

1. Step 1: Identify the type of val after indexing

   val = C(2); uses parentheses, so val is a 1x1 cell array, not a number.

2. Step 2: Understand why adding 1 causes error

   Adding 1 to a cell array is invalid. To add 1 to the content, use curly braces: val = C{2};

3. Final Answer:

   Cannot add 1 to a cell array; use curly braces to get content. -> Option A

4. Quick Check:

   Parentheses return cell, cannot add number [OK]

Hint: Add numbers only to content, not cells [OK]

Common Mistakes:

• Using parentheses when curly braces needed
• Assuming disp cannot show numbers
• Thinking parentheses indexing is syntax error

5. You have a cell array C = {1, {2, 3}, 4}; and want to extract the number 3 inside the nested cell. Which indexing expression correctly extracts it?

hard

A. C(2){2}

B. C{2}{2}

C. C{2}(2)

D. C(2)(2)

Solution

1. Step 1: Understand the structure of C

   C is a 1x3 cell array: first cell is 1, second cell is another cell array {2,3}, third cell is 4.

2. Step 2: Extract nested content step-by-step

   Use C{2} to get the nested cell array {2,3}, then {2} to get the second element inside it, which is 3. So C{2}{2} extracts 3.

3. Final Answer:

   C{2}{2} -> Option B

4. Quick Check:

   Curly braces twice for nested content [OK]

Hint: Use curly braces twice for nested cell content [OK]

Common Mistakes:

• Using parentheses which return cells, not content
• Mixing parentheses and curly braces incorrectly
• Trying to index nested cell without extracting first