Cell array indexing (curly vs parentheses) in MATLAB - Quick Revision & Key Differences

Recall & Review

beginner

What does using parentheses () do when indexing a cell array in MATLAB?

Parentheses () return a subset of the cell array as another cell array, preserving the cell structure.

beginner

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

Curly braces {} extract the contents inside the cell, returning the actual data stored in that cell.

beginner

Given a cell array C = {1, 'hello', [2 3]}, what is the result of C(2)?

C(2) returns a 1x1 cell array containing the string 'hello'.

beginner

Given the same cell array C = {1, 'hello', [2 3]}, what is the result of C{2}?

C{2} returns the string 'hello' directly, not inside a cell.

intermediate

Why would you use parentheses () instead of curly braces {} when indexing a cell array?

Use parentheses () when you want to keep the result as a cell array, for example, to pass a subset of cells to another function expecting cells.

What type of output do you get when you use curly braces {} to index a cell array?

A A cell array containing the cell

B An error

C A numeric array

D The contents inside the cell

Curly braces {} extract the actual contents inside the cell.

What does C(1:2) return if C is a cell array?

A A cell array containing the first two cells

B The contents of the first two cells concatenated

C The contents of the first cell only

D An error

Parentheses () return a subset of the cell array as a cell array.

If C = {10, 20, 30}, what does C{3} return?

A [30]

B {30}

C 30

D A cell array with 30

Curly braces {} return the content inside the cell, here the number 30.

Which indexing method would you use to get a cell array containing only the second cell of C?

A C.2

B C(2)

C C[2]

D C{2}

Parentheses () return a cell array subset, so C(2) returns a cell array with the second cell.

What happens if you try to assign a value to C{2} when C is a cell array?

A The content inside the second cell is replaced

B The second cell is replaced with a new cell

C An error occurs

D The entire cell array is replaced

Using curly braces {} on the left side replaces the content inside that cell.

Explain the difference between using parentheses () and curly braces {} when indexing a cell array in MATLAB.

Think about whether you want the cell itself or what is inside it.

Describe a situation where you would prefer to use parentheses () over curly braces {} when working with cell arrays.

Consider when the output needs to remain a cell array.

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