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

Practice - 5 Tasks

1fill in blank

easy

Complete the code to access the content of the first cell in the cell array.

MATLAB

value = C[1];

Drag options to blanks, or click blank then click option'

A1

B(1)

C[1]

D{1}

Attempts:

3 left

💡 Hint

Common Mistakes

Using parentheses instead of curly braces returns a cell, not the content.

Using square brackets causes syntax errors.

Curly braces {} are used to access the content inside a cell in MATLAB.

2fill in blank

medium

Complete the code to extract a sub-cell array containing the first two cells.

MATLAB

subCells = C[1];

Drag options to blanks, or click blank then click option'

A1:2

B{1:2}

C(1:2)

D[1:2]

Attempts:

3 left

💡 Hint

Common Mistakes

Using curly braces returns contents, not a cell array.

Using square brackets causes syntax errors.

Parentheses () are used to extract a sub-cell array from a cell array.

3fill in blank

hard

Fix the error in the code to assign the content of the second cell to variable x.

MATLAB

x = C[1];

Drag options to blanks, or click blank then click option'

A{2}

B(2)

C[2]

D2

Attempts:

3 left

💡 Hint

Common Mistakes

Using parentheses returns a cell array, not the content.

Using square brackets causes syntax errors.

Curly braces {} are needed to access the content inside the second cell.

4fill in blank

hard

Fill both blanks to create a cell array with two cells and access the content of the first cell.

MATLAB

C = [1], [2];
value = C[3];

Drag options to blanks, or click blank then click option'

A'apple'

B'banana'

C1

D2

Attempts:

3 left

💡 Hint

Common Mistakes

Using parentheses instead of curly braces to access content.

Using numeric values without quotes to create string cells.

The cell array is created with two string cells. Curly braces with 1 access the first cell's content.

5fill in blank

hard

Fill all three blanks to create a cell array, extract a sub-cell array, and get the content of the second cell in the sub-array.

MATLAB

C = [1], [2], [3];
subC = C[4];
value = subC[5];

Drag options to blanks, or click blank then click option'

A'cat'

B'dog'

C'bird'

D(2:3)

E(1:2)

F{2}

G{1}

H{3}

Attempts:

3 left

💡 Hint

Common Mistakes

Using curly braces instead of parentheses to get sub-cell array.

Using parentheses instead of curly braces to get content.

The cell array has three string cells. Parentheses with 1:2 extract the first two cells as a sub-cell array. Curly braces with 2 get the content of the second cell in the sub-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