Cell array creation in MATLAB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a cell array in MATLAB?

A cell array is a data type in MATLAB that can hold different types of data in each cell, such as numbers, strings, or even other arrays.

beginner

How do you create an empty cell array of size 3-by-2?

Use the syntax cell(3,2) to create a 3-by-2 empty cell array.

intermediate

What is the difference between {} and () when creating cell arrays?

{} is used to create or access cell contents, while () is used to access cells themselves as containers.

beginner

How do you create a cell array with mixed data types, for example a number and a string?

Use curly braces with comma-separated values, like {42, 'hello'} to create a cell array with a number and a string.

intermediate

What happens if you assign a value to a cell array element that does not exist yet?

MATLAB automatically expands the cell array size to include the new element, filling any gaps with empty cells.

Which syntax creates a 2-by-2 cell array with elements 1, 'a', [3 4], and true?

A cell{2,2}

B [1, 'a'; [3 4], true]

C cell(2,2)

D {1, 'a'; [3 4], true}

Curly braces with semicolon and comma separate rows and columns to create a cell array with mixed types.

What does cell(4,1) create in MATLAB?

A A 4-by-1 empty cell array

B A 1-by-4 numeric array

C A 4-by-1 numeric array

D An empty numeric matrix

cell(4,1) creates a cell array with 4 rows and 1 column, all cells empty.

How do you access the content of the first cell in a cell array C?

A C[1]

B C{1}

C C(1)

D C.1

Curly braces {} access the content inside the cell.

What is the result of C = {1, 2; 3, 4}; size(C)?

A [1 4]

B [4 1]

C [2 2]

D [2 1]

The cell array has 2 rows and 2 columns, so size returns [2 2].

If you assign C{3,3} = 'new' to a 2-by-2 cell array C, what happens?

A The cell array expands to 3-by-3, filling new cells with empty cells

B An error occurs because the index is out of bounds

C Only the element at (3,3) is created, others remain unchanged

D The cell array shrinks to 3-by-3

MATLAB automatically expands the cell array to include the new index, filling gaps with empty cells.

Explain how to create a cell array with different types of data and how to access its elements.

Think about how you mix numbers and strings inside curly braces.

Describe what happens when you assign a value to a cell array element outside its current size.

Imagine stretching a container to fit a new item.

Practice

1. What is the correct way to create a cell array in MATLAB that contains a number, a string, and a logical value?

easy

A. myCell = [42, 'hello', true];

B. myCell = (42, 'hello', true);

C. myCell = {42, 'hello', true};

D. myCell = <42, 'hello', true>;

Solution

1. Step 1: Understand cell array syntax

   Cell arrays use curly braces { } to hold mixed data types.

2. Step 2: Identify correct syntax for mixed types

   myCell = {42, 'hello', true}; uses curly braces with different types: number, string, logical.

3. Final Answer:

   myCell = {42, 'hello', true}; -> Option C

4. Quick Check:

   Curly braces create cell arrays [OK]

Hint: Use curly braces { } for cell arrays, not square brackets [ ] [OK]

Common Mistakes:

• Using square brackets instead of curly braces
• Using parentheses instead of curly braces
• Using angle brackets which are invalid

2. Which of the following is the correct syntax to create a 2x2 cell array with elements 1, 'a', 3.14, and false?

easy

A. C = [1, 'a'; 3.14, false];

B. C = {1, 'a'; 3.14, false};

C. C = {1; 'a'; 3.14; false};

D. C = (1, 'a'; 3.14, false);

Solution

1. Step 1: Recall cell array 2D syntax

   Use curly braces { } with semicolons for rows and commas for columns.

2. Step 2: Check each option

   C = {1, 'a'; 3.14, false}; correctly uses { } with two rows and two columns separated by ; and ,.

3. Final Answer:

   C = {1, 'a'; 3.14, false}; -> Option B

4. Quick Check:

   Curly braces + semicolon for rows = correct 2D cell array [OK]

Hint: Use semicolon to separate rows inside curly braces [OK]

Common Mistakes:

• Using square brackets which create numeric arrays
• Using commas instead of semicolons for rows
• Using parentheses which are invalid for arrays

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

myCell = {10, 'MATLAB'; 'false', 3.5};
disp(myCell{2,1});

medium

A. false

B. 3.5

C. 'MATLAB'

D. 10

Solution

1. Step 1: Understand cell content access

   Using curly braces with indices accesses the content inside the cell.

2. Step 2: Identify the element at row 2, column 1

   myCell{2,1} is the element in second row, first column, which is 'false' as a string.

3. Final Answer:

   'false' -> Option A

4. Quick Check:

   myCell{2,1} = 'false' [OK]

Hint: Use curly braces { } to get cell content, not parentheses ( ) [OK]

Common Mistakes:

• Using parentheses instead of curly braces to access content
• Confusing row and column indices
• Expecting the whole cell instead of content

4. Consider this MATLAB code snippet:

C = {1, 2, 3; 4, 5, 6};
val = C(2,3);

What is the issue with this code if you want to get the number 6 as a value?

medium

A. C(2,3) returns a cell, not the content inside the cell

B. C(2,3) is invalid syntax for cell arrays

C. C(2,3) returns the number 6 directly

D. C(2,3) returns an error because of wrong indexing

Solution

1. Step 1: Understand difference between () and {}

   Parentheses () return a cell array subset, curly braces {} return content.

2. Step 2: Analyze C(2,3)

   C(2,3) returns a 1x1 cell containing 6, not the number 6 itself.

3. Final Answer:

   C(2,3) returns a cell, not the content inside the cell -> Option A

4. Quick Check:

   Use curly braces to get content: C{2,3} [OK]

Hint: Use curly braces { } to get cell content, parentheses () return cells [OK]

Common Mistakes:

• Using parentheses expecting content instead of cell
• Confusing cell array indexing with numeric array indexing
• Expecting direct numeric output from parentheses indexing

5. You want to create a cell array that stores a numeric vector, a string, and another cell array inside it. Which of the following correctly creates such a nested cell array?

hard

A. nestedCell = {[1, 2, 3], 'text', [true; false]};

B. nestedCell = {[1, 2, 3], 'text', [true, false]};

C. nestedCell = {[1, 2, 3], 'text', (true, false)};

D. nestedCell = {[1, 2, 3], 'text', {true, false}};

Solution

1. Step 1: Identify nested cell array syntax

   To nest a cell array inside another, use curly braces { } inside the main cell array.

2. Step 2: Check each option for correct nesting

   nestedCell = {[1, 2, 3], 'text', {true, false}}; uses {true, false} as a nested cell array inside the main cell array.

3. Final Answer:

   nestedCell = {[1, 2, 3], 'text', {true, false}}; -> Option D

4. Quick Check:

   Curly braces inside curly braces create nested cells [OK]

Hint: Use curly braces inside curly braces for nested cell arrays [OK]

Common Mistakes:

• Using square brackets for nested cell arrays
• Using parentheses which are invalid
• Using semicolons inside nested cell arrays incorrectly