Cell array indexing (curly vs parentheses) in MATLAB

Introduction

Cell arrays hold different types of data together. Using curly braces or parentheses helps you get data or parts of the cell array in different ways.

When you want to get the actual content inside a cell.

When you want to get a smaller cell array from a bigger one.

When you want to change the content inside a cell.

When you want to keep the data inside cells grouped as cells.

Syntax

MATLAB

classdef CellArrayExample
    properties
        cells
    end
    methods
        function obj = CellArrayExample()
            obj.cells = {1, 'text'; [2 3], magic(2)};
        end
        function content = getContent(obj, row, col)
            content = obj.cells{row, col}; % Curly braces to get content
        end
        function subCells = getSubCells(obj, rows, cols)
            subCells = obj.cells(rows, cols); % Parentheses to get sub-cell array
        end
    end
end

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

Parentheses ( ) extract a cell or cells as a smaller cell array.

Examples

Curly braces get the content inside the cell at row 1, column 2.

MATLAB

myCells = {10, 'hello'; [1 2], {5, 6}};
value = myCells{1,2}; % Gets 'hello' as a string

Parentheses get a smaller cell array, not the content inside.

MATLAB

subCells = myCells(1, 2); % Gets a 1x1 cell array containing 'hello'

Curly braces on empty cell cause error; parentheses return empty cell array.

MATLAB

emptyCells = {};
% Trying to get content from empty cell array will cause error
% content = emptyCells{1}; % This will error
subCells = emptyCells([]); % This returns an empty cell array

With one element, curly braces get the value, parentheses get a cell array.

MATLAB

singleCell = {42};
content = singleCell{1}; % Gets 42
subCell = singleCell(1); % Gets a 1x1 cell array containing 42

Sample Program

This program shows how to use parentheses to get a smaller cell array and curly braces to get or change the actual content inside cells.

MATLAB

clc;
clear;

% Create a cell array with different types of data
myCellArray = {123, 'apple'; [4 5 6], magic(3)};

% Display original cell array
disp('Original cell array:');
disp(myCellArray);

% Use parentheses to get a sub-cell array (1x2)
subCellArray = myCellArray(1, :);
disp('Sub-cell array using parentheses (1, :):');
disp(subCellArray);

% Use curly braces to get content inside a cell
content1 = myCellArray{1, 2}; % 'apple'
content2 = myCellArray{2, 1}; % [4 5 6]

fprintf('Content at (1,2) using curly braces: %s\n', content1);
fprintf('Content at (2,1) using curly braces: [%s]\n', num2str(content2));

% Modify content inside a cell using curly braces
myCellArray{1,1} = 999;
disp('Cell array after changing (1,1) content to 999:');
disp(myCellArray);

Important Notes

Using curly braces to get content is fast and direct, but you get the raw data, not a cell.

Parentheses keep the data inside cells grouped as cells, useful when you want to keep the structure.

Trying to use curly braces on an empty cell or invalid index causes an error.

Use curly braces when you want to work with the actual data inside cells.

Use parentheses when you want to keep the data inside cells grouped or want a smaller cell array.

Summary

Curly braces { } get or set the actual content inside a cell.

Parentheses ( ) get or set a smaller cell array, keeping the cell structure.

Use curly braces to work with data inside cells, parentheses to work with cells themselves.

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