How to Use Indexing in MATLAB: Syntax and Examples

In MATLAB, you use indexing to access or change elements of arrays or matrices by specifying their position inside parentheses, like A(row, column). You can use single indices for vectors or multiple indices for matrices, and also logical or colon operators for flexible selection.

Syntax

Indexing in MATLAB uses parentheses () with one or more indices inside to select elements from arrays or matrices.

• A(i): Access the i-th element of a vector A.
• A(i,j): Access the element in the i-th row and j-th column of matrix A.
• A(start:end): Use colon operator to select a range of elements.
• A(logicalIndex): Use a logical array to select elements where the condition is true.

matlab

A = [10, 20, 30, 40, 50];
element = A(3); % Access third element

M = [1, 2, 3; 4, 5, 6; 7, 8, 9];
element_matrix = M(2,3); % Access element at row 2, column 3

range = A(2:4); % Access elements from index 2 to 4

logicalIndex = A > 25;
selected = A(logicalIndex); % Elements greater than 25

Example

This example shows how to create a matrix and use different types of indexing to access and modify its elements.

matlab

M = [5, 10, 15; 20, 25, 30; 35, 40, 45];

% Access element at row 1, column 2
val1 = M(1, 2);

% Access entire second row
row2 = M(2, :);

% Access entire third column
col3 = M(:, 3);

% Modify element at row 3, column 1
M(3, 1) = 100;

% Logical indexing: elements greater than 20
greaterThan20 = M(M > 20);

Output

val1 = 10
row2 = [20 25 30]
col3 = [15; 30; 45]
M =
    5    10    15
   20    25    30
  100    40    45
greaterThan20 = [25 30 100 40 45]

Common Pitfalls

Common mistakes when using indexing in MATLAB include:

• Using parentheses () instead of curly braces {} for cell arrays.
• Indexing outside the array bounds, which causes errors.
• Confusing linear indexing with row-column indexing in matrices.
• Forgetting that MATLAB indices start at 1, not 0.

matlab

A = [1, 2, 3];

% Wrong: Using 0 as index (MATLAB indices start at 1)
% val = A(0); % This will cause an error

% Correct:
val = A(1); % Access first element

% Wrong: Using parentheses for cell array content
C = {10, 20, 30};
% val = C(2); % Returns a cell, not the content

% Correct:
val_content = C{2}; % Access content inside the cell

Output

val = 1
val_content = 20

Quick Reference

Indexing Type	Syntax	Description
Single element	A(i)	Access the i-th element of a vector or linear index in matrix
Row and column	A(i,j)	Access element at row i and column j in a matrix
Range	A(start:end)	Access elements from start to end indices
All elements in row/column	A(i,:)	All columns in row i
All elements in column	A(:,j)	All rows in column j
Logical indexing	A(logicalArray)	Select elements where logicalArray is true

Key Takeaways

MATLAB indexing uses parentheses with one or more indices to access array elements.

Indices start at 1, not 0, so always use positive integers starting from 1.

Use colon operator ':' to select ranges or entire rows/columns easily.

Logical indexing lets you select elements based on conditions.

For cell arrays, use curly braces '{}' to access content, not parentheses.