4.3: Creating and Indexing a 2D Array

The most common way to create a matrix in MATLAB is to type the values inside square brackets. Values in the same row are separated by spaces or commas. Rows are separated by semicolons.

mat1 = [4, 3, 1; 3, 8, 6]
mat2 = [2 10 5; 6 20 17; 13 2 0]

The first matrix has 2 rows and 3 columns. The second matrix has 3 rows and 3 columns. It is often helpful to line up the entries visually when you type a matrix, especially when the matrix has several rows:

mat2 = [ 2  10   5;
         6  20  17;
        13   2   0 ];

MATLAB does not require the extra spaces, but spacing can make your code easier to read.

Common mistake

Every row of a matrix must have the same number of elements. If one row has 3 values and another row has 4 values, MATLAB cannot create a rectangular 2D array.

Using the Colon Operator to Create Rows

The colon operator can also be used inside a matrix. This is convenient when the rows follow a pattern.

Example \(\PageIndex{1}\)

mat3 = [2:5; 3:6]

Solution

The first row is created from 2 through 5, and the second row is created from 3 through 6. Both rows have four elements, so MATLAB can combine them into a matrix.

mat3 =

   2   3   4   5
   3   4   5   6

Indexing in 2D Arrays

Indexing means referring to a specific part of an array. For a 2D array, we use two indices: one for the row and one for the column.

Example \(\PageIndex{2}\)

Indexing a 2D array.

mat1 = [4, 3, 1; 3, 8, 6];
number = mat1(1,3)

Solution

This asks MATLAB for the value in row 1, column 3. In this case, the answer is 1.

Remember: In MATLAB, matrix indexing is row first, column second: matrix(row, column).

For example, in matrix mat2, the value 17 is in row 2, column 3.

	Column 1	Column 2	Column 3
Row 1	2	10	5
Row 2	6	20	17
Row 3	13	2	0