How to find data in a matrix in Matlab and Octave

Use the find() function to find data in an array on Matlab and Octave

find(condizione);

The argument is a condition to select the elements of the array.

The find () function returns the list of elements that satisfy the selection conditions.

Examples

Example 1

Create a 3x3 matrix

>> M=[1 2 3; 4 5 6; 7 8 9]
M =

1 2 3
4 5 6
7 8 9

Type [row, col]=find(M==5) to find the value 5 in the array

>> [row,col]=find(M==5)

This command finds the value 5 at the coordinates row = 2 and col = 2

row = 2
col = 2

The value 5 is located at the coordinates (2; 2) that is on the second row and second column of the matrix.

Example 2

Type [row,col]=find(M<3) to find values less than 3 in the matrix

>> [row,col]=find(M<3)

In this case the variables row and col are two arrays because the selection finds more than one value that satisfies the condition.

row = [1 1]
col = [1 2]

The first element is located at the coordinates row(1)=1 e col(1)=1 that is to the coordinates (1; 1) of the matrix.

The second element is located at the coordinates row(2)=1 e col(2)=2 that is to the coordinates (1; 2) of the matrix.

Example 3

Type [row,col]=find(M>10) to find values greater than 10 in the matrix

>> [row,col]=find(M>10)

The matrix has no values greater than 10.

The row and col arrays are empty.

row = [](0x1)
col = [](0x1)