How to Find Indices and Values of Nonzero Elements in MATLAB?

MATLAB provides functionality that finds the indices and values of all non-zero elements in a vector or multidimensional array; the find() function. The find function with required parameters gives back a vector containing indices of non-zero elements in the passed array/vector.

Syntax:

vec = [] %some vector or array
indices = find(vec,...);

Here, the vec could be a vector or array. The indices vector stores the indices of non-zero elements in vec. Now, let us see different forms of find with the help of examples.

Finding non-zero elements' indices in a vector.

Example 1:

% MATLAB code for indices
% Vector 
vec = [1 0 2 2 4 3 5 0 76 23 0.1 -1 0];    

% Getting indices
indices = find(vec);

% Displaying the indices
disp(indices)

Output:

When an array is passed to the find() function, it returns a column vector that stores the linear indices of non-zero elements i.e., indexing when counting each element column-wise.

Example 2:

% MATLAB code for vector
% 2D array
vec = [1 0 2 2 4;
       3 5 0 76 23;
       0.1 -1 0 23 0];    

% Getting indices
indices = find(vec);    

% Displaying indices
disp(indices)

Output:

We can find the indices of n non-zero elements from starting from the beginning or end as well.

Example 3:

% MATLAB code for indices with non-zero elements
vec = [1 0 2 2 4;
       3 5 0 76 23;
       0.1 -1 0 23 0];
       
% Getting first 7 non-zero elements' indices
indices_first = find(vec,7,"first");    

% Getting last 7 non-zero elements' indices
indices_last = find(vec,7,"last");        

Output:

Note: In case "first" or "last" is not specified, MATLAB takes "first" as a default setting.

Getting Values of Non-Zero Elements With Indices:

We can also get the values of non-zero elements along with their indices. 

Example 4:

% MATLAB code 
% Array
vec = [1 0 2 2 4;
       3 5 0 76 23;
       0.1 -1 0 23 0];    

% Getting row, column value of non
% zero element with their values
[row_ind, col_ind, values] = find(vec,7,"first");

Output:

This will create three vectors:

1. row_ind will store the row number of non-zero elements.
2. col_ind will store the column number of non-zero elements.
3. values store the values of non-zero elements.

The same can be done with vectors. In that case, one row_ind or col_ind will remain a constant vector. 

Example 5:

% MATLAB code for vector
vec = [1 0 2 2 4 3 5 0 76 23 0.1 -1 0 23 0];    

% Getting the values of row, col, 
% and values of non-zero element
[row_ind, col_ind, values] = find(vec,7,"first");

Output: