find

Find indices and values of nonzero elements

Syntax

• k = find(x)
  [i,j] = find(X)
  [i,j,v] = find(X)
  

Description

k = find(X) returns the indices of the array X that point to nonzero elements. If none is found, find returns an empty matrix.

[i,j] = find(X) returns the row and column indices of the nonzero entries in the matrix X. This is often used with sparse matrices.

[i,j,v] = find(X) returns a column vector v of the nonzero entries in X, as well as row and column indices.

In general, find(X) regards X as X(:), which is the long column vector formed by concatenating the columns of X.

Examples

[i,j,v] = find(X~=0) produces a vector v with all 1s, and returns the row and column indices.

Some operations on a vector

• x = [11  0  33  0  55]';
  find(x)
  
  ans =
  
       1
       3
       5
  
  find(x == 0)
  
  ans =
  
       2
       4
  
  find(0 < x & x < 10*pi)
  
  ans =
  
       1
  

And on a matrix

M = magic(3)

M =

     8     1     6
     3     5     7
     4     9     2

[i,j,v] = find(M > 6)

i =            j =          v =

     1              1            1
     3              2            1
     2              3            1