MATLAB Function Reference

find

Find indices and values of nonzero elements

Syntax

• indices = find(X)
  indices = find(X, k)
  indices = find(X, k, 'first')
  indices = find(X, k, 'last')
  [i,j] = find(...)
  [i,j,v] = find(...)
  

Description

indices = find(X) returns the linear indices corresponding to the nonzero entries of the array X. If none are found, find returns an empty, 0-by-1 matrix. In general, find(X) regards X as X(:), which is the long column vector formed by concatenating the columns of X.

indices = find(X, k) or indices = find(X, k, 'first') returns at most the first k indices corresponding to the nonzero entries of X. k must be a positive integer, but it can be of any numeric data type.

indices = find(X, k, 'last') returns at most the last k indices corresponding to the nonzero entries of X.

[i,j] = find(...) returns the row and column indices of the nonzero entries in the matrix X. This syntax is especially useful when working with sparse matrices. If X is an N-dimensional array with N > 2, j contains linear indices for the dimensions of X other than the first.

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

Examples

• X = [1 0 4 -3 0 0 0 8 6];
  indices = find(X)
  

returns linear indices for the nonzero entries of X.

• indices =
  
       1     3     4     8     9
  

You can use a logical expression to define X. For example,

• find(X > 2)
  

returns linear indices corresponding to the entries of X that are greater than 2.

• ans =
  
       3     8     9
  

The following commands

• X = [3 2 0; -5 0 7; 0 0 1];
  [i,j,v] = find(X) 
  

return

• i =
  
       1
       2
       1
       2
       3
  

a vector of row indices of the nonzero entries of X,

• j =
  
       1
       1
       2
       3
       3
  

a vector of column indices of the nonzero entries of X, and

• v =
  
       3
      -5
       2
       7
       1
  

a vector containing the nonzero entries of X.

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
  

For the matrix

• M = magic(3)
  
  M =
  
       8     1     6
       3     5     7
       4     9     2
  
  find(M > 3, 4)
  

returns the indices of the first four entries of M that are greater than 3.

• ans =
  
       1
       3
       5
       6
  

If X is a vector of all zeros, find(X) returns an empty, 0-by-1 matrix. For example,

• indices = find([0;0;0])
  
  indices =
  
     Empty matrix: 0-by-1 
  

See Also

nonzeros, sparse, colon, logical operators (elementwise and short-circuit), relational operators, ind2sub

filter2

findall

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