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find

Find indices and values of nonzero elements

Syntax

ind = find(X)
ind = find(X, k)
ind = find(X, k, 'first')
ind = find(X, k, 'last')
[row,col] = find(X, ...)
[row,col,v] = find(X, ...)

Description

ind = find(X) locates all nonzero elements of array X, and returns the linear indices of those elements in vector ind. If X is a row vector, then ind is a row vector; otherwise, ind is a column vector. If X contains no nonzero elements or is an empty array, then ind is an empty array.

ind = find(X, k) or ind = 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.

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

[row,col] = find(X, ...) 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, col contains linear indices for the columns. For example, for a 5-by-7-by-3 array X with a nonzero element at X(4,2,3), find returns 4 in row and 16 in col. That is, (7 columns in page 1) + (7 columns in page 2) + (2 columns in page 3) = 16.

[row,col,v] = find(X, ...) returns a column or row vector v of the nonzero entries in X, as well as row and column indices. If X is a logical expression, then v is a logical array. Output v contains the non-zero elements of the logical array obtained by evaluating the expression X. For example,

A= magic(4)
A =
    16     2     3    13
     5    11    10     8
     9     7     6    12
     4    14    15     1

[r,c,v]= find(A>10);

r', c', v'
ans =
     1     2     4     4     1     3
ans =
     1     2     2     3     4     4
ans =
     1     1     1     1     1     1

Here the returned vector v is a logical array that contains the nonzero elements of N where

N=(A>10)

Examples

Example 1

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

Example 2

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

Example 3

The following find command

X = [3 2 0; -5 0 7; 0 0 1];
[r,c,v] = find(X) 

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

r =
     1
     2
     1
     2
     3

a vector of column indices of the nonzero entries of X

c =
     1
     1
     2
     3
     3

and a vector containing the nonzero entries of X.

v =
     3
    -5
     2
     7
     1

Example 4

The expression

X = [3 2 0; -5 0 7; 0 0 1];
[r,c,v] = find(X>2)

returns a vector of row indices of the nonzero entries of N where N=(X>2)

r =
     1
     2

a vector of column indices of the nonzero entries of N where N=(X>2)

c =
     1
     3

and a logical array that contains the nonzero elements of N where N=(X>2).

v =
     1
     1

Recall that when you use find on a logical expression, the output vector v does not contain the nonzero entries of the input array. Instead, it contains the nonzero values returned after evaluating the logical expression.

Example 5

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

Example 6

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

Example 7

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

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

See Also

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