FIND Find Non-zero Elements of An Array

Section: Array Generation and Manipulations

Usage

Returns a vector that contains the indicies of all non-zero elements in an array. The usage is 
   y = find(x)

The indices returned are generalized column indices, meaning that if the array x is of size [d1,d2,...,dn], and the element x(i1,i2,...,in) is nonzero, then y will contain the integer

The second syntax for the find command is 

   [r,c] = find(x)

which returns the row and column index of the nonzero entries of x. The third syntax for the find command also returns the values 

   [r,c,v] = find(x).

Note that if the argument is a row vector, then the returned vectors are also row vectors. This form is particularly useful for converting sparse matrices into IJV form. The find command also supports some additional arguments. Each of the above forms can be combined with an integer indicating how many results to return: 

   y = find(x,k)

where k is the maximum number of results to return. This form will return the first k results. You can also specify an optional flag indicating whether to take the first or last k values: 

   y = find(x,k,'first')
   y = find(x,k,'last')

in the case of the 'last' argument, the last k values are returned.

Example

Some simple examples of its usage, and some common uses of find in FreeMat programs. 
--> a = [1,2,5,2,4];
--> find(a==2)

ans = 
 2 4

Here is an example of using find to replace elements of A that are 0 with the number 5. 

--> A = [1,0,3;0,2,1;3,0,0]

A = 
 1 0 3 
 0 2 1 
 3 0 0 

--> n = find(A==0)

n = 
 2 
 4 
 6 
 9 

--> A(n) = 5

A = 
 1 5 3 
 5 2 1 
 3 5 5

Incidentally, a better way to achieve the same concept is: 

--> A = [1,0,3;0,2,1;3,0,0]

A = 
 1 0 3 
 0 2 1 
 3 0 0 

--> A(A==0) = 5

A = 
 1 5 3 
 5 2 1 
 3 5 5

Now, we can also return the indices as row and column indices using the two argument form of find: 

--> A = [1,0,3;0,2,1;3,0,0]

A = 
 1 0 3 
 0 2 1 
 3 0 0 

--> [r,c] = find(A)
r = 
 1 
 3 
 2 
 1 
 2 

c = 
 1 
 1 
 2 
 3 
 3

Or the three argument form of find, which returns the value also: 

--> [r,c,v] = find(A)
r = 
 1 
 3 
 2 
 1 
 2 

c = 
 1 
 1 
 2 
 3 
 3 

v = 
 1 
 3 
 2 
 3 
 1