FreeMat: FIND Find Non-zero Elements of An Array

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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