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<div class="title">FIND Find Non-zero Elements of An Array </div> </div>
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<div class="textblock"><p>Section: <a class="el" href="sec_array.html">Array Generation and Manipulations</a> </p>
<h1><a class="anchor" id="Usage"></a>
Usage</h1>
<p>Returns a vector that contains the indicies of all non-zero elements in an array. The usage is </p>
<pre class="fragment"> y = find(x)
</pre><p> The indices returned are generalized column indices, meaning that if the array <code>x</code> is of size <code>[d1,d2,...,dn]</code>, and the element <code>x(i1,i2,...,in)</code> is nonzero, then <code>y</code> will contain the integer </p>
<p class="formulaDsp">
<img class="formulaDsp" alt="\[ i_1 + (i_2-1) d_1 + (i_3-1) d_1 d_2 + \dots \]" src="form_3.png"/>
</p>
<p> The second syntax for the <code>find</code> command is </p>
<pre class="fragment"> [r,c] = find(x)
</pre><p> which returns the row and column index of the nonzero entries of <code>x</code>. The third syntax for the <code>find</code> command also returns the values </p>
<pre class="fragment"> [r,c,v] = find(x).
</pre><p> 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.</p>
<p>The <code>find</code> command also supports some additional arguments. Each of the above forms can be combined with an integer indicating how many results to return: </p>
<pre class="fragment"> y = find(x,k)
</pre><p> where <code>k</code> is the maximum number of results to return. This form will return the first <code>k</code> results. You can also specify an optional flag indicating whether to take the first or last <code>k</code> values: </p>
<pre class="fragment"> y = find(x,k,'first')
y = find(x,k,'last')
</pre><p> in the case of the <code>'last'</code> argument, the last <code>k</code> values are returned. </p>
<h1><a class="anchor" id="Example"></a>
Example</h1>
<p>Some simple examples of its usage, and some common uses of <code>find</code> in FreeMat programs.</p>
<pre class="fragment">--> a = [1,2,5,2,4];
--> find(a==2)
ans =
2 4
</pre><p>Here is an example of using find to replace elements of <code>A</code> that are <code>0</code> with the number <code>5</code>.</p>
<pre class="fragment">--> 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
</pre><p>Incidentally, a better way to achieve the same concept is:</p>
<pre class="fragment">--> 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
</pre><p>Now, we can also return the indices as row and column indices using the two argument form of <code>find</code>:</p>
<pre class="fragment">--> 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
</pre><p>Or the three argument form of <code>find</code>, which returns the value also:</p>
<pre class="fragment">--> [r,c,v] = find(A)
r =
1
3
2
1
2
c =
1
1
2
3
3
v =
1
3
2
3
1
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