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    <p>If you're building the matrix <code>output</code> from scratch, you should make sure it hasn't already been initialized to anything. To do this, you can set it to the empty matrix at the beginning of your code:</p> <pre><code>output = []; </code></pre> <p>Also, if you know how large <code>output</code> is going to be, <a href="http://www.mathworks.com/access/helpdesk/help/techdoc/matlab_prog/f8-784135.html#f8-793781" rel="nofollow noreferrer">your code will run more efficiently if you preallocate the array</a> <code>output</code> and index into the array to assign values instead of appending values to it. In your case, <code>output</code> should have the same number of rows as there are unique values in the array <code>ordered1</code>, so you could use the function <a href="http://www.mathworks.com/access/helpdesk/help/techdoc/ref/unique.html" rel="nofollow noreferrer">UNIQUE</a> to preallocate <code>output</code>:</p> <pre><code>nRows = numel(unique(ordered1)); %# Get the number of unique values output = zeros(nRows,2); %# Initialize output </code></pre> <p>You would then have to keep a separate counter (say <code>r</code>) to track which index into <code>output</code> you will be adding to next:</p> <pre><code>... output(r,:) = [count/537 tempnum]; %# Overwrite a row in output r = r+1; %# Increment the row index ... </code></pre> <h2>Some additional advice...</h2> <p>Even if you solve the error you are getting, you're going to run into more with the code you have above:</p> <ul> <li><p>I believe you are actually computing a <a href="http://en.wikipedia.org/wiki/Probability_density_function" rel="nofollow noreferrer">probability density function</a> (or PDF) with your code. In order to get the <a href="http://en.wikipedia.org/wiki/Cumulative_distribution_function" rel="nofollow noreferrer">cumulative distribution function</a> (or CDF), you have to perform a cumulative sum over the final values in the first column of <code>output</code>. You can do this with the function <a href="http://www.mathworks.com/access/helpdesk/help/techdoc/ref/cumsum.html" rel="nofollow noreferrer">CUMSUM</a>:</p> <pre><code>output(:,1) = cumsum(output(:,1)); </code></pre></li> <li><p>Your loop will throw an error when it reaches the last element of <code>ordered1</code>. The value of <code>k</code> can become 538 in your inner while loop, which will then cause an error to be thrown when you try to access <code>ordered1(k)</code> anywhere. To get around this, you will have to add checks to the value of <code>k</code> at a number of points in your code. One such point is your inner while loop, which can be rewritten as:</p> <pre><code>while (k &lt;= 537) &amp;&amp; (ordered1(k) == tempnum) count = count + 1; k = k + 1; end </code></pre> <p>This solution uses the <a href="http://www.mathworks.com/access/helpdesk/help/techdoc/ref/logicaloperatorsshortcircuit.html" rel="nofollow noreferrer">short-circuit AND operator</a> <code>&amp;&amp;</code>, which will first check if <code>(k &lt;= 537)</code> is true or false. If it is false (i.e. <code>k &gt; 537</code>), the second logical check is skipped since its result doesn't matter, and you avoid the error that would result from evaluating <code>ordered1(k)</code>.</p></li> </ul> <h2>Bonus MATLAB coolness...</h2> <p>MATLAB has a lot of cool functions that can do a lot of the work for you. One such function is <a href="http://www.mathworks.com/access/helpdesk/help/techdoc/ref/accumarray.html" rel="nofollow noreferrer">ACCUMARRAY</a>. Your TA may want you to do things using loops like you have above, but you can actually reduce your whole code to just a few lines like so:</p> <pre><code>nValues = numel(ordered1); %# Get the number of values p = accumarray(ordered1,ones(size(ordered1)))./nValues; %# Create a PDF output = [cumsum(p) unique(ordered1)]; %# Create the CDF output </code></pre>
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