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    <p>Your global definitions of <code>row</code>,<code>col</code>,<code>matrix</code>, ... is the problem. </p> <pre><code>int row=0, col=0; vector&lt;vector&lt;int&gt; &gt; matrix(row, vector&lt;int&gt; (col) ); vector&lt;vector&lt;int&gt; &gt; matrix2(row, vector&lt;int&gt; (col) ); vector&lt;vector&lt;int&gt; &gt; matrix3(row, vector&lt;int&gt; (col) ); </code></pre> <p>What's happening here is the following: <code>row</code> and <code>col</code> is now <code>0</code> and therefore all your matrices now have 0 rows and columns.</p> <p>You can fix this by using the <code>vector::resize()</code> function after you get <code>row</code> and <code>col</code> from the user.</p> <pre><code>cout&lt;&lt;"Enter the number of rows for each Matrix: "&lt;&lt;endl; cin&gt;&gt;row; cout&lt;&lt;"Enter the number of columns for each Matrix: "&lt;&lt;endl; cin&gt;&gt;col; // Resize "matrix" matrix.resize(row); for(int i = 0; i &lt; row; ++i) matrix[i].resize(col); // Repeat for "matrix2" and "matrix3" </code></pre> <p>Also, this means you don't have to "initialize" your <code>matrix</code> objects. So now you can just define them as:</p> <pre><code>vector&lt;vector&lt;int&gt; &gt; matrix; vector&lt;vector&lt;int&gt; &gt; matrix2; vector&lt;vector&lt;int&gt; &gt; matrix3; </code></pre> <h3>Note:</h3> <ol> <li>Think about using <a href="http://www.cplusplus.com/doc/tutorial/other_data_types/" rel="nofollow noreferrer"><code>typedef</code></a> to make your code look better.</li> <li>You don't need them to be global variables. You are using a <code>vector</code> and your <code>printMatrix</code> and <code>addMatrix</code> functions can call <code>vector::size()</code> to find out the size of your matrix. You should rewrite those functions to take your matrix as an argument (lots of good advice <a href="https://stackoverflow.com/questions/2139224/how-to-pass-objects-to-functions-in-c">here</a> on that) and then work on them.</li> </ol>
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    1. COThanks, that really helped me. Vectors were explained *very* badly to me. I got the impression they were magic arrays where it didn't matter if they started off as zero/empty and could be manipulated on the fly anyway I want, without using any of the functions from the vector library/class itself. I now know they are more restrictive than previously thought...
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