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    <p>Let me flesh out your question a little bit. You mention:</p> <blockquote> <p>I ask because I want to return an array [...] from a function but I want to build the array without knowing the size beforehand. The size will be passed as an argument. I want to know if there is a way to do this without using dynamic allocation.</p> </blockquote> <p>For the <em>I want to return an array from a function [...] size passed as an argument</em>, it seems reasonable to me that you can use <code>std::vector</code> everywhere, and call its <code>.data()</code> method when you need access to the underlying array (which is guaranteed to be contiguous). For example:</p> <pre><code>std:vector&lt;double&gt; myfun(size_t N) { std::vector&lt;double&gt; r(N); // fill r[0], r[1], ..., r[N-1] return r; } // later on: r.data(); // gives you a pointer to the underlying double[N] </code></pre> <p>And for the <em>I want to to do this without dynamic allocation</em>, that is not possible unless you know the size at compile time. If that is the case, then do exactly as before but use <code>std::array</code>, which can implement optimizations based on known compile-time size:</p> <pre><code>std::array&lt;double, N&gt; myfun() { std::array&lt;double, N&gt; r; // fill r[0], r[1], ..., r[N-1] return r; } // later on: r.data(); // gives you a pointer to the underlying double[N] </code></pre> <p>And to be generic, I would actually use a template function capable of working with arbitrary containers:</p> <pre><code>template&lt;typename T&gt; void myfun(T&amp; data) { for(int k=0; k&lt;data.size(); k++) { // do stuff to data[k] } } // call as, for example: std::vector&lt;double&gt; data(10); myfun(data); // or equally valid: std::array&lt;double, 10&gt; data; myfun(data); </code></pre> <p>Finally, if you are working with two-dimensional data, please remember that when you store the Matrix in row-major order that is:</p> <pre><code>Matrix [1, 2; 3 4] is stored as [1 2 3 4] </code></pre> <p>then you can refer to element <code>(i, j)</code> of the matrix by calling <code>data[i * ncols + j]</code>. For example: consider a three by four matrix:</p> <pre><code>a b c d e f g h i j k l </code></pre> <p>The element <code>(2, 2)</code> (that is: third row, third column because we assume zero-based C-type indexing) is calculated as: <code>M[2][2] = M[2 * 4 + 2] = M[10] = k</code>. This is the case because it was stored as:</p> <pre><code> [a b c d e f g h i j k l] [0 1 2 3 4 5 6 7 8 9 10 11] </code></pre> <p>and <code>k</code> is the element with index <code>10</code>.</p>
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