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    <p>A <code>vector&lt;vector&lt;int&gt; &gt;</code> (note the space in the <code>&gt; &gt;</code>) can work well, but it's not necessarily the most efficient way to do things. Another that can work quite nicely is a wrapper around a single vector, that keeps track of the "shape" of the matrix being represented, and provides a function or overloaded operator to access the data:</p> <pre><code>template &lt;class T&gt; class matrix { int columns_; std::vector&lt;T&gt; data; public: matrix(int columns, int rows) : columns_(columns), data(columns*rows) {} T &amp;operator()(int column, int row) { return data[row*columns_+column]; } }; </code></pre> <p>Note that the C++ standard only allows <code>operator[]</code> to take a single operand, so you can't use it for this job, at least directly. In the example above, I've (obviously enough) used <code>operator()</code> instead, so subscripts look more like Fortran or BASIC than you're accustomed to in C++. If you're really set on using <code>[]</code> notation, you can do it anyway, though it's mildly tricky (you overload it in the matrix class to return a proxy, then have the proxy class also overload <code>operator[]</code> to return (a reference to) the correct element -- it's mildly ugly internally, but works perfectly well anyway).</p> <p>Edit: Since I have it lying around, here's an example of how to implement the latter. I wrote this (quite a while) before most compilers included <code>std::vector</code>, so it statically allocates an array instead of using a vector. It's also for the 3D case (so there are two levels of proxies involved), but with a bit of luck, the basic idea comes through anyway:</p> <pre><code>template&lt;class T, int size&gt; class matrix3 { T data[size][size][size]; friend class proxy; friend class proxy2; class proxy { matrix3 &amp;m_; int index1_, index2_; public: proxy(matrix3 &amp;m, int i1, int i2) : m_(m), index1_(i1), index2_(i2) {} T &amp;operator[](int index3) { return m_.data[index1_][index2_][index3]; } }; class proxy2 { matrix3 &amp;m_; int index_; public: proxy2(matrix3 &amp;m, int d) : m_(m), index_(d) { } proxy operator[](int index2) { return proxy(m_, index_, index2); } }; public: proxy2 operator[](int index) { return proxy2(*this, index); } }; </code></pre> <p>Using this, you can address the array with the normal C++ syntax, such as:</p> <pre><code>matrix3&lt;double, size&gt; m; for (int x=0; x&lt;size; x++) for (int y = 0; y&lt;size; y++) for (int z = 0; z&lt;size; z++) m[x][y][z] = x*100 + y * 10 + z; </code></pre>
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    1. POdynamical two dimension array according to input
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    1. USJerry Coffin
    1. USJerry Coffin
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    1. COCare to shed some light on the inefficiency part of vector of vectors?
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    2. CO@Murali:basically, you've got inefficiency in a couple of ways. First of all, even though all the sub-vectors (so to speak) are going to be the same size, each one stores its own length. Second, a vector is (at least normally) implemented using a pointer to dynamically allocated data, so with a vector of vectors, you need to go through two levels of pointers to get to the real data. Using a single vector involves multiplication instead, which was once a bad tradeoff, but with CPUs faster than memory, it's now almost always a win (extra CPU time vs. possibility of extra memory access).
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    3. COYou could also use std::valarray since it already supports a variety of subset access mechanisms.
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