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POAccess a 1D array as a 2D array in C++
 

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  1. POAccess a 1D array as a 2D array in C++
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    <p>This has bothered me for a while. A lot of times I find myself making a large buffer to hold a "maximum" amount of data. This helps me avoid dynamically allocating and deallocating a buffer each time the size of the next data set changes. </p> <p>For instance say I have an array that is way too large for its actual useful size, but I know the length of the useful data.</p> <pre><code>int amountOfData = 9; char data1D[100] = some data that is only 9 bytes long stored in a 100 byte array </code></pre> <p>Lets say I have an algorithm I want to run on this data set that uses 2D array indexing. So I want to be able to access the data as follows:</p> <pre><code>cout &lt;&lt; "I am accessing this data as a 2D array: " &lt;&lt; data1D[0][1] &lt;&lt; endl; </code></pre> <p>Lets say for this algorithm I know that the xlength and ylength of the 2D array are going to be:</p> <pre><code>int xlength = 3; int ylength = 3; </code></pre> <p>for this iteration, because <code>amountOfData = 9</code>. However, the lengths may be different for the next iteration. Ie. they could be <code>xlength = 4</code> and <code>ylength = 4</code> given <code>amountOfData = 16</code>.</p> <p>I want to do some kind of casting that allows me cast the 1D array using 2D array indexing. I know how long my initial 1D length is, which tells me how long my 2D <code>xlength</code> and <code>ylength</code> are, so this should be easy to do without using <code>new</code> or <code>malloc</code> as long as the initial 100 bytes is long enough to hold any useful data set to me.</p> <p>I realize that:</p> <pre><code>char** data2d = (char**) data1D; </code></pre> <p>will not work because the compiler doesn't know the size of the second dimension. But I will know what it is at runtime!</p> <p>What is the underlying reason for this being the way it is? Are there any workarounds? Am I missing anything?</p>
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    1. CODo look into [vectors](http://en.cppreference.com/w/cpp/container/vector).
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    2. COWhat kind of types are you expecting to use in data1D?
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    3. CO@Tomas Badan I used `char`s as examples, but a specific case of this problem arose today when I wanted to access a 1D array of `unsigned char`s as a 2D array of `unsigned short`s. The 1D array was spit out of proprietary camera software. I want to access the resulting image as 2D pixel data.
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