Why Does Declaring a Float Array with a Dynamic Size Result in an \'Expected Constant Expression\' Error in C ?

Array Size Confusion Causes Error

When declaring a float array with a dynamic size, such as:

<code class="cpp">int size = 100;
float x[size][2];</code>

you may encounter an "expected constant expression" error. This occurs because C requires arrays to have fixed sizes at compile time, not determined during runtime. The size variable here cannot be used as a valid array dimension.

Solution with Vector

To avoid this issue, consider using a vector:

<code class="cpp">std::vector<std::array<float, 2>> x(size);</code>

This creates a vector of arrays, each array containing two floats.

Alternative Solutions

Alternatively, you could use raw memory allocation with new or create your own array type:

<code class="cpp">// With new
float (*px)[2] = new float[size][2];

// With custom array type
template<typename T, size_t N>
struct array {
  T data[N];
};

array<float, 2> myArray[size];</code>

Additional Options

Other options include using a vector of pairs or rolling your own array type with syntax helpers:

<code class="cpp">// Vector of pairs
std::vector<std::pair<float, float>> x(size);

// Custom array type
template<typename T>
struct identity {
  typedef T type;
};

using FloatArray2 = identity<float[2]>::type;
FloatArray2 myArray[size];</code>

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