How do I check if an array element is empty in Cg? (Unity shader development)

Question

In my shader, I declare a non-serialized array of float4s and execute a block of code on the elements:

uniform float4 _HandPos[5];

v2f vert(appdata v) 
{
    float distH;

    for (float i = 0; i < _HandPos.Length; i++)
    {

         /*if element has not been assigned, assuming an empty element is NaN, 
         check distance between vertex and element*/
         if (!isnan(_HandPos[i]))
         {
             distH = distance(_HandPos[i], v.vertex);
         }
         //do something with distH here
    }
}

Every frame, an array in a C# script is sent to _HandPos. It doesn't always send values for all 5 elements, so I have to filter the empty ones out. Here, I assumed that an empty element is NaN, but that doesn't seem to work.

Answer 1

If you don't write a value each frame, whatever value was set last will be preserved. Alternatively, you could use an array of int values that correspond to which _HandPos elements need to be reprocessed.

Keep in mind that a branching statement (such as if()) on the GPU is generally going to cause the GPU to execute both branches then discard one value -- so if this is meant to be an optimization it's probably not going to have the desired effect.