__global__ void Initalize(float* u, float* un, int nx, int nt, float dx)

{

int i = blockIdx.x * blockDim.x + threadIdx.x;

int j = blockIdx.y * blockDim.y + threadIdx.y;

if (i < nx && j < nx)

{

}

}

The old code was:

for (int k = 0; k <= nx - 1; k++)

if (k*dx >= 0.5 && k*dx <= 1)