How to select data randomly in 2-d array

I want to get some data randomly from an 2-d array.

In my partial code given below where cluster_center and sample data both are 2-d array in double type. I want to assign some data randomly from sample_data array to cluster_center array.

for(int i= 0; i< 3; i++)
{
    for(k=0; k<17; k++)
        cluster_center[i][k] = //what will be???;
}

TIA :)