Question

I am trying to convert this C code I have into a python script so it’s readily accessible by more people, but I am having problems understanding this one snippet.

int i, t;
for (i = 0; i < N; i++) {
   t = (int)(T*drand48());
   z[i] = t;
   Nwt[w[i]][t]++;
   Ndt[d[i]][t]++;
   Nt[t]++;
}

N is a value (sum of one column from an array. Elemental corrected me).

T is just a numerical value.

z, w, and d are memory allocations created from the N array. They were created with this method.

w = ivec(N);
d = ivec(N);
z = ivec(N);

int *ivec(int n) //
{
   int *x = (int*)calloc(n,sizeof(int));
   assert(x);
   return x;
}

Nwt & Ndt are both arrays too, with each element being a memory allocation? (Not sure). At least, each one of them was created by using the following method, passing in two different int’s.

Nwt = dmat(W,T);
Ndt = dmat(D,T);

double **dmat(int nr, int nc) //
{
   int N = nr*nc;
   double *tmp = (double*) calloc(N,sizeof(double));
   double **x  = (double**)calloc(nr,sizeof(double*));
   int r;
   assert(tmp);
   assert(x);
   for (r = 0; r < nr; r++) x[r] = tmp + nc*r;
   return x;
}

So looking at the first loop I posted, what are the following lines doing? I would like to accomplish the same thing in python, but since no memory allocation is needed, not sure what those three lines do, or how I would duplicate it in python.

Nwt[w[i]][t]++;
Ndt[d[i]][t]++;
Nt[t]++;

This is what I have so far:

for i in range(self.N):
        t = self.T * random.random()
        self.z[i] = t
        //** INCORRECT BELOW **
        //self.Nwt[self.N[i]] = t + 1 
        //self.Ndt[i] = t + 1
        //self.Nt[t + 1] += 1

Answer 1

In your translation, the first thing I would worry about is making sensical variable names, particularly for those arrays. Regardless, much of that translates directly.

Nwt and Ndt are 2D arrays, Nt is a one dimensional array. It looks like you’re looping over all the ‘columns’ in the z array, and generating a random number for each one. Then you increment whichever column was picked in Nwt (row w[i]), Ndt (row d[i]) and Nt. The actual random value is stashed in z.

#Literal translation
for i in range(N):
    t = Random.randint(0,T) #Not sure on this... but it seems likely.
    z[i] = t
    Nwt[w[i]][t] += 1
    Ndt[d[i]][t] += 1
    Nt[t] += 1

#In place of w= ivec(N);
w = [0]*N
d = [0]*N
z = [0]*N

#In place of Nwt = dmat(W,T)
Nwt = [[0.0] * T] * W
Ndt = [[0.0] * T] * D

EDIT: corrected w/d/z initialization from “n” to “N”

Note that there are still some things wrong here, since it looks like N must equal W, and D… so tread carefully.