Correct way to translate C++ code to Python

Question

I have a C++ code doing the following:

U[0]=rate[0];
map<int,double> Q[N+1];           
map<int,double> r[N+1];           
map<int,double> d[N+1];           
r[0][0]=rate[0];
d[0][0]=1/(1+r[0][0]*dt);
Q[0][0]=1.0;
for(int i=1;i<=N;i++)
{
    for(int j=-i;j<=i;j=j+2)
    {      
        if(j==i)
            {Q[i][i]= 0.5*Q[i-1][i-1]*d[i-1][i-1];}
        if(j==-i)
            {Q[i][-i]=0.5*Q[i-1][-i+1]*d[i-1][-i+1];}

        Q[i][j]=0.5*(Q[i-1][j-1]*d[i-1][j-1]+Q[i-1][j+1]*d[i-1][j+1]);
    }
}

I need to write the equivalent Python version for this as I'm not too comfortable with C++. I have done the following:

Q, U, r, d = [], [], [], []
Q.append([])
Q[0].append(1)
U.append(rate[1])
r.append([])
r[0].append(rate[1])
d.append([])
d[0].append(1/(1+r[0][0]*dt))
for i in range(0, N):
    for j in range(-i, i, 2):
        if j == i:
            Q[i].append(0.5*Q[i-1][i-1]*d[i-1][i-1])
        elif j == -i:
            Q[i].insert(-i, (0.5*Q[i-1][-i+1]*d[i-1][-i+1]))
        Q[i].insert(j, 0.5*(Q[i-1][j-1]*d[i-1][j-1]+Q[i-1][j+1]*d[i-1][j+1]))

However, I'm getting an index out of range error in this line:

Q[i].insert(-i, (0.5*Q[i-1][-i+1]*d[i-1][-i+1]))

How do I correctly translate the C++ code to it's python equivalent?

Answer 1

You get the index error because your arrays aren't prepopulated, as they would be in C++.

Try defining them like so:

from collections import defaultdict
...

#map<int,double> Q[N+1];
Q = [defaultdict(float) for _ in range(N+1)]

#map<int,double> r[N+1];
r = [defaultdict(float) for _ in range(N+1)]

#map<int,double> d[N+1];
d = [defaultdict(float) for _ in range(N+1)]

---

How do I correctly translate the C++ code to it's python equivalent?

If it were me, and I didn't understand the code I was starting with, I'd do a line-by-line, expression-by-expression translation with absolutely minimal changes.

I would also demand a test case that runs correctly with the C++ code and adapt that to confirm the Python code.

Here is my line-by-line, UNTESTED translation:

from collections import defaultdict


#NameError: name 'N' is not defined
N = 12

#NameError: name 'rate' is not defined
rate = [.03]

#NameError: name 'dt' is not defined
dt = 1.0/12


#U[0]=rate[0];
#  unused

#map<int,double> Q[N+1];
Q = [defaultdict(float) for _ in range(N+1)]

#map<int,double> r[N+1];
r = [defaultdict(float) for _ in range(N+1)]

#map<int,double> d[N+1];
d = [defaultdict(float) for _ in range(N+1)]

#r[0][0]=rate[0];
r[0][0] = rate[0]

#d[0][0]=1/(1+r[0][0]*dt);
d[0][0] = 1 / (1 + r[0][0] * dt)

#Q[0][0]=1.0;
Q[0][0] = 1.0

#for(int i=1;i<=N;i++)
#{
for i in range(1, N+1, 1):

#    for(int j=-i;j<=i;j=j+2)
#    {
    for j in range(-i, i+1, 2):

#        if(j==i)
#            {Q[i][i]= 0.5*Q[i-1][i-1]*d[i-1][i-1];}
        if j == i:
            Q[i][i] = 0.5 * Q[i-1][i-1] * d[i-1][i-1]

#        if(j==-i)
#            {Q[i][-i]=0.5*Q[i-1][-i+1]*d[i-1][-i+1];}

        if j == -i:
            Q[i][-i] = 0.5 * Q[i-1][-i+1] * d[i-1][-i+1]

#        Q[i][j]=0.5*(Q[i-1][j-1]*d[i-1][j-1]+Q[i-1][j+1]*d[i-1][j+1]);
        Q[i][j] = 0.5 * (Q[i-1][j-1] * d[i-1][j-1] + Q[i-1][j+1] * d[i-1][j+1])
#    }
#}