Translating C to Python: Random number generation (With conditions)-CodePudding

I'm trying to translate a C file to Python and have ran into a small situation. Adding a snippet of the working C code:

int i,j;

double vx[N]; // x velocity array
double vy[N]; // y velocity array

int npart = 7; // 7 points representing the particles are along each direction

/* Initialize the velocities to start the program*/
/* Start with a little random velocity */
for(i=1; i < npart;   i){
    for(j=1; j < npart;   j){
        I = j   (i-1)*(npart-1);
        vx[I] = (double) rand()/RAND_MAX - 0.5;
        vy[I] = (double) rand()/RAND_MAX - 0.5;
    }
}

I can't figure out how to translate the randomising part of the code and translate to python.

This is what I did, (Adding specific snippet)

import numpy as np
import random

npart = 7 #7 points representing the particles are along each direction

for i in range(1, npart):
    for j in range(1, npart):
        I = j   (i - 1)*(npart - 1)
        vx[I] = random() - 0.5
        vy[I] = random() - 0.5

This gives the error:

NameError: name 'vx' is not defined

I'm a student currently learning these, so any help would be very appreciated.
Thanks in advance.

CodePudding user response：

There are 2 issues in your code:

random module usage:
In order to generate a random integer number the proper way is:

random.randint(from,to)

(random itself is a module, and is not callable via random()).

Lists/Arrays initialization:
Your lists/arrays are not initialized. You must initialize them with a proper size:

N = 100
vx = [0] * N
vy = [0] * N

Set N to the same value you used in c .
See more info about python arrays: How do I declare an array in Python?.

CodePudding user response：

As the error implies, you did not initialize the vx and vy lists before accessing them. Once you initialize and fill them, you can proceed with the loop.

vx = []
vy = []