CODEWITHSUNDEEP
pythonarraysrandominitialization
Aakash
Aakash

Reputation: 5

last array values being repeated for all the previous arrays: Python

I am trying to generate three different arrays of vx[], vy[] and vz[] for 2 atoms (natoms). I am able to print these different values when I initialize the arrays just after the initialization. However, when I try to print the values later it give the same array, ie. vz[] for all the arrays vx[], vy[] and vz[]. Am I missing something? I tried using different loops, but still cannot get it. I also tried copying the arrays in new arrays, as velx (=vx), vely and velz but its of no use. Could someone please tell what am I doing wrong? 

    for i in xrange(0,natoms,1):
            x = random.random()
            vx[i]=4.0*math.sqrt(vtotsq)*(x-0.5)
            print '1st %f' % (vx[i]) # prints the value here
    print (vx)
    velx = vx
    for i in xrange(0,natoms,1):
            y = random.random()
            vy[i]=4.0*math.sqrt(vtotsq)*(y-0.5)
            print '2nd %f' % (vy[i])
    print (vy)
    vely = vy
    for i in xrange(0,natoms,1):
            z = random.random()
            vz[i]=4.0*math.sqrt(vtotsq)*(z-0.5)
            print '3rd %f' % (vz[i])
    print (vz)
    velz = vz

    for i in xrange(0,natoms,1):
            print 'vx %d is %f' %(i,vx[i])

    print (vx)
    print '\n'
    print (vy)
    print '\n'
    print (vz)
    print '\n'
    print (velx)   # unfortunately veld, vely and velz all are still the same equal to vz
    print '\n'
    print (vely)
    print '\n'
    print (velz)

Upvotes: 0

Views: 49

Answers (1)

jonrsharpe
jonrsharpe

Reputation: 122052

All of your lists are simply references to the same list object, so changing one appears to change them all. To actually copy a list, you can slice the whole thing:

velx = vx[:]

or explicitly create a new list:

velx = list(vx)

But the neatest way to do what you want is a list comprehension, which creates and fills the list in one step:

vx = [4.0 * math.sqrt(vtotsq) * (random.random() - 0.5) for _ in range(natoms)]

Or, better:

vtotsqrt = math.sqrt(vtotsq) # sqrt is hard, do this once
vx = [4 * vtotsqrt * (random.random() - 0.5) for _ in range(natoms)]

Note that range(0, x, 1) is equivalent to range(x). Assuming you want the three lists to contain different random values, you should run this three times rather than copying the lists.

Upvotes: 2

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