Iterating within a function using for loop doesn't work?

Question

When I use for loop within a function (see below), the block of statements below the for loop is not iterated,

def F(rho, m, E, v_rho, v_m, v_E):
    for n in xrange(N):
        #Update conserved quantities
        rho = G(Evolve_rho1(rho))
        m = momentum(Evolve_m1(m))
        E = Energy(Evolve_E1(E))
        v_rho = rho_v(Evolve_rho_v(rho))
        v_m = m_v(Evolve_mv(m))
        v_E = E_v(Evolve_Ev(E))

    return (rho, m, E, v_rho, v_m, v_E)

since after calling the function in this way: density, momentum, Energy, dflux, mflux, Eflux = F(rho, m, E, v_rho, v_m, v_E), and print for example density, leads to wrong answer.

But if I only use for loop like below, it works fine.

for n in xrange(N):
    #Update conserved quantities
    rho = G(Evolve_rho1(rho))
    m = momentum(Evolve_m1(m))
    E = Energy(Evolve_E1(E))
    v_rho = rho_v(Evolve_rho_v(rho))
    v_m = m_v(Evolve_mv(m))
    v_E = E_v(Evolve_Ev(E))
print rho
print m

etc., give correct results.

Any suggestion is welcome and appreciated.

This might be a pasting error, but in the function N is not defined. — Chris Pickett
– Chris Pickett, Commented Aug 26, 2011 at 19:41
@Chris Pickett not having N defined would throw an Exception ...though it would be good to know that it wasn't set to 0 — pyInTheSky
– pyInTheSky, Commented Aug 26, 2011 at 19:44
We don't have enough information to help you solve your problem. Please devise a minimal example that demonstrates the problem and post the whole module. — SingleNegationElimination
– SingleNegationElimination, Commented Aug 26, 2011 at 20:32

Ross Patterson · Accepted Answer · 2011-08-27 18:45:41Z

1

I'm guessing your missing the difference between a global and a local vlue for N. As you've written it, the value of N in the F function will be whatever N is when F is called, not when it's defined. So if at the time F is called N==0, then the loop block will never be executed.

answered Aug 27, 2011 at 18:45

Ross Patterson

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Simplicity. · Accepted Answer · 2011-08-30 14:19:40Z

0

The function was using global variables instead of local which were updated in each iteration in F(). The solution is to pass variables as parameters and define `

def F(rho, m, E, v_rho, v_m, v_E):
    for n in xrange(N):                                  
        rho = G(Evolve_rho1(rho,v_rho,m))
        m  = momentum(Evolve_m1(rho,v_rho, m,v_m,E))
        E = Energy(Evolve_E1(rho, m, E, v_E))
        v_rho = rho_v(Evolve_rho_v(rho,v_rho,m))
        v_m =   m_v( Evolve_mv(rho,v_rho, m,v_m,E)) 
        v_E  =  E_v(Evolve_Ev(rho, m, E, v_E))
    return (rho, m, E, v_rho, v_m, v_E)

Thanks Folks for all your contributions.

answered Aug 30, 2011 at 14:19

Simplicity.

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