On 18 Jan 2016, at 15:55, Lawrence Mitchell <lawrence.mitchell@imperial.ac.uk> wrote:

On 18/01/16 14:49, Angwenyi David wrote:

Dear Lawrence,

On 18 Jan 2016, at 15:28, Lawrence Mitchell <lawrence.mitchell@imperial.ac.uk> wrote:

On 18/01/16 14:07, Angwenyi David wrote:

f = Function(V).interpolate(Expression("x[0]*x[1]*x[2]*t"))

This looks wrong.  Expressions by default have access to the
coordinates (x[0..2]), but if you want to provide other values, you
need to do so like so:

expr = Expression("x[0]*x[1]*x[2]*t", t=value_for_t)

f = Function(V).interpolate(expr)

Although it looks like you don't use the function "f" later at all.

I have used f in defining fn and fn_1 such that fn is f evaluated at time tn and fn_1 is f evaluated at time tn-1; both expressions which have been used in the time-loop. Could there be a way of achieving this end?

Oh, I missed that bit.  That didn't do what you wanted, it copied the
values of f into fn and fn_1, when you made the new Function objects.

I would do something like:

fn = Function(V, name="f")
fn_1 = Function(V, name="f last")

t = 0

f_expr = Expression("x[0]*x[1]*x[2]*t", t=t)

# Initially, presumably, fn and fn_1 are both evaluated at t=0

fn.interpolate(f_expr)
fn_1.interpolate(f_expr)

while t < T:

  ...

  t += dt
  fn_1.assign(fn)
  f_expr.t = t
  fn.interpolate(f_expr)

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