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Basic question about forAll loop and volVectorField

Hello all, Pls I have two basic questions.
1.) What's the difference between the forAll loops below i.e. what's the effect of using the variable "A" with the index "celli"?

Code:

forAll (A,celli)

{

  if (A <= 1.0)

  {

  Talpha[A[celli]] = x[A[celli]]*y[A[celli]];

....

compared to solving as

Code:

Talpha[celli] = x[celli]*y[celli];

2.) Given a surfaceVectorField, how can I transform it into a volVectorField? I've tried using fvc::reconstruct() but it makes my case blow up, guess it's only ideal for scalar field.

Thanks in advance for your help

1-
A is array so A[celli] returns the value of A in the celli, for example celli is 1 but the value of A[celli] is 10, then T[A[celli]] equals T[10] and T[celli] equals T[1]

2- you can use forexample: surfaceSum, there are several else in programmer guide

Thanks for your quick response