[Rocky4]

Dear Foamers,

i have a volScalarField (say X) whose value at the patches and cells depends on the cell values of another volScalarField (sayY). i am able to change the value of X at the cells using

forAll(X, celli)
{ if (Y[celli] ==0)
{X[celli] =a;}
else
{X[celli]=b;}}

however i am unable to do the same for the patches of X
i.e if Y[celli] =0 where celli is any boundary cell then X[patchi] should be set to 'a' else it should be set to 'b'.something like this

forAll(X.boundaryField(), patchi)
{ //if (Y[celli] ==0) ,this represents any boundary cell value of field Y
{X.boundaryField()[patchi] =a;}
else
{X.boundaryField()[patchi]=b;}}

can someone can give a detailed code to accomplish this task?
i m new to openfoam. any kind of help is greatly appreciated. thanks in advance.