# Option "rhsvarf" for on() **URL:** \ **Category:** Feature Request\ **Created:** [July 18, 2024, 2:12pm UTC](https://community.freefem.org/t/option-rhsvarf-for-on/3404 "2024-07-18T14:12:49Z")\ **Posts on this page:** 9\ **Page:** 1
**Author:** ![fb77](https://yyz2.discourse-cdn.com/flex030/user_avatar/community.freefem.org/fb77/32/3796_2.png) [@fb77](https://community.freefem.org/u/fb77)\ **Post date:** [July 18, 2024, 2:12pm UTC](https://community.freefem.org/t/option-rhsvarf-for-on/3404/1 "2024-07-18T14:12:49Z")
This proposal is related to the discussion [Behavior of varf with rhs and on()](https://community.freefem.org/t/behavior-of-varf-with-rhs-and-on/3248) I think it would be useful to have a new option “`rhsvarf`” (=true or false) for the `on()` tool that sets Dirichlet BC. – If `on()` is used in the context of “`solve`” or “`problem`”, the option has no effect, and the behavior is the present one. – The default value of `rhsvarf` would be “true”, in this case the behavior of `on()` is the present one. This means that when used within “`varf`”, the imposed value (i.e. `g` for “`on(1,u=g)`” ) is interpreted to the right-hand side. Ex ```auto varf test(u,v) =int2d(Th)(dx(u)*dx(v)+dy(u)*dy(v)) -int2d(Th)(a*v) +on(1,u=g) ; ``` Then `tab=test(0,Vh);` sets `tab` to `+g*tgv` (additionally to the `-int2d(Th)(a*v)` terms) – Setting `rhsvarf=false` would imply that when used within “`varf`”, the imposed value `g` is interpreted in the left-hand side (thus changing its sign). For the previous example it means that `tab=test(0,Vh);` sets` tab` to `-g*tgv` (additionally to the `-int2d(Th)(a*v)` terms) As a consequence, both codes ```auto solve prob(u,v) =int2d(Th)(dx(u)*dx(v)+dy(u)*dy(v)) -int2d(Th)(a*v) +on(1,u=g) ; solprob=u[]; ``` and ```auto varf test(u,v) =int2d(Th)(dx(u)*dx(v)+dy(u)*dy(v)) -int2d(Th)(a*v) +on(1,u=g,rhsvarf=false) ; matrix A=test(Vh,Vh); tab=test(0,Vh);//compute the left-hand side tab=-tab;//transform it into right-hand side sol=A^-1*tab; ``` would give the same solutions `solprob` and `sol`. ---
**Author:** ![prj](https://avatars.discourse-cdn.com/v4/letter/p/ecae2f/32.png) [@prj](https://community.freefem.org/u/prj)\ **Post date:** [July 19, 2024, 5:49am UTC](https://community.freefem.org/t/option-rhsvarf-for-on/3404/2 "2024-07-19T05:49:06Z")
Do you have a proper use case? ---
**Author:** ![fb77](https://yyz2.discourse-cdn.com/flex030/user_avatar/community.freefem.org/fb77/32/3796_2.png) [@fb77](https://community.freefem.org/u/fb77)\ **Post date:** [July 19, 2024, 9:52am UTC](https://community.freefem.org/t/option-rhsvarf-for-on/3404/3 "2024-07-19T09:52:28Z")
I would use it all the time, because I don’t want to ask myself what is the right sign each time I use on()! Indeed the present way it works is a source of error for me. An example where it would really simplify the coding is when I use a macro to define boundary conditions: ```auto macro BCond +int1d(Th,2)(u1*N.x+u2*N.y)+on(3,u1=1.)+on(1,4,u1=0.,u2=-1.)//boundary conditions ``` Then I call `BCond` in many places all along the code, some involving `problem`, some involving `varf` ---
**Author:** ![prj](https://avatars.discourse-cdn.com/v4/letter/p/ecae2f/32.png) [@prj](https://community.freefem.org/u/prj)\ **Post date:** [July 19, 2024, 10:19am UTC](https://community.freefem.org/t/option-rhsvarf-for-on/3404/4 "2024-07-19T10:19:15Z")
If you don’t want to ask yourself what is the proper sign, why do you switch between `varf` and `problem` and not just stick to a single type? ---
**Author:** ![prj](https://avatars.discourse-cdn.com/v4/letter/p/ecae2f/32.png) [@prj](https://community.freefem.org/u/prj)\ **Post date:** [July 19, 2024, 10:23am UTC](https://community.freefem.org/t/option-rhsvarf-for-on/3404/5 "2024-07-19T10:23:23Z")
Also, what kind of guarantees do we have that people will never forget `tab=-tab;//transform it into right-hand side` when needed? You are asking for a way to let users shoot themselves in the foot. ---
**Author:** ![fb77](https://yyz2.discourse-cdn.com/flex030/user_avatar/community.freefem.org/fb77/32/3796_2.png) [@fb77](https://community.freefem.org/u/fb77)\ **Post date:** [July 19, 2024, 11:59am UTC](https://community.freefem.org/t/option-rhsvarf-for-on/3404/6 "2024-07-19T11:59:07Z")
“Why do you switch between varf and problem and not just stick to a single type?” Each of them has its advantages : problem is more simple, and varf has more capabilities (but needs more code lines) “You are asking for a way to let users shoot themselves in the foot.” The issue of not forgetting the minus sign is already present concerning linear terms coming from `int2d`, as in my basic example. This sign is obviously related to considering right-hand sides (solving Ax=b) or left-hand sides (solving Ax+b=0). I think that `rhsvarf=true` is adapted to the use of a two `varf`, one for the bilinear terms and one for the linear terms, whereas `rhsvarf=false` is adapted to the use of a single `varf` with bilinear and linear terms in it. ---
**Author:** ![prj](https://avatars.discourse-cdn.com/v4/letter/p/ecae2f/32.png) [@prj](https://community.freefem.org/u/prj)\ **Post date:** [July 19, 2024, 12:20pm UTC](https://community.freefem.org/t/option-rhsvarf-for-on/3404/7 "2024-07-19T12:20:59Z")
> [@fb77](#): > > The issue of not forgetting the minus sign is already present concerning linear terms coming from `int2d` What does that mean? If you know the proper definition of a `varf`, how could you be confused by the sign in front of the linear terms? ---
**Author:** ![fb77](https://yyz2.discourse-cdn.com/flex030/user_avatar/community.freefem.org/fb77/32/3796_2.png) [@fb77](https://community.freefem.org/u/fb77)\ **Post date:** [July 19, 2024, 12:43pm UTC](https://community.freefem.org/t/option-rhsvarf-for-on/3404/8 "2024-07-19T12:43:10Z")
There is no confusion: it follows that one needs to write `tab=-tab;` because we solve Ax+b=0, and this is not a difficulty. I am sorry to bother you, I see that you do not like my proposal. ---
**Author:** ![prj](https://avatars.discourse-cdn.com/v4/letter/p/ecae2f/32.png) [@prj](https://community.freefem.org/u/prj)\ **Post date:** [July 19, 2024, 1:00pm UTC](https://community.freefem.org/t/option-rhsvarf-for-on/3404/9 "2024-07-19T13:00:43Z")
You are not bothering me, this is a difficult (and frequent) issue, so it’s always worthwhile discussing this. That being said, you are correct, I don’t think this change would be useful for common users of FreeFEM (I personally never teach about `problem`, always `varf`, so there is less room for confusion). But if you make a merge request and find someone that would be OK to merge this, I won’t object.