# Quick Starts with singularity

## 1. Installation Quick Start

Using Feel++ inside Docker is the recommended and fastest way to use Feel++. The Docker chapter is dedicated to Docker and using Feel++ in Docker.

We strongly encourage you to follow these steps if you begin with Feel++ in particular as an end-user.

People who would like to develop with and in Feel++ should read through the remaining sections of this chapter.

## 2. Usage Start

Start the Docker container `feelpp/feelpp-base` or `feelpp/feelpp-toolboxes` as follows

``> docker run -it -v $HOME/feel:/feel feelpp/feelpp-toolboxes``  these steps are explained in the chapter on Feel++ containers. Then run e.g. the Quickstart Laplacian that solves the Laplacian problem in Quickstart Laplacian sequential or in Quickstart Laplacian on 4 cores in parallel. Quickstart Laplacian sequential ``> feelpp_qs_laplacian_2d --config-file Testcases/quickstart/laplacian/feelpp2d/feelpp2d.cfg`` The results are stored in Docker in `/feel/qs_laplacian/feelpp2d/np_1/exports/ensightgold/qs_laplacian/` and on your computer `$HOME/feel/qs_laplacian/feelpp2d/np_1/exports/ensightgold/qs_laplacian/`

The mesh and solutions can be visualized using e.g. Parariew or Visit.

 ParaView (recommended) is an open-source, multi-platform data analysis and visualization application. Visit is a distributed, parallel visualization and graphical analysis tool for data defined on two- and three-dimensional (2D and 3D) meshes
Quickstart Laplacian on 4 cores in parallel
``> mpirun -np 4 feelpp_qs_laplacian_2d --config-file Testcases/quickstart/laplacian/feelpp2d/feelpp2d.cfg``

The results are stored in a simular place as above: just replace `np_1` by `np_4` in the paths above. The results should look like

 Solution Mesh

## 3. Syntax Start

Here are some excerpts from Quickstart Laplacian that solves the Laplacian problem. It shows some of the features of Feel++ and in particular the domain specific language for Galerkin methods.

First we load the mesh, define the function space define some expressions

``````    tic();

tic();
auto Vh = Pch<2>( mesh ); (1)
auto u = Vh->element("u"); (2)
auto mu = expr(soption(_name="functions.mu")); // diffusion term (3)
auto f = expr( soption(_name="functions.f"), "f" ); (4)
auto r_1 = expr( soption(_name="functions.a"), "a" ); // Robin left hand side expression (5)
auto r_2 = expr( soption(_name="functions.b"), "b" ); // Robin right hand side expression (6)
auto n = expr( soption(_name="functions.c"), "c" ); // Neumann expression (7)
auto solution = expr( checker().solution(), "solution" ); (8)
auto g = checker().check()?solution:expr( soption(_name="functions.g"), "g" ); (9)
auto v = Vh->element( g, "g" ); (3)
toc("Vh");``````

Second we define the linear and bilinear forms to solve the problem

Laplacian problem in an arbitrary geometry, defining forms and solving
``````    tic();
auto l = form1( _test=Vh );
l = integrate(_range=elements(mesh),
_expr=f*id(v));
l+=integrate(_range=markedfaces(mesh,"Robin"), _expr=r_2*id(v));
l+=integrate(_range=markedfaces(mesh,"Neumann"), _expr=n*id(v));
toc("l");

tic();
auto a = form2( _trial=Vh, _test=Vh);
tic();
a = integrate(_range=elements(mesh),