# Writing Gaudi Algorithms

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:::{admonition} Page contributors
:class: callout dropdown

Valentin Volkl, Gerardo Ganis, Juraj Smiesko
:::
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:::{admonition} Learning Objectives
:class: objectives

This tutorial will teach you how to:

* write an algorithm for FCCSW
* interact with the cmake based build system 
* use other Gaudi components in the algorithms 
:::


## Getting Started

Writing Gaudi components requires a bit of boilerplate code.
Often it is easiest to start from existing files and modify them as needed.
For this tutorial, there is a dedicated repository that contains an example.
Start by cloning it locally:

```bash
git clone https://github.com/key4hep/k4-project-template
```

It contains a CMake configuration (as described in more detail in the previous tutorial) so it can be built with:

```bash
cd k4-project-template
mkdir build install
cd build
cmake .. -DCMAKE_INSTALL_PREFIX=../install
make -j 4
```

To run the algorithms contained in this repository, it is not necesary to run

```
make install
```

you can use the `run` script in the `build` directory, like:

```bash
./run fccrun ../K4TestFWCore/options/createExampleEventData.py 

```


## Exercise: Adding an Algorithm

The repository contains an `EmptyAlg` in `K4TestFWCore/src/components`.


* As a first exercise, copy and modify this algorithm to print out the current event number.

* Second step: If you used `std::cout` in the first step, try to use the gaudi logging service instead.

* Third Step: Print out a string before the event number that should be configurable at runtime.

* Finally: use the Gaudi Random Number Generator Service to approximate pi with a [Monte Carlo Integration](https://en.wikipedia.org/wiki/Monte_Carlo_integration)