# Visualizing a Decision Tree
> Google Machine Learning Recipes 2
> 官方中文博客 http://chinagdg.org/2016/03/machine-learning-recipes-for-new-developers/
> 视频地址 http://v.youku.com/v_show/id_XMTUzNDE5Mzg0MA==.html?f=26979872&from=y1.2-3.4.3
> Github工程地址 https://github.com/ahangchen/GoogleML
> 欢迎Star,也欢迎到[Issue区讨论](https://github.com/ahangchen/GoogleML/issues)
我们从Iris问题,学习决策树可视化,了解决策树工作过程。
## Why decision Tree
有很多分类器
- Artificial neural network
- Support Vector Machine
- Logistics
### 决策树好处
- Easy to read and understand
- 仅有的可解释的几种模型之一(能理解分类器做决策的过程)
决策树就是一系列关于feature的判断作为结点,以label为叶子的一棵树。因此feature越好,结果也越好。
## Iris
经典机器学习问题:[识别三种Iris](https://en.wikipedia.org/wiki/Iris_flower_data_set)
可以在维基看到这个数据集的详细信息,共 50 * 3 = 150 条记录
四个feature:Sepal length, Sepal width, Petal length, Petal width
三个label:setosa, versicolor, virginica。
可以从sklearn中直接导入。
**组成**
- metadata: feature_names, target_names(这个其实就是label names),描述数据用
- data: 具体feature数据,是一个数组,数组中的每个元素是dataset中的一条数据
- target: 具体label数据,是一个数组
## 目标
1. 导入数据
2. 训练分类器
3. 预测新的花的label
4. 查看决策树
## 测试数据
- 非训练数据的真实数据,测试分类器的准确度,
- 这里从dataset中抽出第0,第50,第100条作为测试数据
- numpy是一个Python的数据处理库,查看官方[Tutorial](https://docs.scipy.org/doc/numpy-dev/user/quickstart.html)学习更多
- 测试有很多内容,后面还会有。
## 环境
可视化使用了pydot,但Pycharm会升级anaconda中的包,导致找不到,我执行了
```
sudo /home/cwh/anaconda2/bin/conda install -p /home/cwh/anaconda2 pydot -y
```
重新安装pydot修复pydot找不到的问题;
另外pydot会找不到Graphviz,需要再安装
```
sudo /home/cwh/anaconda2/bin/conda install -p /home/cwh/anaconda2 Graphviz -y
```
然后将Graphviz添加到环境变量中,修改/etc/environment为以下内容,重启系统(我的系统是Ubuntu14.04LTS):
```
PATH="/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/home/cwh/android-sdk-linux/ndk-bundle:/home/cwh/android-sdk-linux/platform-tools:/home/cwh/anaconda2/pkgs/graphviz-2.38.0-1/bin"
```
然后又会有Graphviz中找不到libgvplugin_pango.so.6的问题,根据[官网Issue的解答](http://www.graphviz.org/content/issue-warning-could-not-load-usrlibgraphvizlibgvpluginrsvgso6),应该是少了依赖库
```
ldd /home/cwh/anaconda2/pkgs/graphviz-2.38.0-1/lib/graphviz/libgvplugin_pango.so.6
```
发现libpng16 not found,于是安装libpng16,在[这里](https://sourceforge.net/projects/libpng/?source=directory)下载,然后安装,
```
./configure
make
sudo make install
sudo ldconfig
```
再运行代码即可。
## 代码
[Viz](../../src/viz.py):以Iris为例,导入数据,训练分类器,预测,查看决策树
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