玩kaggle时听说这个库很牛逼，准确率很高，于是打算学学看看。xgboost是extreme gradient boosting的缩写，gradient boosting是一类思想方法，这里不多赘述，可以参考这里：gradient boosting简介
本文主要解析实例，理论以后慢慢补上。
binary classification的例子官方原文在此： 点击打开链接
先说例子做的是一件什么事情，很简单，给你一堆有label的包含各种属性的蘑菇数据集，label只有两种： 有毒p或者可食用e。蘑菇有22种属性，每种属性有若干可取值，官方描述如下：
7. Attribute Information: (classes: edible=e, poisonous=p)
1. cap-shape: bell=b,conical=c,convex=x,flat=f,
knobbed=k,sunken=s
2. cap-surface: fibrous=f,grooves=g,scaly=y,smooth=s
3. cap-color: brown=n,buff=b,cinnamon=c,gray=g,green=r,
pink=p,purple=u,red=e,white=w,yellow=y
4. bruises?: bruises=t,no=f
5. odor: almond=a,anise=l,creosote=c,fishy=y,foul=f,
musty=m,none=n,pungent=p,spicy=s
6. gill-attachment: attached=a,descending=d,free=f,notched=n
7. gill-spacing: close=c,crowded=w,distant=d
8. gill-size: broad=b,narrow=n
9. gill-color: black=k,brown=n,buff=b,chocolate=h,gray=g,
green=r,orange=o,pink=p,purple=u,red=e,
white=w,yellow=y
10. stalk-shape: enlarging=e,tapering=t
11. stalk-root: bulbous=b,club=c,cup=u,equal=e,
rhizomorphs=z,rooted=r,missing=?
12. stalk-surface-above-ring: fibrous=f,scaly=y,silky=k,smooth=s
13. stalk-surface-below-ring: fibrous=f,scaly=y,silky=k,smooth=s
14. stalk-color-above-ring: brown=n,buff=b,cinnamon=c,gray=g,orange=o,
pink=p,red=e,white=w,yellow=y
15. stalk-color-below-ring: brown=n,buff=b,cinnamon=c,gray=g,orange=o,
pink=p,red=e,white=w,yellow=y
16. veil-type: partial=p,universal=u
17. veil-color: brown=n,orange=o,white=w,yellow=y
18. ring-number: none=n,one=o,two=t
19. ring-type: cobwebby=c,evanescent=e,flaring=f,large=l,
none=n,pendant=p,sheathing=s,zone=z
20. spore-print-color: black=k,brown=n,buff=b,chocolate=h,green=r,
orange=o,purple=u,white=w,yellow=y
21. population: abundant=a,clustered=c,numerous=n,
scattered=s,several=v,solitary=y
22. habitat: grasses=g,leaves=l,meadows=m,paths=p,
urban=u,waste=w,woods=d
官方格式的数据集长这样：
e,x,y,y,t,l,f,c,b,n,e,r,s,y,w,w,p,w,o,p,n,s,g p,f,s,n,t,p,f,c,n,p,e,e,s,s,w,w,p,w,o,p,k,s,g e,f,f,w,f,n,f,w,b,h,t,e,f,s,w,w,p,w,o,e,n,s,g e,f,f,n,f,n,f,w,b,n,t,e,f,f,w,w,p,w,o,e,k,a,g p,f,s,n,t,p,f,c,n,w,e,e,s,s,w,w,p,w,o,p,n,v,g e,x,s,n,f,n,f,w,b,k,t,e,f,f,w,w,p,w,o,e,n,a,g e,x,f,n,f,n,f,w,b,n,t,e,s,f,w,w,p,w,o,e,k,s,g e,x,f,e,t,n,f,c,b,u,t,b,s,s,g,w,p,w,o,p,k,v,d e,f,f,n,f,n,f,w,b,k,t,e,s,s,w,w,p,w,o,e,k,a,g
第一列为lable，后面22列为属性值
看起来很规整对吧？对于xgboost cli来说，还不够规整，xgboost期望的数据格式长这样：
0 3:1 9:1 20:1 21:1 24:1 34:1 36:1 39:1 45:1 53:1 56:1 65:1 69:1 77:1 86:1 88:1 92:1 95:1 102:1 106:1 116:1 120:1 0 3:1 9:1 20:1 21:1 23:1 34:1 36:1 39:1 42:1 53:1 56:1 65:1 69:1 77:1 86:1 88:1 92:1 95:1 102:1 105:1 117:1 122:1 0 1:1 10:1 20:1 21:1 23:1 34:1 36:1 39:1 51:1 53:1 56:1 65:1 69:1 77:1 86:1 88:1 92:1 95:1 102:1 106:1 117:1 120:1 1 3:1 9:1 19:1 21:1 30:1 34:1 36:1 40:1 41:1 53:1 58:1 65:1 69:1 77:1 86:1 88:1 92:1 95:1 102:1 106:1 118:1 124:1 0 3:1 7:1 11:1 22:1 29:1 34:1 37:1 39:1 42:1 54:1 58:1 65:1 66:1 77:1 86:1 88:1 92:1 95:1 98:1 105:1 114:1 120:1
这是什么鬼？看过的朋友就知道这是libsvm格式，简单来说第一列为label，后面的 A:B中的A代表feature index, B代表feature值，值为0的直接被略掉了，所以称为稀疏表示法。具体可以参考这里
而由于每个蘑菇feature可以有超过2种可能值，为了方便，我们先转换成独热编码的形式，再转化成上面这种需要的形式，思路上第一步就是这样，上代码：
mapfeat.py:
-
#!/usr/bin/python
-
-
def loadfmap( fname ):
-
fmap = {}
-
nmap = {}
-
-
for l in open( fname ):
-
arr = l.split()
-
if arr[ 0].find( '.') != -1:
-
idx = int( arr[ 0].strip( '.') )
-
assert idx not in fmap
-
fmap[ idx ] = {}
-
ftype = arr[ 1].strip( ':')
-
content = arr[ 2]
-
else:
-
content = arr[ 0]
-
for it in content.split( ','):
-
if it.strip() == '':
-
continue
-
k , v = it.split( '=')
-
fmap[ idx ][ v ] = len(nmap) + 1
-
nmap[ len(nmap) ] = ftype+ '='+k
-
return fmap, nmap
-
-
def write_nmap( fo, nmap ):
-
for i in range( len(nmap) ):
-
fo.write( '%d\t%s\ti\n' % (i, nmap[i]) )
-
-
# start here
-
fmap, nmap = loadfmap( 'agaricus-lepiota.fmap' )
-
fo = open( 'featmap.txt', 'w' )
-
write_nmap( fo, nmap )
-
fo.close()
-
-
fo = open( 'agaricus.txt', 'w' )
-
for l in open( 'agaricus-lepiota.data' ):
-
arr = l.split( ',')
-
if arr[ 0] == 'p':
-
fo.write( '1')
-
else:
-
assert arr[ 0] == 'e'
-
fo.write( '0')
-
for i in range( 1,len(arr) ):
-
fo.write( ' %d:1' % fmap[i][arr[i].strip()] )
-
fo.write( '\n')
-
-
fo.close()
不长，也很简单，就是转成独热编码的libsvm表示，都是些字符串处理的逻辑，不多废话了，结果存储到agaricus-lepiota.data。
接下来，把所有的输入数据拆分成training set 和 test set
mknfold.py:
-
#!/usr/bin/python
-
import sys
-
import random
-
-
#==fc==
-
if len(sys.argv) < 2:
-
print ( 'Usage:<filename> <k> [nfold = 5]')
-
exit( 0)
-
-
random.seed( 10 )
-
-
k = int( sys.argv[ 2] )
-
if len(sys.argv) > 3:
-
nfold = int( sys.argv[ 3] )
-
else:
-
nfold = 5
-
-
fi = open( sys.argv[ 1], 'r' )
-
ftr = open( sys.argv[ 1]+ '.train', 'w' )
-
fte = open( sys.argv[ 1]+ '.test', 'w' )
-
for l in fi:
-
if random.randint( 1 , nfold ) == k:
-
fte.write( l )
-
else:
-
ftr.write( l )
-
-
fi.close()
-
ftr.close()
-
fte.close()
-
逻辑也不复杂，就是通过随机数的一种巧妙方法把数据集做一个随机拆分，分别写到两个文件里。
至此，数据集准备完毕。下面开始用xgboost训练， 这里用cli版本的调用如下：
../../xgboost mushroom.conf其实就是一个跑xgboost的配置文件，里面包含这些等等：
# General Parameters, see comment for each definition # choose the booster, can be gbtree or gblinear booster = gbtree # choose logistic regression loss function for binary classification objective = binary:logistic # Tree Booster Parameters # step size shrinkage eta = 1.0 # minimum loss reduction required to make a further partition gamma = 1.0 # minimum sum of instance weight(hessian) needed in a child min_child_weight = 1 # maximum depth of a tree max_depth = 3
完整的参数配置参考 这里
接下来可以用这种命令：../../xgboost mushroom.conf task=dump model_in=0002.model name_dump=dump.raw.txt 把训练出来的树dump出来， 树大概长这样：
booster[0]:
0:[f29<-9.53674e-07] yes=1,no=2,missing=1
1:[f56<-9.53674e-07] yes=3,no=4,missing=3
3:[f60<-9.53674e-07] yes=7,no=8,missing=7
7:leaf=1.90175
8:leaf=-1.95062
4:[f21<-9.53674e-07] yes=9,no=10,missing=9
9:leaf=1.77778
10:leaf=-1.98104
2:[f109<-9.53674e-07] yes=5,no=6,missing=5
5:[f67<-9.53674e-07] yes=11,no=12,missing=11
11:leaf=-1.98531
12:leaf=0.808511
6:leaf=1.85965
booster[1]:
0:[f29<-9.53674e-07] yes=1,no=2,missing=1
1:[f21<-9.53674e-07] yes=3,no=4,missing=3
3:leaf=1.1457
4:[f36<-9.53674e-07] yes=7,no=8,missing=7
7:leaf=-6.87558
8:leaf=-0.127376
2:[f109<-9.53674e-07] yes=5,no=6,missing=5
5:[f39<-9.53674e-07] yes=9,no=10,missing=9
9:leaf=-0.0386054
10:leaf=-1.15275
6:leaf=0.994744
完整的执行顺序如下：
#!/bin/bash
# map feature using indicator encoding, also produce featmap.txt
python mapfeat.py
# split train and test
python mknfold.py agaricus.txt 1
# training and output the models
../../xgboost mushroom.conf
# output prediction task=pred
../../xgboost mushroom.conf task=pred model_in=0002.model
# print the boosters of 00002.model in dump.raw.txt
../../xgboost mushroom.conf task=dump model_in=0002.model name_dump=dump.raw.txt
# use the feature map in printing for better visualization
../../xgboost mushroom.conf task=dump model_in=0002.model fmap=featmap.txt name_dump=dump.nice.txt
cat dump.nice.txt