四类创建方法:
# Series方法创建
pd.Series(["a", "b", "c", "a"], dtype="category")
# 对DataFrame指定类型
temp_df = pd.DataFrame({'A':pd.Series(["a", "b", "c", "a"], dtype="category"),'B':list('abcd')})
# 使用内置Categorical类型
cat = pd.Categorical(["a", "b", "c", "a"], categories=['a','b','c'])
pd.Series(cat)
# 使用cut函数,默认使用区间类型为标签
pd.cut(np.random.randint(0,60,5), [0,10,30,60])
# 可指定字符为标签
pd.cut(np.random.randint(0,60,5), [0,10,30,60], right=False, labels=['0-10','10-30','30-60'])
包括三个部分,元素值(values)、分类类别(categories)、是否有序(order)。使用cut函数创建的分类变量默认为有序分类变量。
如何获取或修改这些属性?
(a)describe方法
该方法描述了一个分类序列的情况,包括非缺失值个数、元素值类别数(不是分类类别数)、最多次出现的元素及其频数
s = pd.Series(pd.Categorical(["a", "b", "c", "a",np.nan], categories=['a','b','c','d']))
s.describe()
count 4
unique 3
top a
freq 2
dtype: object
(b)categories和ordered属性
查看分类类别和是否排序
s.cat.categories
Index(['a', 'b', 'c', 'd'], dtype='object')
s.cat.ordered
False
(a)利用set_categories修改
修改分类,但本身值不会变化
s = pd.Series(pd.Categorical(["a", "b", "c", "a",np.nan], categories=['a','b','c','d']))
s.cat.set_categories(['new_a','c'])
0 NaN
1 NaN
2 c
3 NaN
4 NaN
dtype: category
Categories (2, object): [new_a, c]
(b)利用rename_categories修改
注意:该方法会把值和分类同时修改
s = pd.Series(pd.Categorical(["a", "b", "c", "a",np.nan], categories=['a','b','c','d']))
s.cat.rename_categories(['new_%s'%i for i in s.cat.categories])
0 new_a
1 new_b
2 new_c
3 new_a
4 NaN
dtype: category
Categories (4, object): [new_a, new_b, new_c, new_d]
利用字典修改值
s.cat.rename_categories({'a':'new_a','b':'new_b'})
0 new_a
1 new_b
2 c
3 new_a
4 NaN
dtype: category
Categories (4, object): [new_a, new_b, c, d]
(c)利用add_categories添加
s = pd.Series(pd.Categorical(["a", "b", "c", "a",np.nan], categories=['a','b','c','d']))
s.cat.add_categories(['e'])
0 a
1 b
2 c
3 a
4 NaN
dtype: category
Categories (5, object): [a, b, c, d, e]
(d)利用remove_categories移除
s = pd.Series(pd.Categorical(["a", "b", "c", "a",np.nan], categories=['a','b','c','d']))
s.cat.remove_categories(['d'])
0 a
1 b
2 c
3 a
4 NaN
dtype: category
Categories (3, object): [a, b, c]
(e)删除元素值未出现的分类类型
s = pd.Series(pd.Categorical(["a", "b", "c", "a",np.nan], categories=['a','b','c','d']))
s.cat.remove_unused_categories()
0 a
1 b
2 c
3 a
4 NaN
dtype: category
Categories (3, object): [a, b, c]
前面提到,分类数据类型被分为有序和无序,这非常好理解,例如分数区间的高低是有序变量,考试科目的类别一般看做无序变量
将一个序列转为有序变量
s = pd.Series(["a", "d", "c", "a"]).astype('category').cat.as_ordered()
s
0 a
1 d
2 c
3 a
dtype: category
Categories (3, object): [a < c < d]
将一个有序序列退化为无序变量
s.cat.as_unordered()
0 a
1 d
2 c
3 a
dtype: category
Categories (3, object): [a, c, d]
pd.Series(["a", "d", "c", "a"]).astype('category').cat.set_categories(['a','c','d'],ordered=True)
0 a
1 d
2 c
3 a
dtype: category
Categories (3, object): [a < c < d]
这个方法的特点在于,新设置的分类必须与原分类为同一集合
s = pd.Series(["a", "d", "c", "a"]).astype('category')
s.cat.reorder_categories(['a','c','d'],ordered=True)
0 a
1 d
2 c
3 a
dtype: category
Categories (3, object): [a < c < d]
#s.cat.reorder_categories(['a','c'],ordered=True) #报错
#s.cat.reorder_categories(['a','c','d','e'],ordered=True) #报错
pandas基础操作中介绍的值排序和索引排序都是适用的
s = pd.Series(np.random.choice(['perfect','good','fair','bad','awful'],50)).astype('category')
s.cat.set_categories(['perfect','good','fair','bad','awful'][::-1],ordered=True).head()
0 good
1 fair
2 bad
3 perfect
4 perfect
dtype: category
Categories (5, object): [awful < bad < fair < good < perfect]
s.sort_values(ascending=False).head()
29 perfect
17 perfect
31 perfect
3 perfect
4 perfect
dtype: category
Categories (5, object): [awful, bad, fair, good, perfect]
df_sort = pd.DataFrame({'cat':s.values,'value':np.random.randn(50)}).set_index('cat')
df_sort.head()
value | |
---|---|
cat | |
good | -1.746975 |
fair | 0.836732 |
bad | 0.094912 |
perfect | -0.724338 |
perfect | -1.456362 |
df_sort.sort_index().head()
value | |
---|---|
cat | |
awful | 0.245782 |
awful | 0.063991 |
awful | 1.541862 |
awful | -0.062976 |
awful | 0.472542 |
s = pd.Series(["a", "d", "c", "a"]).astype('category')
s == 'a'
0 True
1 False
2 False
3 True
dtype: bool
s == list('abcd')
0 True
1 False
2 True
3 False
dtype: bool
两个分类变量的等式判别需要满足分类完全相同
s = pd.Series(["a", "d", "c", "a"]).astype('category')
s == s
0 True
1 True
2 True
3 True
dtype: bool
s != s
0 False
1 False
2 False
3 False
dtype: bool
s_new = s.cat.set_categories(['a','d','e'])
#s == s_new #报错
两个分类变量的不等式判别需要满足两个条件:
s = pd.Series(["a", "d", "c", "a"]).astype('category')
#s >= s #报错
s = pd.Series(["a", "d", "c", "a"]).astype('category').cat.reorder_categories(['a','c','d'],ordered=True)
s >= s
0 True
1 True
2 True
3 True
dtype: bool
如果您觉得文章对您有用,请给我点个赞吧!
您的肯定是对我最大的鼓励。