我如何添加一个颜色列到下面的数据框架，使颜色='绿色'如果设置== 'Z'，和颜色='红色'否则?
Type Set
1 A Z
2 B Z
3 B X
4 C Y
当前回答
如果你在处理海量数据，记忆方法是最好的:
# First create a dictionary of manually stored values
color_dict = {'Z':'red'}
# Second, build a dictionary of "other" values
color_dict_other = {x:'green' for x in df['Set'].unique() if x not in color_dict.keys()}
# Next, merge the two
color_dict.update(color_dict_other)
# Finally, map it to your column
df['color'] = df['Set'].map(color_dict)
当您有许多重复的值时，这种方法将是最快的。我的一般经验法则是记住data_size > 10**4 & n_distinct < data_size/4
在一种情况下，记忆10,000行，不同值不超过2,500。
其他回答
如果你在处理海量数据，记忆方法是最好的:
# First create a dictionary of manually stored values
color_dict = {'Z':'red'}
# Second, build a dictionary of "other" values
color_dict_other = {x:'green' for x in df['Set'].unique() if x not in color_dict.keys()}
# Next, merge the two
color_dict.update(color_dict_other)
# Finally, map it to your column
df['color'] = df['Set'].map(color_dict)
当您有许多重复的值时，这种方法将是最快的。我的一般经验法则是记住data_size > 10**4 & n_distinct < data_size/4
在一种情况下，记忆10,000行，不同值不超过2,500。
一个使用np.select的更简洁的方法:
a = np.array([['A','Z'],['B','Z'],['B','X'],['C','Y']])
df = pd.DataFrame(a,columns=['Type','Set'])
conditions = [
df['Set'] == 'Z'
]
outputs = [
'Green'
]
# conditions Z is Green, Red Otherwise.
res = np.select(conditions, outputs, 'Red')
res
array(['Green', 'Green', 'Red', 'Red'], dtype='<U5')
df.insert(2, 'new_column',res)
df
Type Set new_column
0 A Z Green
1 B Z Green
2 B X Red
3 C Y Red
df.to_numpy()
array([['A', 'Z', 'Green'],
['B', 'Z', 'Green'],
['B', 'X', 'Red'],
['C', 'Y', 'Red']], dtype=object)
%%timeit conditions = [df['Set'] == 'Z']
outputs = ['Green']
np.select(conditions, outputs, 'Red')
134 µs ± 9.71 µs per loop (mean ± std. dev. of 7 runs, 10000 loops each)
df2 = pd.DataFrame({'Type':list('ABBC')*1000000, 'Set':list('ZZXY')*1000000})
%%timeit conditions = [df2['Set'] == 'Z']
outputs = ['Green']
np.select(conditions, outputs, 'Red')
188 ms ± 26.5 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)
pyjanitor中的case_when函数是pd.Series.mask的包装器，并为多种条件提供了可链接/方便的形式:
对于单一条件:
df.case_when(
df.col1 == "Z", # condition
"green", # value if True
"red", # value if False
column_name = "color"
)
Type Set color
1 A Z green
2 B Z green
3 B X red
4 C Y red
适用于多种情况:
df.case_when(
df.Set.eq('Z') & df.Type.eq('A'), 'yellow', # condition, result
df.Set.eq('Z') & df.Type.eq('B'), 'blue', # condition, result
df.Type.eq('B'), 'purple', # condition, result
'black', # default if none of the conditions evaluate to True
column_name = 'color'
)
Type Set color
1 A Z yellow
2 B Z blue
3 B X purple
4 C Y black
更多的例子可以在这里找到
你可以使用pandas方法:
df['color'] = 'green'
df['color'] = df['color'].where(df['Set']=='Z', other='red')
# Replace values where the condition is False
or
df['color'] = 'red'
df['color'] = df['color'].mask(df['Set']=='Z', other='green')
# Replace values where the condition is True
或者，你也可以使用lambda函数的transform方法:
df['color'] = df['Set'].transform(lambda x: 'green' if x == 'Z' else 'red')
输出:
Type Set color
1 A Z green
2 B Z green
3 B X red
4 C Y red
@chai的性能比较:
import pandas as pd
import numpy as np
df = pd.DataFrame({'Type':list('ABBC')*1000000, 'Set':list('ZZXY')*1000000})
%timeit df['color1'] = 'red'; df['color1'].where(df['Set']=='Z','green')
%timeit df['color2'] = ['red' if x == 'Z' else 'green' for x in df['Set']]
%timeit df['color3'] = np.where(df['Set']=='Z', 'red', 'green')
%timeit df['color4'] = df.Set.map(lambda x: 'red' if x == 'Z' else 'green')
397 ms ± 101 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)
976 ms ± 241 ms per loop
673 ms ± 139 ms per loop
796 ms ± 182 ms per loop
使用.apply()方法的一行代码如下:
df['color'] = df['Set'].apply(lambda set_: 'green' if set_=='Z' else 'red')
之后，df数据帧是这样的:
>>> print(df)
Type Set color
0 A Z green
1 B Z green
2 B X red
3 C Y red
