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
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)
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。
列表推导式是有条件地创建另一列的另一种方法。如果您在列中使用对象dtype，就像您的示例一样，列表推导式通常优于大多数其他方法。
示例列表理解:
df['color'] = ['red' if x == 'Z' else 'green' for x in df['Set']]
%时间它测试：
import pandas as pd
import numpy as np
df = pd.DataFrame({'Type':list('ABBC'), 'Set':list('ZZXY')})
%timeit df['color'] = ['red' if x == 'Z' else 'green' for x in df['Set']]
%timeit df['color'] = np.where(df['Set']=='Z', 'green', 'red')
%timeit df['color'] = df.Set.map( lambda x: 'red' if x == 'Z' else 'green')
1000 loops, best of 3: 239 µs per loop
1000 loops, best of 3: 523 µs per loop
1000 loops, best of 3: 263 µs per loop
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)
当你有一个或几个条件时，可以使用下面的简单语句:
df['color'] = np.select(condlist=[df['Set']=="Z", df['Set']=="Y"], choicelist=["green", "yellow"], default="red")
容易，很好去!
更多信息请访问:https://numpy.org/doc/stable/reference/generated/numpy.select.html
另一种实现这一目标的方法是
df['color'] = df.Set.map( lambda x: 'red' if x == 'Z' else 'green')
