如果等于另一列中的值，则熊猫从多列返回值

Question

我有一个像这样的 Pandas 数据框：

  A      B        C         D
0 month   month+1 quarter+1 season+1
1 season  month+5 quarter+3 season+2
2 day     month+1 quarter+2 season+1
3 year    month+3 quarter+4 season+2
4 quarter month+2 quarter+1 season+1
5 month   month+4 quarter+1 season+2

我想根据几个 IF 条件插入一个名为“E”的新列。 如果“A”列等于“月”，则返回“B”中的值，如果“A”列等于“季度”，则返回“C”中的值，如果“A”列等于“季节”，则返回“D”中的值，如果不是，则返回“A”列中的值

  A      B        C         D        E
0 month   month+1 quarter+1 season+1 month+1
1 season  month+5 quarter+3 season+2 season+2
2 day     month+1 quarter+2 season+1 day
3 year    month+3 quarter+4 season+2 year
4 quarter month+2 quarter+1 season+1 quarter+1
5 month   month+4 quarter+1 season+2 month+4

我在做这件事时遇到了麻烦。 我试过玩弄一个函数，但它没有用。 看我的尝试：

def f(row):
    if row['A'] == 'month':
        val = ['B']
    elif row['A'] == 'quarter':
        val = ['C']
    elif row['A'] == 'season':
        val = ['D']
    else:
        val = ['A']
    return val

df['E'] = df.apply(f, axis=1)

编辑：将最后一个else更改为“A”列

Answer 1

首先，我建议你看看： When should I want to use apply() in my code.

我会使用Series.replace

df['E'] = df['A'].replace(['month','quarter','season'],
                          [df['B'], df['C'], df['D']]) 

或numpy.select

cond = [df['A'].eq('month'), df['A'].eq('quarter'), df['A'].eq('season')]
values= [df['B'], df['C'], df['D']]
df['E']=np.select(cond,values,default=df['A'])

---

  A      B        C         D        E
0 month   month+1 quarter+1 season+1 month+1
1 season  month+5 quarter+3 season+2 season+2
2 day     month+1 quarter+2 season+1 day
3 year    month+3 quarter+4 season+2 year
4 quarter month+2 quarter+1 season+1 quarter+1
5 month   month+4 quarter+1 season+2 month+4

Answer 2

只需使用np.select

c1 = df['A'] == 'month'
c2 = df['A'] == 'quarter'
c3 = df['A'] == 'season'

df['E'] = np.select([c1, c2, c3], [df['B'], df['C'], df['D']], df['A'])

Out[271]:
         A        B          C         D          E
0    month  month+1  quarter+1  season+1    month+1
1   season  month+5  quarter+3  season+2   season+2
2      day  month+1  quarter+2  season+1        day
3     year  month+3  quarter+4  season+2       year
4  quarter  month+2  quarter+1  season+1  quarter+1
5    month  month+4  quarter+1  season+2    month+4

Answer 3

您可能需要像这样修复您的代码：

def f(row):
    if row['A'] == 'month':
        val = row['B']
    elif row['A'] == 'quarter':
        val = row['C']
    elif row['A'] == 'season':
        val = row['D']
    else:
        val = row['D']
    return val

df['E'] = df.apply(f, axis=1)

注意：你忘了包括row

val = ['B'] # before
val = row['B'] # after

编辑：这只是为了指出代码中的问题，为了更好的方法，请查看与使用numpy.select相关的其他答案