python - 通过替换计算两个数据框中两列的减法

Question

我有df1其中包含:

IDs    values
E21    32
DD12   82
K99    222

和df2包含:

IDs   values
GU1   87
K99   93
E21   48

我需要的是检查 ID 是否在df2存在于 df1 , 减去 value的 df1 - df2为此ID并更新 value在df2 .

如果ID的 df2在 df1 中不存在，该值 ID在df2保持不变。

所以，上面例子的结果(基本上df2会被更新):

IDs    values
GU1    87 #the same not changed since it not exist in df1
K99    129 #exist in df1, do the subtraction 222-93=129
E21    -16 #exist in df1, do the subtraction 32-48=129

有什么帮助吗？

Answer 1

# create new column in df2 with name 'new'
df2['new'] = df2['values']
# loop on the values of 'IDs' column
for i, element in enumerate(df2.IDs):
    # condition to check if an element exists in df1  
    if element in df1.IDs.values:
        df2['new'][i]  = df1['values'][df1.index[df1.IDs == element][0]] - df2['values'][i]
# drop the old values column 
df2.drop('values', axis = 1, inplace= True)
# rename the new values column
df2.rename(columns={'new': 'values'}, inplace= True)