python 2.7 - Strange behaviour when adding columns -

i'm using python 2.7.8 |anaconda 2.1.0. i'm wondering why strange behavior below occurs

i create pandas dataframe 2 columns, add third column summing first 2 columns

x = pd.dataframe(np.random.randn(5, 2), columns = ['a', 'b']) x['c'] = x[['a', 'b']].sum(axis = 1) #or x['c'] = x['a'] + x['b']  out[7]:                    b         c 0 -1.644246  0.851602 -0.792644 1 -0.129092  0.237140  0.108049 2  0.623160  0.105494  0.728654 3  0.737803 -1.612189 -0.874386 4  0.340671 -0.113334  0.227337 

all far. want set values of column c 0 if negative

x[x['c']<0] = 0 out[9]:                    b         c 0  0.000000  0.000000  0.000000 1 -0.129092  0.237140  0.108049 2  0.623160  0.105494  0.728654 3  0.000000  0.000000  0.000000 4  0.340671 -0.113334  0.227337 

this gives desired result in column 'c', reason columns 'a' , 'b' have been modified - don't want happen. wondering why happening , how can fix behavior?

you have specify want 'c' column:

x.loc[x['c']<0, 'c'] = 0 

when index boolean array/series, select full rows, can see in example:

in [46]: x['c']<0 out[46]: 0     true 1    false 2    false 3     true 4    false name: c, dtype: bool  in [47]: x[x['c']<0] out[47]:                   b         c 0 -0.444493 -0.592318 -1.036811 3 -1.363727 -1.572558 -2.936285