CODEWITHSUNDEEP
pythonpandas
Nima
Nima

Reputation: 122

How to change one columns values based another column via the index

I have a data frame that has two columns:

A    B

0    0
0    1
0    0
0    0 
0    0
0    1

I want to have some code that checks the B column and when a 1 is found, the values of column A from that INDEX on changes to 1 like this;

A    B
0    0
1    1
1    0
1    0
1    0
1    1

My code is like this but it is not what I want:

df['A']= np.where(df['B'] == 1,df['A'], '1')

Upvotes: 3

Views: 51

Answers (2)

BENY
BENY

Reputation: 323226

Let us try cummax

df['A']=df.B.cummax()
df
Out[302]: 
   A  B
0  0  0
1  1  1
2  1  0
3  1  0
4  1  0
5  1  1

Upvotes: 2

Dani Mesejo
Dani Mesejo

Reputation: 61910

You could do:

df.loc[df['B'].idxmax():, 'A'] = 1
print(df)

Output

   A  B
0  0  0
1  1  1
2  1  0
3  1  0
4  1  0
5  1  1

If you want to use np.where, do:

df['A'] = np.where(df.index < df['B'].idxmax(), df['A'], 1)
print(df)

If the column B can have other values besides 0s and 1s, do:

df['A'] = np.where(df.index < df['B'].eq(1).idxmax(), df['A'], 1)

Upvotes: 5

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