How to make current row value equal to previous row value if a column equals a value in Pandas?

Question

I have the dataframe as below:

df

ID  val

1   0.0

2   yes

3   1.0

4   0.0

5   yes

How do I match my previous value with my current value if the column val equals "yes"

I tried using the code below:

 df['val'] = df['val'].replace('yes', np.nan).bfill().astype(str) 

I am getting this output which is not what I wanted to.

ID  val

1   yes

2   yes

3   1.0

4   yes

5   yes

can we use np.where along with the bfill? 

Answer 1

You can use the below code:

df.loc[df['val'].shift(-1).eq('yes'), 'val'] = 'yes'

Output:

   ID  val

0   1  yes

1   2  yes

2   3  1.0

3   4  yes

4   5  yes

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