Getting "cannot set using a multi-index selection indexer with a different length than the value" error while using np.where function

Stack Overflow Asked by ashok747 on December 7, 2020

I am trying to append two data frames row wise iteratively. After that I am trying fill 0 values in one column with the values in other columns and vice versa. I am using np.where function to fill the 0 values. When I am doing it separately it is giving correct result but when I am using it in a loop it is throwing "cannot set using a multi-index selection indexer with a different length than the value" error. My code looks like below.

def myfunc(dd1,dd2,dfc):
for i in range(n):
for j in range(m):
return dfc

Where dd1 and dd2 are my dataframes, I am appending rows in them iteratively to a empty dataframe dfc. Here I am using row and column indices to fill the values. Any help on this will be appreciated.

One Answer

This is not how np.where works. The input of np.where is a list-like object. Instead of looping every data in the dataframe and fed it into the np.where, you should input the entire array to the np where.

dfc.iloc[:,2:3] = np.where(dfc.iloc[:,2:3]==0,dfc.iloc[:,3:4].shift(-1),dfc.iloc[:,2:3])
dfc.iloc[:,3:4] = np.where(dfc.iloc[:,3:4]==0,dfc.iloc[:,2:3],dfc.iloc[:,3:4].shift(-1))

This should work now. Be careful about the pd.DataFrame.iloc and avoid it if you are assigning it to new values. I would recommend you to use loc instead. My script may have potential bug depends on your pandas version.

Correct answer by Fergus Kwan on December 7, 2020

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