How Can I Fill Missing Values in One Column with Values from Another Column in Pandas?

Filling Missing Values with Values from Another Column using fillna()

Problem:

I need to fill missing values in one column (e.g., Cat2) with values from another column (e.g., Cat1) using Pandas' fillna() method. It's recommended to avoid inefficient row-wise looping and perform the operation in a single step.

Solution:

Pandas allows you to pass an entire column as an argument to fillna(). It will align the values based on matching indexes and fill the missing values accordingly.

df['Cat2'].fillna(df['Cat1'])

Example:

Consider the following data:

Day	Cat1	Cat2
1	cat	mouse
2	dog	elephant
3	cat	giraf
4	NaN	ant

After applying the fillna() method, the missing value in Cat2 for Day 4 will be filled with ant:

Day	Cat1	Cat2
1	cat	mouse
2	dog	elephant
3	cat	giraf
4	ant	ant

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