PySpark replace null in column with value in other column

Question

I want to replace null values in one column with the values in an adjacent column ,for example if i have

A|B
0,1
2,null
3,null
4,2

I want i

Answer 1

df.rdd.map(lambda row: row if row[1] else Row(a=row[0],b=row[0])).toDF().show()