How do I get the last item from a list using pyspark?

Question

Why does column 1st_from_end contain null:

from pyspark.sql.functions import split
df = sqlContext.createDataFrame([('a b c d',)], ['s',])
df.select(   split(df.s, ' ')[0].alias('0th'),
             split(df.s, ' ')[3].alias('3rd'),
             split(df.s, ' ')[-1].alias('1st_from_end')
         ).show()

I thought using [-1] was a pythonic way to get the last item in a list. How come it doesn't work in pyspark?

Answer 1

For Spark 2.4+, use pyspark.sql.functions.element_at, see below from the documentation:

element_at(array, index) - Returns element of array at given (1-based) index. If index < 0, accesses elements from the last to the first. Returns NULL if the index exceeds the length of the array.

from pyspark.sql.functions import element_at, split, col

df = spark.createDataFrame([('a b c d',)], ['s',])

df.withColumn('arr', split(df.s, ' ')) \
  .select( col('arr')[0].alias('0th')
         , col('arr')[3].alias('3rd')
         , element_at(col('arr'), -1).alias('1st_from_end')
     ).show()

+---+---+------------+
|0th|3rd|1st_from_end|
+---+---+------------+
|  a|  d|           d|
+---+---+------------+

Answer 2

If you're using Spark >= 2.4.0 see jxc's answer below.

In Spark < 2.4.0, dataframes API didn't support -1 indexing on arrays, but you could write your own UDF or use built-in size() function, for example:

>>> from pyspark.sql.functions import size
>>> splitted = df.select(split(df.s, ' ').alias('arr'))
>>> splitted.select(splitted.arr[size(splitted.arr)-1]).show()
+--------------------+
|arr[(size(arr) - 1)]|
+--------------------+
|                   d|
+--------------------+