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You can use the following syntax to calculate a rolling mean in a PySpark DataFrame:

from pyspark.sql import Window
from pyspark.sql import functions as F

#define window for calculating rolling mean
w = (Window.orderBy('day').rowsBetween(-3, 0))

#create new DataFrame that contains 4-day rolling mean column
df_new = df.withColumn('rolling_mean', F.avg('sales').over(w))

This particular example creates a new column that contains the 4-day rolling average of values in the sales column of the DataFrame.

The following example shows how to use this syntax in practice.

Example: How to Calculate a Rolling Mean in PySpark

Suppose we have the following PySpark DataFrame that contains information about the sales made during 10 consecutive days at some grocery store:

from pyspark.sql import SparkSession
spark = SparkSession.builder.getOrCreate()

#define data
data = [[1, 11], 
        [2, 8], 
        [3, 4], 
        [4, 5], 
        [5, 5], 
        [6, 8],
        [7, 7],
        [8, 7],
        [9, 6],
        [10, 4]] 
  
#define column names
columns = ['day', 'sales']

#create dataframe using data and column names
df = spark.createDataFrame(data, columns) 
  
#view dataframe
df.show()

+---+-----+
|day|sales|
+---+-----+
|  1|   11|
|  2|    8|
|  3|    4|
|  4|    5|
|  5|    5|
|  6|    8|
|  7|    7|
|  8|    7|
|  9|    6|
| 10|    4|
+---+-----+

We can use the following syntax to calculate the 4-day rolling mean of values in the sales column:

from pyspark.sql import Window
from pyspark.sql import functions as F

#define window for calculating rolling mean
w = (Window.orderBy('day').rowsBetween(-3, 0))

#create new DataFrame that contains 4-day rolling mean column
df_new = df.withColumn('rolling_mean', F.avg('sales').over(w))

#view new DataFrame
df_new.show()

+---+-----+-----------------+
|day|sales|     rolling_mean|
+---+-----+-----------------+
|  1|   11|             11.0|
|  2|    8|              9.5|
|  3|    4|7.666666666666667|
|  4|    5|              7.0|
|  5|    5|              5.5|
|  6|    8|              5.5|
|  7|    7|             6.25|
|  8|    7|             6.75|
|  9|    6|              7.0|
| 10|    4|              6.0|
+---+-----+-----------------+

The resulting DataFrame contains a new column called rolling_mean that shows the rolling mean of the values in the sales column for the most recent 4 days.

For example, the rolling mean of values in the sales column on day 4 is calculated as: 

Rolling Mean = (11 + 8 + 4 + 5) / 4 = 7

And the rolling mean of values in the sales column on day 5 is calculated as: 

Rolling Mean = (8 + 4 + 5 + 5) / 4 = 5.5

And so on.

Note that you can calculate a rolling average using a different number of previous periods by simply changing the first value in the rowsBetween function.

For example, you could instead calculate a 5-day rolling average by using rowsBetween(-4, 0) instead.

Additional Resources

The following tutorials explain how to perform other common tasks in PySpark: