Trimming Characters from Strings¶
Let us go through how to trim unwanted characters using Spark Functions.
We typically use trimming to remove unnecessary characters from fixed length records.
Fixed length records are extensively used in Mainframes and we might have to process it using Spark.
As part of processing we might want to remove leading or trailing characters such as 0 in case of numeric types and space or some standard character in case of alphanumeric types.
As of now Spark trim functions take the column as argument and remove leading or trailing spaces. However, we can use
exprorselectExprto use Spark SQL based trim functions to remove leading or trailing spaces or any other such characters.Trim spaces towards left -
ltrimTrim spaces towards right -
rtrimTrim spaces on both sides -
trim
Let us start spark context for this Notebook so that we can execute the code provided. You can sign up for our 10 node state of the art cluster/labs to learn Spark SQL using our unique integrated LMS.
from pyspark.sql import SparkSession
import getpass
username = getpass.getuser()
spark = SparkSession. \
builder. \
config('spark.ui.port', '0'). \
config("spark.sql.warehouse.dir", f"/user/{username}/warehouse"). \
enableHiveSupport(). \
appName(f'{username} | Python - Processing Column Data'). \
master('yarn'). \
getOrCreate()
If you are going to use CLIs, you can use Spark SQL using one of the 3 approaches.
Using Spark SQL
spark2-sql \
--master yarn \
--conf spark.ui.port=0 \
--conf spark.sql.warehouse.dir=/user/${USER}/warehouse
Using Scala
spark2-shell \
--master yarn \
--conf spark.ui.port=0 \
--conf spark.sql.warehouse.dir=/user/${USER}/warehouse
Using Pyspark
pyspark2 \
--master yarn \
--conf spark.ui.port=0 \
--conf spark.sql.warehouse.dir=/user/${USER}/warehouse
Tasks - Trimming Strings¶
Let us understand how to use trim functions to remove spaces on left or right or both.
Create a Dataframe with one column and one record.
Apply trim functions to trim spaces.
l = [(" Hello. ",) ]
df = spark.createDataFrame(l).toDF("dummy")
df.show()
+-------------+
| dummy|
+-------------+
| Hello. |
+-------------+
from pyspark.sql.functions import col, ltrim, rtrim, trim
df.withColumn("ltrim", ltrim(col("dummy"))). \
withColumn("rtrim", rtrim(col("dummy"))). \
withColumn("trim", trim(col("dummy"))). \
show()
+-------------+----------+---------+------+
| dummy| ltrim| rtrim| trim|
+-------------+----------+---------+------+
| Hello. |Hello. | Hello.|Hello.|
+-------------+----------+---------+------+
from pyspark.sql.functions import expr
spark.sql('DESCRIBE FUNCTION rtrim').show(truncate=False)
+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
|function_desc |
+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
|Function: rtrim |
|Class: org.apache.spark.sql.catalyst.expressions.StringTrimRight |
|Usage:
rtrim(str) - Removes the trailing space characters from `str`.
rtrim(trimStr, str) - Removes the trailing string which contains the characters from the trim string from the `str`
|
+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
# if we do not specify trimStr, it will be defaulted to space
df.withColumn("ltrim", expr("ltrim(dummy)")). \
withColumn("rtrim", expr("rtrim('.', rtrim(dummy))")). \
withColumn("trim", trim(col("dummy"))). \
show()
+-------------+----------+--------+------+
| dummy| ltrim| rtrim| trim|
+-------------+----------+--------+------+
| Hello. |Hello. | Hello|Hello.|
+-------------+----------+--------+------+
spark.sql('DESCRIBE FUNCTION trim').show(truncate=False)
+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
|function_desc |
+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
|Function: trim |
|Class: org.apache.spark.sql.catalyst.expressions.StringTrim |
|Usage:
trim(str) - Removes the leading and trailing space characters from `str`.
trim(BOTH trimStr FROM str) - Remove the leading and trailing `trimStr` characters from `str`
trim(LEADING trimStr FROM str) - Remove the leading `trimStr` characters from `str`
trim(TRAILING trimStr FROM str) - Remove the trailing `trimStr` characters from `str`
|
+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
df.withColumn("ltrim", expr("trim(LEADING ' ' FROM dummy)")). \
withColumn("rtrim", expr("trim(TRAILING '.' FROM rtrim(dummy))")). \
withColumn("trim", expr("trim(BOTH ' ' FROM dummy)")). \
show()
+-------------+----------+--------+------+
| dummy| ltrim| rtrim| trim|
+-------------+----------+--------+------+
| Hello. |Hello. | Hello|Hello.|
+-------------+----------+--------+------+