In our last tutorial section, we looked at more data transformations with Spark. In this tutorial, we will continue with data transformations on Spark RDD before we move on to Actions. Today, we will focus on map and intersect keywords and apply them to Spark RDDs.
A intersection between two different datasets only returns that values that – well – intersect. This means that only those values are returned if their values correspond. Only one element is returned, even though there are multiple elements in both datasets. Intersection is used like this:
ds_one = sc.parallelize([("Mark", 1984), ("Lisa", 1985)]) ds_two = sc.parallelize([("Mark", 1984), ("Anastasia", 2017)]) sorted(ds_one.intersection(ds_two).collect())
The output should look like this now:
Please note that “Mark” is only considered by the intersection keyword if all keys match. In our case, this means the year and name.
Map returns a new RDD, that was transformed by applying a function to it. This is very useful if you want to check a dataset for different things or modify data. You can also add entries by this. The following example is using a little more complex transformation where we create our own function and calculate the age and gender of the person:
import datetime def doTheTrans(values): age = datetime.datetime.now().year - values gender = "" if values.endswith("a"): gender = "f" else: gender = "m" return ([values, age, gender]) ds_one = sc.parallelize([("Mark", 1984), ("Lisa", 1985)]) sorted(ds_one.map(doTheTrans).collect())
What happens in the code?
First, we import datetime. We need this to get the current year (we could also hardcode “2019”, but then I would have to re-write the entire tutorial in january!). Next, we create our function which we call “doTheTrans”. This gets tuples from the map function (which we will use later). Note that tuples are immutable, therefore we need to create new variables and return a new tuple.
First, we calculate the age. This is easily done by “datetime.now().year – values”. We have the year the person is born in the second index (number 1) in the tuple. Then, we check if the name ends with “a” and define if the person is female or male. Please note here that this isn’t sufficient normally, we just use it for our sample.
Once we are done with this, we can call the “map” function with the function we have used. Now, the map function applies the new function which we created to each item in the rdd.
Now, the output of this should look like the following:
[['Lisa', 34, 'f'], ['Mark', 35, 'm']]
Data Transformations on Spark with a Lambda Function
It is also possible to write the function inline as lambda, if your transformations aren’t that complex. Suppose we only want to return the age of a person; here, the code would look like the following:
sorted(ds_one.map(lambda x: (x, datetime.datetime.now().year - x +1)).collect())
And the output should be this:
[('Lisa', 35), ('Mark', 36)]
Now, after 3 intense tutorials on Data transformations on Spark RDD, we will focus on Actions in Spark in the next tutorial.
There is of course much more to learn about Spark, so make sure to read the entire Apache Spark Tutorial. I regularly update this tutorial with new content. Also, I created several other tutorials, such as the Machine Learning Tutorial and the Python for Spark Tutorial. The official Apache Spark page can intensify your experience. Your learning journey can still continue.
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