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PySpark DataFrame | sample method

schedule Aug 12, 2023
Last updated
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PySpark DataFrame's sample(~) method returns a random subset of rows of the DataFrame.

Parameters

1. withReplacement | boolean | optional

  • If True, then sample with replacement, that is, allow for duplicate rows.

  • If False, then sample without replacement, that is, do not allow for duplicate rows.

By default, withReplacement=False.

WARNING

If withReplacement=False, then Bernoulli sampling is performed, which is a technique where we iterate over each element and we include the element into sample with a probability of fraction. On the other hand, withReplacemnt=True will use Poisson sampling. I actually don't quite understand this, and if you have any idea as to what this is, please let me know!

2. fraction | float

A number between 0 and 1, which represents the probability that a value will be included in the sample. For instance, if fraction=0.5, then each element will be included in the sample with a probability of 0.5.

WARNING

The sample size of the subset will be random since the sampling is performed using Bernoulli sampling (if withReplacement=True). This means that even setting fraction=0.5 may result in a sample without any rows! On average though, the supplied fraction value will reflect the number of rows returned.

3. seed | int | optional

The seed for reproducibility. By default, no seed will be set which means that the derived samples will be random each time.

Return Value

A PySpark DataFrame (pyspark.sql.dataframe.DataFrame).

Examples

Consider the following PySpark DataFrame:

df = spark.createDataFrame([["Alex", 20],\
["Bob", 24],\
["Cathy", 22],\
["Doge", 22]],\
["name", "age"])
df.show()
+-----+---+
| name|age|
+-----+---+
| Alex| 20|
| Bob| 24|
|Cathy| 22|
| Doge| 22|
+-----+---+

Sampling random rows from a PySpark DataFrame (Bernoulli sampling)

To get a random sample in which the probability that an element is included in the sample is 0.5:

df.sample(fraction=0.5).show()
+----+---+
|name|age|
+----+---+
|Doge| 22|
+----+---+

Running the code once again may yield a sample of different size:

df.sample(fraction=0.5).show()
+-----+---+
| name|age|
+-----+---+
| Alex| 20|
|Cathy| 22|
+-----+---+

This is because the sampling is based on Bernoulli sampling as explained in the beginning.

Sampling with replacement (Poisson Sampling)

Once again, consider the following PySpark DataFrame:

df = spark.createDataFrame([["Alex", 20],\
["Bob", 24],\
["Cathy", 22],\
["Doge", 22]],\
["name", "age"])
df.show()
+-----+---+
| name|age|
+-----+---+
| Alex| 20|
| Bob| 24|
|Cathy| 22|
| Doge| 22|
+-----+---+

To sample with replacement (using Poisson sampling), use withReplacement=True:

df.sample(fraction=0.5, withReplacement=True).show()
+-----+---+
| name|age|
+-----+---+
| Alex| 20|
| Bob| 24|
| Bob| 24|
| Bob| 24|
|Cathy| 22|
+-----+---+

Notice how the sample size can exceed the original dataset size.

robocat
Published by Isshin Inada
Edited by 0 others
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