get value out of dataframe

问题:

In Scala I can do get(#) or getAs[Type](#) to get values out of a dataframe. How should I do it in pyspark?

I have a two columns DataFrame: item(string) and salesNum(integers). I do a groupby and mean to get a mean of those numbers like this:

saleDF.groupBy("salesNum").mean()).collect()

and it works. Now I have the mean in a dataframe with one value.

How can I get that value out of the dataframe to get the mean as a float number?

回答1:

collect() returns your results as a python list. To get the value out of the list you just need to take the first element like this:

saleDF.groupBy("salesNum").mean()).collect()[0] 

回答2:

To be precise collect returns a list whose elements are of type class 'pyspark.sql.types.Row'.

In your case to extract the real value you should do:

saleDF.groupBy("salesNum").mean()).collect()[0]["avg(yourColumnName)"]

where yourColumnName is the name of the column you are taking the mean of (pyspark when applying mean, rename the resulting column in this way by default).

As an example I ran the following code, look at the types and outputs of each step.

>>> columns = ['id', 'dogs', 'cats', 'nation']
>>> vals = [
...      (2, 0, 1, 'italy'),
...      (1, 2, 0, 'italy'),
...      (3, 4, 0, 'france')
... ]
>>> df = sqlContext.createDataFrame(vals, columns)
>>> df.groupBy("nation").mean("dogs").collect()
[Row(nation=u'france', avg(dogs)=4.0), Row(nation=u'italy', avg(dogs)=1.0)]
>>> df.groupBy("nation").mean("dogs").collect()[0]
Row(nation=u'france', avg(dogs)=4.0))
>>> df.groupBy("nation").mean("dogs").collect()[0]["avg(dogs)"]
4.0
>>> type(df.groupBy("nation").mean("dogs").collect())
<type 'list'>
>>> type(df.groupBy("nation").mean("dogs").collect()[0])
<class 'pyspark.sql.types.Row'>
>>> type(df.groupBy("nation").mean("dogs").collect()[0]["avg(dogs)"])
<type 'float'>
>>> 
>>>