从RDD创建DataFrame

1.pandas df 与 spark df的相互转换

df_s=spark.createDataFrame(df_p)

df_p=df_s.toPandas()

import pandas as pd
import numpy as np
arr = np.arange(6).reshape(-1,3)

df_p=pd.DataFrame(arr)
df_p

df_p.columns=['a','b','c']
df_p

df_s=spark.createDataFrame(df_p)
df_s.show()

 

df_s.collect()

df_s.toPandas()

2. Spark与Pandas中DataFrame对比

http://www.lining0806.com/spark%E4%B8%8Epandas%E4%B8%ADdataframe%E5%AF%B9%E6%AF%94/

 

3.1 利用反射机制推断RDD模式

• sc创建RDD
• 转换成Row元素，列名=值
• spark.createDataFrame生成df
• df.show(), df.printSchema()

from pyspark.sql import Row
people = spark.sparkContext.textFile('file:///usr/local/spark/examples/src/main/resources/people.txt').map(lambda line:line.split(',')).map(lambda w:Row(name=w[0],age=int(w[1])))
sPeople = spark.createDataFrame(people)
sPeople.createOrReplaceTempView('people')

personDF = spark.sql('select name,age from people where age>20')
personRDD = personDF.rdd.map(lambda p:"Name:"+p.name+","+"Age:"+str(p.age))
personRDD.foreach(print)

 

sPeople.show()

sPeople.printSchema()

3.2 使用编程方式定义RDD模式

• 生成“表头”
  • fields = [StructField(field_name, StringType(), True) ,...]
  • schema = StructType(fields)

from pyspark.sql.types import *
from pyspark.sql import Row
schemaString = 'name age'
fields = [StructField(field_name,StringType(),True) for field_name in schemaString.split(' ')]
schema = StructType(fields)

• 生成“表中的记录”
  • 创建RDD
  • 转换成Row元素，列名=值

lines = spark.sparkContext.textFile('file:///usr/local/spark/examples/src/main/resources/people.txt')
part = lines.map(lambda w:w.split(","))
peoples = part.map(lambda p:Row(p[0],p[1].strip()))
peoples.collect()

• 把“表头”和“表中的记录”拼装在一起
  • = spark.createDataFrame(RDD, schema)

schemaPeople = spark.createDataFrame(people,schema)
schemaPeople.show()
schemaPeople.printSchema()

 4. DataFrame保存为文件

df.write.json(dir)

schemaPeople.write.json('file:///home/hadoop/schema_out')