Select columns in Pyspark Dataframe

问题

I am looking for a way to select columns of my dataframe in pyspark. For the first row, I know I can use df.first() but not sure about columns given that they do not have column names.

I have 5 columns and want to loop through each one of them.

+--+---+---+---+---+---+---+
|_1| _2| _3| _4| _5| _6| _7|
+--+---+---+---+---+---+---+
|1 |0.0|0.0|0.0|1.0|0.0|0.0|
|2 |1.0|0.0|0.0|0.0|0.0|0.0|
|3 |0.0|0.0|1.0|0.0|0.0|0.0|

回答1:

Try something like this:

df.select([c for c in df.columns if c in ['_2','_4','_5']]).show()

回答2:

First two columns and 5 rows

 df.select(df.columns[:2]).take(5)

回答3:

You can use an array and unpack it inside the select:

cols = ['_2','_4','_5']
df.select(*cols).show()

回答4:

Use df.schema.names:

spark.version
# u'2.2.0'

df = spark.createDataFrame([("foo", 1), ("bar", 2)])
df.show()
# +---+---+ 
# | _1| _2|
# +---+---+
# |foo|  1| 
# |bar|  2|
# +---+---+

df.schema.names
# ['_1', '_2']

for i in df.schema.names:
  # df_new = df.withColumn(i, [do-something])
  print i
# _1
# _2

回答5:

The dataset in ss.csv contains some columns I am interested in:

ss_ = spark.read.csv("ss.csv", header= True, 
                      inferSchema = True)
ss_.columns

['Reporting Area', 'MMWR Year', 'MMWR Week', 'Salmonellosis (excluding Paratyphoid fever andTyphoid fever)†, Current week', 'Salmonellosis (excluding Paratyphoid fever andTyphoid fever)†, Current week, flag', 'Salmonellosis (excluding Paratyphoid fever andTyphoid fever)†, Previous 52 weeks Med', 'Salmonellosis (excluding Paratyphoid fever andTyphoid fever)†, Previous 52 weeks Med, flag', 'Salmonellosis (excluding Paratyphoid fever andTyphoid fever)†, Previous 52 weeks Max', 'Salmonellosis (excluding Paratyphoid fever andTyphoid fever)†, Previous 52 weeks Max, flag', 'Salmonellosis (excluding Paratyphoid fever andTyphoid fever)†, Cum 2018', 'Salmonellosis (excluding Paratyphoid fever andTyphoid fever)†, Cum 2018, flag', 'Salmonellosis (excluding Paratyphoid fever andTyphoid fever)†, Cum 2017', 'Salmonellosis (excluding Paratyphoid fever andTyphoid fever)†, Cum 2017, flag', 'Shiga toxin-producing Escherichia coli, Current week', 'Shiga toxin-producing Escherichia coli, Current week, flag', 'Shiga toxin-producing Escherichia coli, Previous 52 weeks Med', 'Shiga toxin-producing Escherichia coli, Previous 52 weeks Med, flag', 'Shiga toxin-producing Escherichia coli, Previous 52 weeks Max', 'Shiga toxin-producing Escherichia coli, Previous 52 weeks Max, flag', 'Shiga toxin-producing Escherichia coli, Cum 2018', 'Shiga toxin-producing Escherichia coli, Cum 2018, flag', 'Shiga toxin-producing Escherichia coli, Cum 2017', 'Shiga toxin-producing Escherichia coli, Cum 2017, flag', 'Shigellosis, Current week', 'Shigellosis, Current week, flag', 'Shigellosis, Previous 52 weeks Med', 'Shigellosis, Previous 52 weeks Med, flag', 'Shigellosis, Previous 52 weeks Max', 'Shigellosis, Previous 52 weeks Max, flag', 'Shigellosis, Cum 2018', 'Shigellosis, Cum 2018, flag', 'Shigellosis, Cum 2017', 'Shigellosis, Cum 2017, flag']

but I only need a few:

columns_lambda = lambda k: k.endswith(', Current week') or k == 'Reporting Area' or k == 'MMWR Year' or  k == 'MMWR Week'

The filter returns the list of desired columns, list is evaluated:

sss = filter(columns_lambda, ss_.columns)
to_keep = list(sss)

the list of desired columns is unpacked as arguments to dataframe select function that return dataset containing only columns in the list:

dfss = ss_.select(*to_keep)
dfss.columns

The result:

['Reporting Area',
 'MMWR Year',
 'MMWR Week',
 'Salmonellosis (excluding Paratyphoid fever andTyphoid fever)†, Current week',
 'Shiga toxin-producing Escherichia coli, Current week',
 'Shigellosis, Current week']

The df.select() has a complementary pair: http://spark.apache.org/docs/2.4.1/api/python/pyspark.sql.html#pyspark.sql.DataFrame.drop

to drop the list of columns.

来源：https://stackoverflow.com/questions/46813283/select-columns-in-pyspark-dataframe